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Contents
Overview
Safety
System Components
Declaration of Conformity
System Setup
Software Installation
Equipment Setup
Getting Started
MAX (Measurement & Automated Explorer)
Front Panel
Taking a Measurement
Acquisition Process
Carrier Mobility
Measurement Accuracy
List of Symbols and Acronyms
Ossila FACT1 Troubleshooting
Warranty information and contact details
List of compatible components
Appendices
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Overview
The Ossila Fast Automated Characterisation and Test (FACT1) system is designed to make testing of
organic field effect transistors as simple as possible when combined with Ossila's OFET substrate
systems. Fabrication of devices can be as simple as spin-coating a material on a test-chip and
inserting it into the system. Alongside this, automated test routines perform output and/or transfer
sweeps and calculate the key parameters.
Each substrate contains five individual OFETs with connections made by using gold-coated springloaded connectors. The automatic selection of a device uses a high performance switching network.
At the core of the measurement system is a computer controlled NI PXI dual source-measure unit
with maximum output of ±100 V and accuracy up to 10 pA. This combined high level of output
voltage and accuracy makes for maximum experimental versatility and speed.
Finally, a touch screen graphical user interface makes control and measurement as simple as
possible.
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Safety
Warning
To avoid safety hazards obey the following:

Do not leave devices with applied bias or current unattended as a power failure may
result in board damage or device damage and potentially hazardous situations.
Caution
To avoid damaging devices or equipment obey the following:





Avoid electrostatic discharge (ESD) as this may damage the device. To avoid damage,
use static discharge and prevention equipment where necessary.
Only use the device for the purposes intended (described in this document)
Do not expose the device to any cleaning fluids or solvents.
Ensure that the PXI system is kept away from the CPU to avoid overheating.
Follow good practice when setting up the test system. Avoid placing mobile phones on top
of the system as this can cause interference.
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System Components
NI PXIe-1071 Chassis.
NI PXIe-PCIe8361 (mounted in slot 1 of the
Chassis) and NI PCIe-8361 card (mounted
inside the computer case).
Two NI PXI-4132 SMU.
Ossila Measurement Box containing five
probes and relative relays, NI USB-6501 OEM,
and control electronics.
CSL Speed A25231uH
(Window®7 operative system) endowed with
Core®i5 Intel microprocessors.
Touch Screen monitor
Keyboard
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Mouse
Two flexible coaxial cables endowed
with NI PXI-4132 Front Panel I/O
Connector.
Two USB® cables with Standard A and
Standard B plugs on the opposite ends
Also included

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Three Power Supply Leads (Cords) for the touch screen, computer and PXIe-1071
Chassis.
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EC Declaration of Conformity
In line with directive 2004/108/EC of the European Parliament and of the Council and directive
2006/95/EC of the European Parliament and of the Council.
-Manufacturer Name: Ossila Limited
Manufacturer Address: Kroto Innovation Centre, North Campus, Broad Lane, Sheffield, S3
7HQ
-Item: OFET testing rig for use with PXI system
Model number: FACT1
Software release: FACT1.2
-Specifications of product under harmonised standards:
2004/108/EC
EN 61326-1:2006 Electrical equipment for measurement, control and laboratory use – EMC
requirements – Part 1: General requirements IEC 61326-1:2005
EN 61326-2-1:2006 Electrical equipment for measurement, control and laboratory use – EMC
requirements – Part 2-1: Particular requirements – Test configurations, operational conditions
and performance criteria for sensitive test and measurement equipment for EMC unprotected
applications IEC 61326-2-1:2005
2006/95/EC
EN 61010-1:2010 Safety requirements for electrical equipment for measurement, control,
and laboratory use - Part 1: General requirements IEC 61010-1:2010
EN 61010-2-030:2010 Safety requirements for electrical equipment for measurement,
control, and laboratory use - Part 2-030: Particular requirements for testing and measuring
circuits IEC 61010-2-030:2010
EN 61140:2002 Protection against electric shock - Common aspects for installation and
equipment IEC 61140:2001
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EN 61187:1994 Electrical and electronic measuring equipment - Documentation IEC
61187:1993 (Modified)
EN 61010-2-081:2002 Safety requirements for electrical equipment for measurement,
control and laboratory use - Part 2-081: Particular requirements for automatic and semiautomatic laboratory equipment for analysis and other purposes IEC 61010-2-081:2001
Declaration:
I hereby declare that the equipment named above has been designed to comply with the
relevant sections of the above referenced specifications. The unit complies with all applicable
Essential Requirements of the Directives.
-Signed:
Name: Dr James Kingsley
Date: 23/11/2014
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System Setup
Ossila FACT1 is easy and straightforward to set up. Only a few cable connections are required to
make the system fully operational. Nonetheless, the User must read carefully the following
instructions in order to safely and efficiently exploit the performance and accuracy of Ossila FACT1.
Ossila FACT1 is an integrated measurement system composed of the following parts:





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
Ossila Measurement Box containing five probes and relative relays, NI USB-6501 OEM, and
control electronics.
NI PXIe-1071 Chassis.
NI PXIe-PCIe8361 (mounted in slot 1 of the Chassis) and NI PCIe-8361 card (mounted inside
the computer case).
Two NI PXI-4132 SMU.
Touch Screen monitor
CSL Speed A25231uH desk computer (Window®7 operative system) endowed with Core®i5
Intel microprocessors.
Two flexible coaxial cables endowed with NI PXI-4132 Front Panel I/O Connector.
Two USB® cables with Standard A and Standard B plugs on the opposite ends.
Three Power Supply Leads (Cords) for the touch screen, computer and PXIe-1071 Chassis.
Before starting to assemble the components of Ossila FACT1, make sure that the cables and
connections do not present any damage or alteration that can compromise the safety of the
personnel in charge of the measurement unit and/or affect the performance of the device. In
addition, you must read the NI PXI-4132 SMU, NI PXIe-PCIe8361 and NI PXIe-1071 User’s manual
first, which are provided with this document, with particular reference to the Safety Information and
System Specifications.
Do not operate any third party devices provided with Ossila FACT1 outside the operational range
specified by their respective manufacturers. Do not dislodge the two PXI-4132 SMU units and the NI
PXIe-PCIe8361 from the NI PXIe-1071 Chassis without consulting their respective User’s Manuals.
Ossila must not be held responsible for any injury or damage due to the non strict observance of the
guideline outlined in the User’s Manuals of any third party equipment provided as part of Ossila
FACT1.
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Software Installation
In the case Ossila FACT1 software and drivers have not been previously installed (or in case of
reinstallation of Ossila FACT1 software package), you must install the Installer first.
Insert the CD/DVD provided in the CD/DVD reader and wait for the auto-installation window to
appear and follow the on screen instructions. The installer will automatically install the main and
ancillary software and the drivers. If the auto-run is not functional, you may need to open the
CD/DVD and locate the executable (named setup) and double-click its icon to start the self –
installation.
The software package is composed of the following parts:
1)
2)
3)
4)
5)
6)
7)
8)
Ossila FACT1 (head file)
LabVIEW® Run-Time Engine 2012 f3 or later version.
NI-DAQmx Core Runtime 9.5.5.
NI-DAQmx MAX Configuration Support 9.5.5.
NI-DCPower 1.7.
NI-DCPower Run-Time 1.7 f1.
NI-VISA Configuration Support 5.2.
NI-VISA Runtime 5.2.
Ossila FACT1 is the Ossila developed application controlling (through the drivers) measurement
units, Ossila Box and the output\input functionalities. Items 2) to 8) are the National Instrument
software providing the support to Ossila FACT1 (LabVIEW® Run-Time Engine) and the drivers
required to operate the hardware components of Ossila FACT1. The NI support software and
drivers can be freely downloaded from the National Instruments website (www.ni.com), under
the condition that the User complies with the National Instruments Licence Agreement for each
downloaded software.
In case Ossila FACT1 installation CD/DVD package does not contain all the required software, you
must install Ossila FACT1 first, and then the ancillary software and drivers. Please, make sure
that you are installing the correct version of the required software as specified in this User’s
Manual. If any of the ancillary software and drivers is missing, Ossila FACT1 will prompt a pop-up
window with a list of the missing item(s) and a reference to the National Instrument website as
software repository.
Please contact Ossila if you experience any problem during the software installation.
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Equipment setup
1) Once the software has been installed, you must switch off the computer before you start to
set up Ossila FACT1. With all the devices switched off, place the components of Ossila FACT1
using Figure 1 as a guideline. In order to achieve high precision measurement, it is important
to reduce any source of electromagnetic interference: avoid operating the system with a
mobile phone or any other electronic devices in the proximity of the PXI SMUs and Ossila
Box; maximise the distance between the measurement units (SMUs and Ossila Box) and the
computer case (as depicted in Figure 1).
2) Connect the two coaxial cables connectors (Gate and Drain) to their respective female
sockets on the Ossila Box front panel, see Figure 2. Channel A is the Gate channel, while
Channel B is the Drain.
Plug the NI PXI-4132 Front Panel I/O Connector located at the other ends of the coaxial cable
to the NI PXI-4132. The PXI in slot 2 is the Gate source, while slot 3 is the Drain source. Be
careful to plug the connectors correctly, take advantage of the labels at both ends of the
cables to double-check the connections.
NOTE The I/O PXI connectors must be plugged in a manner such that the warning label is on
the right side of both connectors (In case you plug the two I/O PXI connectors the other way
around, no output power will be supplied to the Ossila Box and, consequently, no
measurement can be carried out), see Figure 3. In FACT1, these connections allow the two
PXIs to supply the source output (Gate and Drain voltages) to the measurement targets
(OFETs) and to measure the input (Drain and Gate currents).
3) Connect the serial port of the NI PXIe-8361 (slot 1 of the PXI Chassis) to the NI PCIe-8361
card (mounted on the back of the computer case) using the serial port cable provided, see
Figure 4.
4) Use the longest USB cable provided to connect the Ossila Box with the computer case. To
this purpose, plug the Standard B USB plug to the Standard B USB receptacle located on the
front panel of Ossila Box, see Figure 5. Plug the Standard A USB end of this cable to any USB
socket on the computer case capable to deliver enough electrical current to efficiently and
stably operate the relays mounted inside the Ossila Box. The purpose of this connection is
two-fold: it allows the communication necessary to programmatically control the state of
the relays while supplying the electrical power necessary to switch on/off the relays.
NOTE The USB ports on the back of the desktop computer may supply more electrical power
that the ones located on the front.
5) Connect the touch-screen display to the computer case using the second USB connection
provided. Finally, connect the computer case, touch screen display and PXI chassis to the
power grid.
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Figure 1Update photos: Ossila FACT1 set up.
Figure 2. Coaxial Cables connection: Channel A is the Gate, Channel B is the Drain.
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Figure 3. I/O PXI connectors. The left side PXI (slot2) is the Gate, the right side one (slot 3) cis the drain. Pay attention to
how you plug thses connections: the warning labels has to be on the right of the connetor.
Figure 4. Connect the PXI chassis to the computer case using the serial port cable provided.
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Figure 5. Connect the Ossila BOX to the desktop computer
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Getting started: MAX© (Measurement and Automation Explorer1)
Legal Notice. This section is neither a comprehensive nor a partial User’s guide to Measurement and
Automation Explorer or any other National Instruments (hereafter NI) products. The scope of this
section is to provide the Users with the information necessary to install, test and maintain the
accuracy level of Ossila FACT1 measurement system. Accordingly, this section is exclusively intended
for Ossila FACT1 users and any other use is explicitly forbidden.
Please, refer to National Instruments Measurement and Automation Explorer Help for
comprehensive documentation and User’s guide.
MAX is an NI product covered by one or more of the following Patents: U.S. Patent No(s) 6,690,390;
7,130,760; 7,134,109; 7,152,116.
Introduction
MAX (Measurement and Automation Explorer) is the National Instruments application that provides
direct access to NI hardware. According to NI specifications2, MAX can be used to






Configure your National Instruments hardware and software
Back up or replicate configuration data
Create and edit channels, tasks, interfaces, scales, and virtual instruments
Execute system diagnostics
View devices and instruments connected to your system
Update your National Instruments software
In the following, we refer to the Source Measurement Units (SMU) dedicated to source and measure
the gate and drain voltage/current of transistor as GATE and DRAIN, respectively. Gate and drain
(lower-case letter) refers to the transistor gate and drain, respectively.
Configure and Test National Instrument Hardware used by Ossila FACT
Make sure that all the connections between the Ossila FACT1 units are well-connected, all the power
cords are plugged in and the Ossila Box toggle switch is ON. Before booting the computer you must
switch on the PXI chassis (otherwise the computer will be unable to communicate with the PXIs).
1. Open NI MAX;
2. On the left side MAX subpanel, click on My System.
NI USB-6501 Settings
3. Click on Device and Interface: a list of devices installed or simulated on the computer will
appear, see Fig. 6;
If the USB cable is properly connected to both PC and FACT Box, the NI USB-6501 icon is
green. The icon of a device installed but not currently present or properly connected is
grey with a superimposed red cross;
4. Check if the device alias name is “SwitchControl”;
1
2
MAX, Measurement and Automation Explorer, ©1999-20013 National Instruments. All right reserved
Measurement and Automation Explorer Help, Version 370694R-01, June 2013.
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Ossila FACT uses the case-sensitivity alias name to communicate and control the NI
hardware. Missing or miswritten alias prevents the programme from functioning and no
measurements can be acquired;
When first installed, MAX assigns a standard alias name to MAX-compatible hardware. For
NI USB-6501, this is “DevN”, where N is a integer indicating the number of device
currently installed on the computer.
To change the alias right-click on the alias name, select Rename on the drop-down menu
and input the new alias.
5. Familiarise with the tool bar commands on the central subpanel of MAX, see Fig. 6,
6. Use Reset to reset SwitchControl and Self-Test to test the hardware.
SMU Settings
7. On the left side panel, click on the arrow next to NI PXIe-1071 “Chassis 1” to expand the
associated tree menu, see Fig 7. The PXI interface card and the two PXI SMUs installed are
listed as
1: NI PXIe-83060 “Remote Control”,
2: NI PXI 4132 “PXIGATE”,
3: NI PXI 4132 “PXIDRAIN” .
8. Rename the alias if necessary.
9. Click on “PXIGATE”.
10. Check in Settings that the Slot Number is 2.
11. Locate this slot on the PXI Chassis (the slot numbers are written on the bottom frame of
the PXI Chassis, just below the PXI slots).
12. Click Reset and Self-Test to reset and check that PXIGATE is properly installed and
communicates correctly with the computer.
13. Click Refresh and check the Current Temperature.
Temperatures outside the operational recommended range (23 oC ± 5 oC ) can affect the
accuracy of both output and input of Source Measurements Units, refer to “Measurement
Accuracy” section of this User’s Guide and NI PXI-4132 Specifications for further
information on this topic.
14. Click on Self-Calibration to execute a self calibration to increases the accuracy of the SMU.
Make sure that the temperature of the SMU is in a five degree Celsius interval of the selfcalibration temperature. For more information on self-calibration and accuracy refer to
the “Self-Calibration” section of this User’s Guide.
15. Click on Test Panel to open the NI-DCPOWER Soft Front Panel user interface, see Fig. 8.
The Soft Panel can be used to perform single point current or voltage measurement.
16. Select DC Voltage, 10 V Range, Voltage level = 5 V and local sense, see Fig 8.
17. Select Output Enabled to enable the output and measure the current. Check that the
Output Status led on the PXI GATE switches on when output is enabled.
18. Disable the output and close the Soft Panel.
Note. When Reset is executed, the output is automatically disabled and the PXI is reset to
a well-defined internal status.
19. Select PXI DRAIN and repeat steps 7 to 11; check that PXIDRAIN is mounted in Slot
Number 3.
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Note. Providing the gate and drain input of Ossila FACT Box are connected to the correct
SMU, the two PXIGATE and DRAIN are interchangeable. However, in order to optimise
measurement accuracy, it is advised to use slot 2 for PXIGATE and slot 3 for PXIDRAIN.
Figure 6. SwitchControl (NI USB 6501) MAX front panel.
Figure 7. SMU (PXIDRAIN, PXIGATE and Remote Controller) MAX interface
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Figure 8. NI-DC Soft Front Panel.
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Getting started: Front Panel
Make sure that all the connections between the Ossila FACT1 units are well-connected, all the power
cords are plugged in and the Ossila Box toggle switch is ON. Before booting the computer you must
switch on the PXI chassis (otherwise the computer will be unable to communicate with the PXIs and
Ossila FACT1 will generate a fatal error when opened). After switching on the PXI, wait a few
seconds, then boot the computer and double click on the Ossila FACT1 blue icon on the Desktop.
The main User Interface (henceforth referred as UI) of Ossila FACT1 should now appear on the
computer screen, while the LED mounted on the Ossila BOX sequentially switch ON/OFF signalling
that the relays are properly controlled and powered by the USB connection.
NOTE In order to improve the User’s experience, it is advisable to maximise the window size of the
main UI using the Maximise control on the control bar.
The main UI (see Figure 9) is organized in two panes. The narrow left side pane is the input pane,
where the main controls and setting buttons are located; the larger right side pane is the output
pane and is reserved to the visual output of the measurement and indicators.
Input Pane
From top to bottom,
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


SETTING. When pressed, a second pop-up window UI prompts the User to input the
measurement setting; see the section “Getting started: Taking a measurement,” for more
details.
CANCEL. This command interrupts the acquisition.
SKIP. This control stops the current Device Under Measurement (DUM) acquisition and force
the programme to advance the acquisition to the successive device.
PXI SETTING. This is used to set the parameter controlling the PXI acquisition, see “PXI
Advanced Setting” in “Measurement Accuracy”section for the relevant information on how
to configure the PXI correctly.
PXI RESET. Press this controls to execute a check on the hardware installed. Upon pressing
PXI RESET, the programme checks for the hardware installed and the state of its connection.
In addition, it executes a complete sequential switch ON/OFF of the five relays and relative
LEDs mounted on the Ossila Box. This functionality executes automatically when Ossila
FACT1 is opened. The result of the hardware check is written in the box “Hardware
Installed”.
Note If the PXI Chassis is disconnected, the computer must be rebooted to relink the Chassis
to the computer. If the USB becomes accidentally disconnected, Ossila FACT1 automatically
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relink the USB channel upon pressing PXI RESET or during the initialisation step of the
measurement procedure.
Output Pane

I-V Characteristic Curves and Transfer Characteristic Curves. These two graphs depict the
two main raw data sets acquired by Ossila FACT1. The first shows the Output Characteristic
(drain current versus drain voltage, I-V for short) of the transistor, while the second graph
represents the Transfer Characteristic (TC) curves for the transistor operated in the Linear
and Saturation regions, i.e. IDS(VGS).

Leakage Current. This ancillary plot shows the IGS versus VDS for any I-V curves.

Mobility, Best Fit Saturation Mode, Best Fit Linear Region. Differently from the previous
output plots, these graphs are obtained through data analysis subroutine and their meaning
will be detailed in the section “Carrier Mobility Estimation.”

Compliance LEDs. The compliance LEDs inform the User whether the PXI Source
Measurement Unit hits compliance, i.e. whether the measured current exceed the Current
Limit.

Gate Current Threshold. Gate Current Threshold is the User’s selected threshold setting, a
limit to the gate current (Leakage Current) that the PXI Gate is allowed to measure as the PXI
Drain sweeps through the Drain Voltages.
Figure 9. Main Ossila FACT1 User Interface (UI).
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Getting Started: Taking a Measurement
When Ossila FACT1 opens, and the system is correctly set up, the system enters the idle state and is
ready to acquire the first set of measurements. The bullet points below outline how to enter the
measurement parameters.

Select SETTING on the main UI (Figure 9).
A second UI appears on the screen as pop-up window, Select Devices to Measure and
Measurement Type UI, see Figure 11I.
NOTE The main UI is now irresponsive, you must close this pop-up window to return the
control to the main UI.
Select Devices to Measure and Measurement Type
Select MEASURE ALL to acquire both I-V and Transfer Characteristic Curves (including
mobility estimation) for all five devices. The UI closes and the programme is now committed
to automatically execute a complete measurement cycle using the parameters that the User
will input through the subsequent pop-up window, Enter Parameters (see below).
The User can also customise the measurement by selecting which device(s) to measure and
whether to measure I-V, TC curves or both. The devices to measure are selected on the
lower section of the pop-up window through the square buttons (1 to 5). The rectangular
buttons, I-V CURVES and TC CURVES, select the measurement type. The User must then
press SUBMIT in order to commit the customised measurement, see Figure 11.
Note. MEASURE ALL overrides any selection made on the Customize Your Measurement subpane.
Press CANCEL to interrupt the measurement setting and return the control to the main UI.
Enter Measurement Input and Device Parameters.
Enter Measurement Input and Device Parameters (pop-up window) UI can be used to enter
the measurement parameters and the input necessary to extract the carrier mobility, see
Figure 12.
The input field of this UI is dived in two main sub-panes. The upper pane is used to enter the
parameters controlling the acquisition of the I-V curves or OUTPUT CHARACTERISTICS of
transistors. The lower sub-pane is dedicated to the setting of the TRANSFER
CHARACTERISTICS (TC) measurement plus CARRIER MOBILITY estimation. For both subpanes, the left side (red colour buttons) is controlling the GATE (PXI in slot 2), while the left
side (blue colour) is reserved to the DRAIN controls (PXI in slot 3).
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Note. Both input and output quantities are represented in SI (International System) prefix on
Ossila FACT1 user interface, see table below
SI prefix
pico
nano
micro
milli
Factor
10-12
10-9
10-6
10-3
Symbol
p
n
u
m
Figure 10. Select Devices and Measurement Type UI. This is a pop-up window which allows you to select which devices
to measure and what to measure
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Figure 11. In this example, I-V and TC curves are acquired for Device 1, 4 and 5. You must press the SUBMIT button to
commit your customised measurement. In the case you customise the measurement you must select the measurement
type (I-V CURVES and/or TC CURVES), otherwise no measurement is carried out. MEASURE ALL will automatically commit a
complete measurement cycle on any devices. MEASURE ALL overrides any selection made on the Customize your
Measurement section.
OUTPUT CHARACTERISTICS (GATE). Select the I-V curves to sweep by choosing Gate Voltage
Start, Gate Voltage End and Gate Sweeps, respectively. The last selection determines how
many curves are swept for each selected device. For example, with the selection on Figure
12, Ossila FACT1 sweeps four I-V curves , IDS (VDS), at VGS = (0 V, -20 V, -40 V, -60 V), with
It follows that if
is known, the number of Gate Sweeps is given by
Gate Sweeps allows the User to select any number of I-V curves from 1 to 100.
NOTE Due to hardware limitations, the maximum voltage that is possible to safely handle
with Ossila FACT1 is ±100 V, therefore the controls were programmed not to accept values
exceeding this threshold.
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NOTE Both current and voltage is measured by the SMUs. Consequently, the actual values
logged in the data file can show up to few tens of mV discrepancy with the voltage input
depending on the voltage range being generated, temperature and measurement setting.
Delay is the total settling time and determines for how long an output is applied before the
next step of the measurement subroutine is executed. Specifically, GATE delay is the lapse
time between the application of the gate voltage and the first drain voltage of the sweep.
Delay must be larger than the time required to the system to deliver a stable output and the
duration of the transient of the transistor under measurement, see Appendix V.
According to the National Instruments specification, the typical settling time of the PXI-4132
is 300 µS on a 1 V step and a load of 50 percent of the current range setting. When the
capacitance of Ossila Box is taken into account, and for short BNC cables (i.e.≤ 1 m), the
Delay for fast inorganic transistor can be set at 1 ms for 1 V step or lower.
Organic transistors have much longer transient time. Accordingly, IEEE3 standard for OFETS
characterisation recommends a minimum settling time (dwell time) of 10 ms up to 100 ms
for each data point. The appropriate settling time depends on the dielectric, organic
semiconductor, OFET architecture and measurement settings.
For accurate measurements, indicative Delay values are 100/200 ms for each gate sweeps
(GATE Delay, with
V) and 10 to 20 ms for each drain current acquired (DRAIN
Delay, with
). These values refer to OFF  ON sweep. For ON OFF sweep, the
required delay times are higher. Refer to the Measurement and Accuracy section for more
detailed information on Settling Time.
Current Limit specifies the maximum gate current range (IGS) that the GATE source is allowed
to induce across the target (OFET, for example). If the current exceed Current Limit, the PXI
will complete the acquisition without increasing the voltage further to avoid damaging the
internal circuitry. Refer to the Measurement and Accuracy section for the appropriate
setting of Current Limit and its relation with measurement accuracy.
Max Gate Current. Differently from Current Limit, Max Gate Current is a User’s selected
threshold on IGS as leakage current. More specifically, Max Gate Current is the maximum
IGS(VDS) that the User deems acceptable for the DUM. If Ileak> IGS, Ossila FACT1 stops acquiring
the I-V curves for the particular VGS for which the limit has been exceeded and moves to
acquire the next (if any) I-V curve. If the threshold is exceeded during the last I-V sweep, the
Transfer Characteristics are not acquired and the programme proceeds to characterise the
successive DUM, if any. Use Max Gate Current to speed up data acquisition by avoiding
measuring transistors which present very high gate current leakage.
Note. Set Max Gate Current to be lower than Current Limit. IEEE recommends gate current
leakage to be less than 1% of the drain current IDS(VDS).
3
IEEE Standard for Test Methods for the Characterization of Organic Transistors and Materials; IEEE
Std 1620-2008.
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SMU Name is the alias name of the PXI SMU that executes the sourcing and measurement
task.
Channel Name is the input/output channel executing the acquisition task. Select 0 for each
channel.
Auto Zero enables and disables Auto Zero functionality. Select Auto Zero Once to execute it
only for the first data point for each I-V curves (recommended setting). Use Auto Zero
Enabled to execute Auto Zero correction for each acquired data point. Refer to
Measurement and Accuracy for detailed information on Auto Zero functionality and duration
of the acquisition.
Figure 12. Use this UI pop-up window to select the measurement parameters the input required to estimate the
mobility.
OUTPUT CHARACTERISTICS (DRAIN). The drain section of the OUTPUT CHARACTERISTICS
sub-pane governs the DRAIN source, and allows the User to choose start, end and number of
drain current points to acquire through the controls Drain Voltage Start, Drain Voltage End,
and ΔVDS, respectively. For any VGS, the I-V output characteristics curves IDS(VDS) can then be
acquired in correspondence of the following Drain voltages
:
,
where
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and N is the number of data points (number of steps) with
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Note. Let us suppose that the User wants to acquire a data point for each steps ΔVDS = 1
over the interval Drain Voltage Start =0 V to Drain Voltage End = 10 V, the equation above
gives then N = 11, which fulfils the User’s request since the acquisition is carried out from 0
to 10 V over 11 voltages separated by 1 V step.
Current Limit specifies the maximum drain current (IDS) that the DRAIN source is allowed to
induce in the target (OFET, for example) as the User’s selected gate voltages are applied to
the target itself, i.e. Current Limit refers to maximum allowed IDS(VDS) for any selected VGS .
Delay. DRAIN Delay is the dwell time between the application of a specific drain voltage and
the acquisition of the respective drain and gate current data points. Delay must be greater
than the total settling time, which is given by the sum of the system settling time and the
DUM transient for drain voltage sweeps, see GATE Delay above.
Sweep Back is selected to “sweep back” each I-V curves: for any VGS, Ossila FACT1 measures
the drain current from Drain Voltage Start to Drain Voltage End and hence backward from
Drain Voltage End back to Drain Voltage Start.
SMU Name. Refer to GATE SMU Name.
Channel Name. Refer to GATE Channel Name.
Auto Zero. Refer to GATE Auto Zero.
TRANSFER CHARACTERISTIC (DRAIN). For the Transfer Characteristic two TC curves, IDS(VGS),
are acquired for two different values of drain source voltage (linear and saturation transfer
characteristics).
Drain Voltage (Linear Region) selects the value VDS that the DRAIN source applies to the
target, with VDS drain voltage for which the target transistor is operating in Linear Mode, as it
appears in the I-V curves when IDS = αVDS, with α proportionality constant. Usually, Drain
Voltage (Linear Region) for organic transistors is approximately 5% to 10% of the drain
voltage at which the transistor is operating in saturation mode.
Drain Voltage (Saturation Mode) is the drain voltage at which the transistor operates in
saturation mode, i.e. IDS(VDS) = Isat, with Isat constant drain saturation current reads from the
I-V Output Characteristic curves.
Delay is the PXI DRAIN settling time. For Transfer Characteristic OFF  ON measurement
indicative DRAIN and GATE Delay time are 10 to 200 ms and 10 to 100 ms, respectively. The
actual delay time depends on dielectric capacitance, organic semiconductor, device
architecture and measurement settings.
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Current Limit (not in use for OFET characterisation) specifies the maximum drain current
(IDS) that the DRAIN source is allowed to generate across the target (OFET, for example) as
the User’s selected gate voltages are applied to the target itself, i.e. Current Limit refers to
IDS(VGS) for any VDS applied.
TRANSFER CHARACTERISTIC (GATE). For both VDS (Linear and Saturation regime) the Gate
sub-panel is used to determine the start, end and measurement points of the Transfer
Characteristic curves IDS(VGS) by means of the controls Gate Voltage Start, Gate Voltage End,
and ΔVGS, respectively. IDS(VGS) is measured at
, with
,
with
and N number of data points (number of steps). N is given by
Note Since the TC data is used to estimate the mobility and due to the nature of the
estimation algorithm, see the Sec. “Mobility Estimation” below, it is advisable to measure
the drain current for (at least) each Volts change in the gate potential, i.e.
≤ and, in
any case, acquire no less than 50/60 data points.
Current Limit specifies the maximum gate current (IGS) that the GATE source generated
across the target (OFET, for example) as the User’s selected drain voltages are applied to the
target itself, i.e. Current Limit refers to IGS(VGS) for any VDS applied.
Delay is the PXIGATE settling time. For each data point, the current
time t=Delay has elapsed from the application of gate voltage
.
is acquired after a
CARRIER MOBILITY. This pane field is used to input the parameters required to calculate the
field effect mobility in both Linear and Saturation mode (read carefully the Sec. “Mobility
Estimation” for details on how the mobility is calculated and how to interpret the results).
The Mobility is expressed in
.
Fitting Method is used to select the linear regression method used to build the linear fit of
IDS(VGS), or the fit of the square root of IDS versus VDS. The possible selections are Least
Square, Least Absolute Residual and Bisquare method.
Tolerance is the tolerance for the fitting method. It applies to the Least Square and Bisquare
methods.
Channel Length is the length, expressed in μm, of the transistor channel. By default, Ossila
OFET considers support where all five devices have the same channel length. In this case,
the common channel length must be inserted in the Channel Length-Fixed input field. In the
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case the transistors on the support have variable channel lengths, the User must disable the
Fixed input and enable the variable Channel Length located next to the SUBMIT button. This
operation is accomplished through the Variable blue button. Upon pressing Variable, the
Fixed input field is disable and greyed out, while the variable Channel Length array input
field is enabled. Each entry of this array corresponds to the device specified by the caption
printed below the entry itself.
Capacitance is the capacitance per unit area of transistor expressed in
(Farad per
centimetre square).
Channel Width is the width of the transistor channel expressed in cm.
Note The Channel Length is the distance between the Source and the Drain (it refers to the
gate-voltage-induced conducting channel that allows the carriers to move from Source to
Drain), while the Channel Width is the transversal dimension of the conducting channel.
Commit the Measurement Setting and Data Logging
Upon pressing SUBMIT, the measurement settings UI is replaced by the pop-up window prompting
the User to enter the file name. The User is advised to enter a short, alphanumeric name/code which
identifies the support under study in the File Name filed. The complete name is composed of the
time-stamp, User’s File Name, a device label (such as Dev1), a reference to the measurement type
(IV or TC for I-V and Transfer Characteristics curves, respectively). Ossila FACT1 generates three files
per devices: two xls or spreadsheet files (one for I-V data, the second for TC and mobility); and a
screenshot in png format for the six output plots. By default, the data files are enclosed in an
appositely generated folder and saved on the User’s Desktop. Upon pressing SAVE, the acquisition
starts, while CANCEL returns the control to the main UI
Option. The User can disable the create folder and the time-stamp option by using FOLDER and TS
buttons, respectively. The location of data logging can be change by selecting a different location
with the Folder/File Location ring selector.
NOTE. The File Name field does not accept an empty name and colon character as file name.
Special characters (#,@,&) and punctuation marks should not be used as file name, while dash and
underscore are admissible. Do not add any file extension (such as .txt). The data file can be open
with MS Excel© or any other open source spreadsheet reader.
Acquisition Process
After pressing Save, the acquisition starts and the data is plotted on the graphs on the main UI. In
particular, the I-V, TC and Leakage Current graphs are updated immediately after the measurement
of each curve is completed. Depending upon the number of point measured and settling time, the
acquisition of a single curve may take up to few minutes to be completed and the programme may
seem unresponsive. To inform the User that the programme is acquiring data, a round, blinking LED
is switched ON during the acquisition state. In addition, the active acquisition state of Ossila FACT1 is
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verifiable by the inspection of the Output state on the PXIs: a yellow LED on the front panel of both
PXI-4132 informs the User that both sources (GATE and DRAIN) are effectively applying a potential
difference (VGS and VDS) to the target.
Figure 13 shows the main UI output panel of Ossila FACT1. Each device has its data output located
on a TAB labelled with the device number. You can navigate the table during the acquisition.
The User is given the possibility to stop the acquisition pressing the button CANCEL. The CANCEL
button should disable and reset the hardware so that the hardware is found in the correct internal
state.
SKIP button stops the acquisition of the DUM and move to the successive device, if any; otherwise it
stops the acquisition and returns the control to the main UI.
During the acquisition all the buttons, with the exception of CANCEL and SKIP, are disabled.
Warning! When the Output Enabled is ON, the User must be aware that Shock Hazard may exist. Do
not touch any part of the Ossila FACT1 that is in direct contact with the voltage sources. To this
purpose, if the lid of Ossila Box is open, the contact between the measurement unit (i.e. the board
hosting the target) and the source is interrupted. Do not open the lid of Ossila Box when the
Output Enable LED is ON, this may force the PXI to generate a Hardware error which, in turn, will
stop Ossila FACT1.
If for any reason Ossila FACT1 terminated the acquisition prematurely, or the User decides to stop
the acquisition, but Output Enabled is still ON, the User must press PXI RESET in order to allow the
programme to shut down the hardware gracefully.
Warning! An inappropriate disabling of the hardware output may results in hardware units
characterised by unknown internal state which, in turn, may translate in unexpected behaviour,
hardware failure and hence unreliable measurement outcomes. USE PXI RESET to reset the PXI and
the USB multiplexer.
The completion of the measurement is signalled by a pop-up window informing the User that the
acquisition is complete. Upon selecting OK, the User may navigate through the TABs and export the
graphs to other applications such as Microsoft Word or Excel.
The application can be close by pressing the X button on the right left side of the toolbar (exactly as
for any other Windows/Linux/Mac applications).
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Figure 13. Screenshot of the output of the main UI output panel.
Carrier Mobility
Ossila FACT1 estimates the carrier mobility using the Saturation Mode and Linear Region methods.
Saturation Mode. The potential VDS for which the transistor is operating in saturation mode is
applied to the transistor by the DRAIN source, while the GATE source sweeps across a range of
negative or positive voltages (for p-type or n-type semiconductor, respectively). As a result, a
Transfer Characteristic curve in saturation mode, IDS(VGS) at fixed VDS, is acquired.
The saturation mode current is approximately governed by the well-known equation,
,
(1)
where L and W are the length and width of the conductive channel, respectively; μ the carrier
mobility, and C the capacitance per unit area (in F/cm2). VT, threshold voltage, is the voltage at which
the formation of an inversion layer between the insulating layer and the substrate allows for a nonnegligible drain-source current. For VGS < VT, the transistor is said to be switched off since IDS ≈ 0.
However, in real devices, IDS below the threshold is small, but different from zero, and, more
importantly, the drain current is also a function of VDS.
The mobility in saturation mode
is obtained inverting Eq. (1),
,
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(2)
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with
slope of the root square of the drain current, i.e.
.
Linear Regime. From the analysis of the I-V output characteristic, it can be deduced that the
transistor is operating in a linear regime for small values of VDS. Indicatively, for VDS equals to 5% of
VDS at saturation regime, the mathematical relation between IDS and VDS is linear, i.e. IDS = VDS, with
constant of proportionality. When this VDS is applied to the transistor by the drain source, while
the gate sweeps the device, the Linear Regime transfer characteristic curve for the transistor, IDS(VGS)
for constant VDS, is obtained.
Using the equation governing the transistor operating in Linear Regime, it is possible to derive an
equation for the mobility similar to Eq. (2). In details, assuming
the slope of IDS (VGS) at constant
VDS in Linear Regime, the mobility reads,
.
(3)
Linear Fit
In order to extrapolate the mobility from the experimental data, the slope of the respective Transfer
Characteristic curves must be calculated. However, since the measurement data are a set of discrete
points, {IDS,i (VGS,i)}, it is necessary to “fit” the experimental data to a straight line first (Linear Fit) and
hence calculate the slope of the resulting line.
The Linear Fit or Linear Regression is a mathematical tool that, given a set of measurement points
such as {IDS,i (VGS,i)}, is looking for the straight line that best fits the data.
Schematically,
Linear Fit
{IDS,i (VGS,i)} →
Measurement data
(Set of discrete points)
→ IDS (VGS) = VGS
Straight line
with slope
The linear regression is meaningful only when the mathematical (theoretical) relation between the
set of experimental data is linear. In the case of a TC transistor curves in Saturation mode, a linear
relation between the square root of IDS and VGS holds only for values of VGS much greater than VT, the
threshold voltage. Therefore, for p(n)-type
(VGS) =
VGS when VGS is large and
negative(positive). The same for transistor operating in Linear Regime with,
(VGS) = VGS for VGS
greater than VT. In addition, for organic transistor, the mobility tends to be a function of the gate
voltage, i.e. μ=μ(VGS). Therefore, Eq 2 and 3 should more appropriately be written as
(4)
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and
.
(5)
Ossila FACT1 estimates the mobility as function of the gate voltage automatically.
In order to automatically estimate the mobility, Ossila FACT1 executes the following algorithm,
1. The input and output points defining the Transfer Characteristics curves (VGS,i, IDS,i) are dived
into subsets of ten point each.
2. For each subset, the Best Linear Fit and its respective slope are estimated.
3. The mobility is then computed using Eq (2) for the Transfer Characteristic in Saturation
mode, and Eq (3) for the Transfer Characteristic in Linear Regime.
4. The mobility is plotted for each ten-point subsets.
For both linear and saturation regime, the maximum mobility is highlighted on the mobility plot and
reported on the data file providing that the linear fit R squared4 parameter is larger than 0.999 (0.99
for the Linear Regime mobility). If no data-point set satisfies this condition, the mobility plot turns
red with an exclamation mark in the plot legend flagging which mobility does not fulfil this
requirement. A warning message is also logged in the data file.
Example. Let us suppose that a TC curve is acquired for Gate voltages ranging from 0 to -80 V at a
constant Drain voltage of -60 V, where the transistor is operating in Saturation mode. Let also
suppose that the drain current is measured for any voltage change, i.e. ΔVGS=1V. When the
measurement is completed, the TC data set is represented by 81 couples (VGS,i, IDS,i), with i index
labelling the input/output couples. The first couple, (VGS,0=0, IDS,0) is discarded, while the rest of the
data is organized in ten-point subset. The first subset is formed by the gate voltages (-1, -2, -3, ..., -9,
-10) V and the respective measured currents. The second is composed by the gate voltages (-11, -12,
-13, ..., -19, -20) V with the current measured at these gate voltages and so on. For each subset, the
linear fit is calculated and its slope is used to calculate the mobility through Eq (2). The motilities so
obtained, are then plotted as function of the average gate voltage for each subset, which in these
example are -5.5 V, -15.5 V, -25.5 V, and so on till -75.5 V. Fig 14 shows the mobility calculated with
the modified linear fit method (using Eq 2 for each data set) and the explicit definition of gate
dependent mobility (Eq 4). The derivative in Eq 4 is calculated using the backward numerical
differentiation (BW) and the more error-robust two-point method (TW). Since the numerical
differentiation amplifies the measurement uncertainty (noise), the resulting mobility curve can be
non-smooth and “noisy”. On the other hand, the modified linear fit method extrapolates the
4
2
2
R squared (R ) is a statistic ranging from 0 to 1 which assume the value R = 1 in case the linear fit exactly
2
describes the relation between the data points, and R = 0if no (linear) relation between current and voltage
exists.
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mobility over ten data-point set and is therefore intrinsically more robust against measurement
noise while still providing the correct gate voltage dependence of the mobility.
Figure 14 Comparison between the field effect saturation mobility calculated from the modified Eq 2 (Linear Fit), and Eq
4 (BW, TP) using two different numerical derivative methods. For each data set, the Linear Fit mobility is plotted versus
the mid-point of the interval, i,e. For VGS = -25 V, - 35 V and so on.
This method has the advantage of not requiring any arbitrary selection of the of input/output to
discard because falling outside the validity range of Eq 2 and 3.
Issue with OFET mobility.
Equations 2 to 5 are only approximation of the actual OFET behaviour. Due to the complex nature
of charge transport, dielectric effects, surface traps, environmental degradation and environmental
electromagnetic noise affecting the measurement of low currents, the mobility extrapolated from
the equations above is not always reliable. In the following, non exhaustive list, typical common
issues with interpretation of mobility data is presented.
1) Mobility values extrapolated outside the range of validity of Eqs 2 and 3 can be erroneously
given as acceptable maximum mobility by Ossila FACT because they passed the R squared
condition. The User should always check that the maximum mobility is extrapolated in
correspondence of data set for which the validity of Eq 2 and 3 is assumed.
2) Due to impurity, degradation etc, a linear relation might not exist between the voltage and
the current in both saturation and liner a regime. Consequently, the mobility cannot be
reliably calculated using the Eqs 2 to 4 above.
3) In principle, linear and saturation mobility should give approximately the same result;
however, due to unavoidable measurement inaccuracy, and the approximate nature of Eqs
(2) and (3), the two methods may lead to quite different numerical values for the mobility of
organic transistors.
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Fitting Method
Ossila FACT1 gives the User the possibility of choosing among three fitting methods:



Least Square
Least Absolute Residual
Bisquare (also known as Tukey's biweight)
If the noise on the measured values (IDS,i) is described by a Gaussian distribution, the Least Square
method should suffice. The Bisquare method is a robust fitting method to be applied if outliers are
present. Outliers are point that lies far away from the majority of the measured data, a
phenomenon that may be caused by short and strong external interferences during the
measurement acquisition. Whereas the Bisquare method is robust against outliers (the resulting
Linear Fit does not strongly depends on few points located far away from the rest of the data set),
the Least Square is severely affected by the these points.
The Tolerance (applicable to Least Square and Bisquare method only) determines when stop the
iterative adjustment of the slope and intercept of the Linear Fit. The Linear Fit is extrapolated from
the data set using iterative methods. At each interaction, the slope and the intercept of the Linear
Fit approaches the Best Linear Fit. If the residual between two iterations is less than Tolerance, the
iteration is terminated and the slope and intercept of the resulting Linear Fit are returned. The
minimum value of Tolerance is 0.0001.
Please, refer to any statistical data analysis manual/textbook for any detailed explanation of the
Linear Fit methods, limitations etc.
Goodness of the Fit
Ossila FACT1 computes the confidence interval for each Linear Regression. In statistics, the
confidence interval defines how reliable the Linear Fit is; more precisely, there is a 95% probability
that the “actual” Best Fit lies inside the confidence interval calculated by Ossila FACT1. A large
confidence interval can signify that or the “Linear Assumption” is not correct (the measurement are
not linearly dependent on the input, IDS (VGS) ≠ VGS ) or the measured data are strongly affected by
errors and external noise.
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The boundaries of the confidence interval are called Upper and Lower Bound, which are plotted on
the Best Linear Fit graphs of Ossila FACT1 UI for both Saturation and Linear Region, see Figure 2.
Figure 6. Linear Fit (Red, solid line) and Upper and Lower Bound defining the confidence interval (dashed blue line) for
two subsets of input (Gate Voltages) and measured point (Drain Current) for TC in Saturation Mode.
Ossila FACT1 computes and logged on the TC output files, two extra statistical parameters useful to
assess the goodness of the Linear Fit. These are,


R-square - The closer to 1 the R-square, the better the fit.
RMSE (Root Mean Square Error) The smaller the RMSE, the better the fit
Please, refer to any statistical data analysis manual/textbook for any detailed explanation of these
parameters.
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Measurement and Accuracy
The two PXI Source Measurement Units (SMUs) are capable of very accurate voltage and current
measurements, see “SMU Specifications” section in the NI PXI-4132 Specifications documents.
The sensitivity and accuracy of the SMU is strongly affected by electromagnetic noise (such as the
one generated by mobile phones, computers too close to the SMU, etc), temperature and, most
important, the PXI measurement settings. In particular, when low currents (of the order of
magnitude of few tens of nano Ampere or less) are measured, external interferences and
inappropriate setting may sensibly reduce the accuracy or (if the order of magnitude of the “noise”
is comparable or larger than the current being measured) even hamper the measurement.
Temperature and Self-Calibration
As specified by the NI-4132 Specifications, PXI SMU should be operated at a temperature of 23 0C ± 5
0
C. Outside this temperature range, the nominal resolution of both output and measurement cannot
be guaranteed.
Operating under high humidity (>90%) or dusty conditions may cause increased leakage between
circuit components and can result in additional sourcing and measurement errors.
To limit the detrimental effect of temperature, humidity and ageing is advisable to execute a SelfCalibration regularly through Measurement and Automation (MAX). In particular, PXIGATE and
DRAIN self-calibration is recommended every time the environment conditions (temperature and
humidity) change considerably.
Note. Execute the self-calibration after 10/15 minutes the PXIs are switched on so to allow enough
time for both SMUs to reach a stable internal temperature.
Set the chassis fan (toggle on the back panel of the chassis) on the high position to allow better
control of the internal temperature and reduce output/input noise.
Calculation of the Measurement Accuracy
For a SMU, the transfer function is given by5
y = gx +b,
Eq 6
where g is the gain, b the offset, x the input and y the output. For an ideal SMU b=0 and m=1 and
therefore y=x. For an actual SMU, however, m ≠1 and b≠0. For the NI PXI 4132, current
5
NI DC power Supply SMU Help.
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measurement accuracy 6 at 23 0C ± 5 0C is reported in the table below (from NI PXI 4132
Specification)
Range
10 μA
100 μA
1 mA
10 mA
100 mA
Current Measurement Accuracy and Resolution
Resolution
Accuracy ± (% of reading + offset)
10 pA
0.028% + 1.0 nA
100 pA
0.028% + 10 nA
1 nA
0.028% + 0.1 μA
10 nA
0.028% + 1.0 μA
100 nA
0.028% + 10 μA
Here resolution means the smallest difference in current that the SMU is capable of discriminate,
while accuracy is the measurement uncertainty.
From this table and Eq 6, the accuracy of a current I = 2mA measured in the 10 mA range is then
given by (m=0.028%; x= 2mA and b =1.0 μA)
Accuracy = 2 x (0.028)/100 mA + 1.0 μA
= 0.56 μA + 1.0 μA
= 1.56 μA
Therefore, the current and its uncertainty are I = 2mA ± 1.56 μA.
Ossila FACT1 automatically selects the appropriate measurement range depending on the PXI
Current Limit chosen by the User.
For each current acquisition chose the appropriate Current Limit, i.e. a value that is greater than
the maximum (expected) current under measurement but not greater than the lowest SMU
current range required for the measurement.
For example, if PXIDRAIN Current Limit for the OFET Characteristic Curves is set to 10 uA or less, the
PXIDRAIN carries out the measurement in the 10 μA range. On the other hand, if the maximum
current is expected to be 50/60 μA, choose as Current Limit any value greater than 60 and not larger
than 100 uA (100 uA is acceptable).
For the sake of easy reference, the PXI voltage output and voltage measurement accuracy and
resolution are also reported in the two tables below.
Range
±10 V
±100 V
Voltage Output Accuracy and Resolution
Resolution
Accuracy ± (%of reading +offset)
50 μV
0.025% + 3.0 mV
500 μV
0.025% + 10 mV
6
Refer to NI PXI 4132 Specification for more detailed information on output and measurement accuracy and
resolution.
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Range
±10 V
±100 V
Voltage Measurement Accuracy and Resolution
Resolution
Accuracy ± (%of reading +offset)
10 μV
0.02% + 3.0 mV
100 μV
0.02% + 5.0 mV
Temperature Effects on SMU Accuracy
Devices operating outside the five degree range of the last calibration temperature, with the
calibration carried out no later than one year from the time the acquisition is carried out, have an
accuracy that is given by the sum of the “factory standard accuracy” (see above) with an extra term
depending on the Tempco (temperature coefficient), where Tempco is express as % of the factory
accuracy specification per degree oC. For NI PXI 4132, Tempco is 15 %.
As an example, let us suppose that a PXI calibrated at T = 25 oC is operated at 35 oC. The PXI is use as
SMU with output voltage Vout = 20 V.
From the Voltage Output accuracy table and Eq. 6, the uncertainty on the output is given by
Accuracy = 20 x ( 0.025)/100 + 10 mV
= 15 mV.
The correction factor due to the temperature is
Temp_Accuracy = Accuracy x (Tempo)/100 *|Calibration_temp – Meas_Temperature|
= 15 x (15)/100 x 10
= 22.5 mV.
The PXI output is then given by Vout = 20 V ± 37.5 mV.
PXI Advanced Setting
The PSI SMU measurement setting can be programmatically controlled using the PXI SETTING
functionality. Upon selecting PXI SETTING on the main UI of Ossila FACT1, the PXI Fine Setting input
field is activated and the User can change the default measurement configurations by modifying the
PLC, Sample To Average and Aperture Time.
PLC. Power Line Cycle is the oscillation frequency of the AC current supplied to the end-users
through the electrical power grid. In the EU, Australia, part of Japan and several other countries, the
PLC is 50 Hz with a voltage range of 220-240 V, while in Taiwan, USA and Canada, part of Japan, etc
the PLC is 60 Hz with a voltage range of 100-120 V.
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It is of paramount importance that the PLC selected in PXI Setting is the PLC of the power grid used
to power the SMUs, otherwise small current measurement will be strongly affected by the
oscillatory nature of the AC supply current.
Sample To Average. If Sample To Average is set to N, the SMU takes N independent measurements,
which are then averaged so that a single (averaged) output is returned. For the PXI-4132, N can be
any integer number from 1 to 127. Increasing N improves accuracy and reduces the noise; however,
in order to optimise the PXI performance, it may also be necessary to modify the Aperture Time
accordingly.
Aperture Time defines the measurement, i.e. the time that the SMU allocates to the acquisition of a
single measurement output. Aperture Time is expressed in unit of PLC. The actual value of Aperture
Time therefore depends on the PLC frequency, as detailed in the following table where Aperture
Time is expressed in term of PLC units and seconds (From NI PXI-4132 Specifications),
PLC
8
4
2
1
1/2
1/4
1/8
1/16
1/32
1/64
Aperture Time
PLC Input
60 Hz
8
133 ms
4
66.6 ms
2
33.3 ms
1
16.6 ms
0.5
8.33 ms
0.25
4.16 ms
0.125
133 ms
0.0625
1.04 ms
0.031250
520 µs
0.015625
260 µs
50 Hz
160 ms
80.0 ms
40.0 ms
20.0 ms
10.0 ms
5.00 ms
2.50 ms
1.25 ms
625 µs
312 µs
Increasing Sample to Average and Aperture Time accordingly allows for noise reduction and optimal
AC Power Line Rejection: the SMU unit can be made insensitive to the PLC oscillations of the
electrical power grid.
NOTE. Aperture time = 1 (1 PLC) and Sample To Average = 1 are the standard measurement setting.
Use Sample To Average = 2 (or higher Aperture Time and/or Sample To Average) to increase
accuracy at the expense of acquisition time. Lower Aperture Times allow for faster acquisition at the
expense of poorer PLC noise rejection.
Auto Zero
High accuracy measurement requires the Auto Zero function to be enabled. With Auto Zero
enabled, the PXI takes an “internal measurement” each time it samples the target. The internal
measurement is then subtracted from the raw target output in order to reduce the effect of the
internal PXI noise on the currents. Auto Zero is used to increase the accuracy of the measurement
and in particular current offset due to temperature, humidity, aging and environmental noise.
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Let us suppose that Sample to Average is set to N=2, while Auto Zero is enabled. The output (IDS or
IGS) is then acquired through the following sequence,
Sample 1
Settling
time
Auto Zero
Settling
time
S1 = Sample 1 – Auto Zero
Sample 2
Settling
time
Auto Zero
Settling
time
S2 = Sample 2 – Auto Zero
The output delivered by the PXIs (gate current, for example) is then IGS = (S1 + S2)/2.
If the Auto Zero is set to once, Auto Zero will be carried out only after the first measurement for
each acquired voltage-current curve.
Auto Zero Enable almost doubles the acquisition time. Auto Zero Once is the recommended setting
for CC and TC curves, while Auto Zero Enable is preferable for high precision acquisition of TC curves
parameters, such as ON/OFF ratio and OFF current, for which very low current measurements are
required.
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List of Symbols and Acronyms
VGS : Gate–Source potential difference
VDS : Drain–Source potential difference
IGS : Gate–Source current
IDS : Darin–Source current
ΔVGS : Gate Voltage step width
ΔVDS : Gate Voltage step width
ΔVGS : Drain Voltage step width
f(x) : f is a function of the independent variable x. Therefore,
IDS(VGS) : IDS is a function of (depends on) the independent variable VDS
IDS(VDS) : IDS is a function of (depends on) the independent variable VDS
IGS,i(VGS,i): Current measured at VGS,i =i×ΔVGS, with i = {0,1,2...,Measurements Points}; the same for the
drain current.
I-V curves: Output Characteristic curves IDS(VDS)
SMU: Source Measurement Units
TC: Transfer Characteristic IDS(VGS)
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Ossila FACT1 Troubleshooting

Ossila FACT1 does not start. Check if the programme is installed.
Action Required: Go to Start>>Control Panel>>Programs>>Programs and Features. If Ossila
FACT1 does not appear in the list of installed programs, reinstall Ossila FACT1.

Ossila FACT1 opens but it closes immediately with an error message.
Check the error message and take action accordingly. Possible reasons for this malfunction
are
1) Drivers or Run-time Engine 2013 (or later) not installed.
Action Required: Install the correct drivers and/or the Run-Time Engine 2012,
the list of required ancillary software and drivers can be found in the Setting up
section of the User’s Manual.
2) USB cable between the computer case and Ossila BOX disconnected or not
properly connected.
Action Required: Plug the Standard A and Standard B sockets of the USB
connections properly. Reboot the computer.

Ossila FACT1 opens but it stops when the measurement is initiated.
Check the error message, if any. Check on “Hardware Installed” box of the main UI of Ossila
FATC whether at least one PXI-4132 is identified as “CONNECTED” (the second PXI is never
recognised as connected).
Action Required: If both PXI-4132 are identified as “NON-CONNECTED”, check the serial
connection between the PXIe-PCIe8361 on the Chassis and the PXIe-PCIe8361 Card mounted
on the back side of the Dell Vostro computer case. Reboot the computer afterwards.

Ossila FACT1 runs, it performs the measurement cycle, the output is constituted by noise
current.
1) Ossila Box toggle switch is OFF
Action Required: switch ON Ossila Box
2) The Lid of Ossila Box is open
Action Required: Close the Lid, make sure that the Lid presses against the safety
relay so that the relay is ON when the Lid is shut.
3) The push-fit connector of the test board (inside Ossila Box) is not present or not
properly fitted.
Action Required: Make sure that the push-fit connector is tightly arranged on
the top of the test board.
4) The coaxial cables connecting the PXI-4132s with the Gate and Drain input of
Ossila Box (Channel A and B) are not properly connected.
Action Required: SWITCH OFF BOTH PXIs and check the connections. Make sure
that the Front Panel I/O PXI connectors (Backshell) are both tightly plugged to
the PXI receptacles, and that the “WARNING” label is on right side of the I/O PXI
Backshell.
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If the problem persists, unplug the Backshell, use a screwdriver to remove their
lids and check if the Central Core (D in Figure 1) and the metallic shield (B in
Figure 1) of the coaxial cable are connected to the first and fourth pin inside the
Backshell. The pin are numbered from top to bottom, with the “WARNING” label
of the Backshell on the right side , see NI PXI-4132 Specifications. Make sure that
the Central Core and the metallic shield are connected (through the Backshell)
to the “High” and “Low” output for both PXIs. The output type of the PXIs can be
read directly on their front panels.
If the Backshell are opened for maintenance, before use make sure they are
properly sealed with both the Central Core and metallic shield tightly wired
and completely enclosed inside the Backshell. Ask the assistance of qualified
technical support if not acquaintance with the maintenance of high-voltage
electrical equipment.
Figure 7. Coaxial cable: internal details

Ossila Ltd
Ossila FACT1 opens, runs and acquire measurement properly but, suddenly, it
stops.
Check the content of the error message (if any) and take action accordingly.
Possible reasons for a sudden interruption of the acquisition and/or stop of the
programme are the following:
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1)
The lid of Ossila Box is open. As the lid open, the security system interrupts the
connection with the PXI. The aperture of the lid and the consequential sudden
abrupt variation of the PXI load can force one of the PXI to generate an error
which, in turn, stops Ossila FACT1.
2)
One or more cables are inadvertently unplugged. Depending on which
connection is unplugged, Ossila FACT1 may stop with an error message or
acquire the background noise only.
Required Action: Refer to the Required Actions for the same issues detailed
above.
Warning! If Ossila FACT1 stops, but the “Output Enabled” LED of one or both PXI is still ON, you must
programmatically shut down the PXI. Run a dummy acquisition (few point for a single device) so that
the PXI(s) with Output Enabled active are gracefully and safely switched off.
When the “Output Enabled” led is ON, Shock Hazard exists.
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Warranty information and contact details
Ossila warrants that:
(a) Ossila has (or will have at the relevant time) the right to sell the Products;
(b) the Customer shall enjoy quiet possession of the Products, subject to the rights
referred to in Clause [8.1(b)];
(c) the Products correspond to any description of the Products supplied by Ossila to
the Customer;
(d) the Products are of satisFACT1ory quality;
(e) the Products are fit for any purpose expressly (but not merely impliedly) made
known by the Customer to Ossila before the relevant Contract is made;
(f) the Products correspond to any sample of the Products supplied by Ossila to the
Customer, and will be free from any defect making their quality unsatisFACT1ory,
which would not be apparent on reasonable examination of the sample;
(g) the Products will comply with all laws, rules, regulations applicable to the
marketing and sale of the Products in United Kingdom.
(h) if, within one year of purchase, the customer experiences system failure or
damage within reasonable constraints the system may be returned to Ossila for
maintenance. Please note that if any system compartment is opened within this
period, warranty will be voided.
To the best of our knowledge the technical information provided here is accurate. However,
Ossila assume no liability for the accuracy of this information. The values provided here are
typical at the time of manuFACT1ure and may vary over time and from batch to batch.
Ossila Contact Details:
Kroto Innovation Centre
North Campus
Broad Lane
Sheffield
S3 7HQ
UK
Technical Contact:
Dr James Kingsley
[email protected]
Customer Services Contact:
Lucy Pickford
[email protected]
Phone no: +44 (0) 114 213 2770
Fax no: +44 (0)800 098 8142
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List of compatible components
Low Density OFET Evaporation Stack
The evaporation stack holds the OFET source-drain
shadow masks and substrates in close contact for
thermal evaporation.
For use with the Ossila Low Density OFET sourcedrain evaporation masks (sold separately, please see
below).
Low Density OFET Source-Drain Shadow Mask
Two types of linear source-drain evaporation masks.
For use with the low density OFET evaporation stack
(above).
Low Density OFET Gate Mask
An evaporation mask for deposition of gate
contacts.
For use with the low density OFET system.
Low Density OFET Active Area Mask
An evaporation mask for deposition on to the active
area.
For use with the low density OFET system.
OFET Gate Mask
An evaporation mask for deposition of gate contacts
onto Ossila's pre-patterned ITO substrates (S161).
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ITO OFET Substrates
Unpatterned ITO substrates. Ideal for spin-coating
gate dielectrics on.
Compatible when used with the Low Density
Fabrication system.
Pre-patterned ITO OFET Substrates
The Ossila pre-patterned ITO OFET substrates have
been designed to enable the fabrication and
characterisation of transistors without the need for
vacuum evaporations or probe stations.
Silicon Oxide OFET Substrates
A range of silicon substrates with thermal oxides on.
Pre-cut to fit the Ossila OFET fabrication systems but
also used for a number of other applications
including ellipsometry and X-ray measurements.
Compatible when used with the Low Density
Fabrication system.
Synthetic Quartz Coated Substrates
Flat glass substrates coated with 20 nm of SiO2 to
help with surface wetting and prevent ion migration
from the glass to the active layer.
Compatible when used with the Low Density
Fabrication system.
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Appendices
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Appendix I
Cleaning Routine
The starting point for any good organic devices is a pristinely clean substrate. However, physical
cleanliness i.e. the absence of dust and particulate matter is often different to a chemically clean
surface free from residues and contaminants. To ensure that substrates are pristinely clean we
generally start by using a hot Hellmanex III solution which acts as an electronic grade detergent.
Keeping the substrates vertical during cleaning also helps to ensure that any particulate matter falls
off the substrate. When using Hellmanex III it is also important to use a hot "dump rinse" and an IPA
sonication to ensure that no residues are left behind.
In addition, we also find that for ITO based substrates it is useful to use a hot sodium hydroxide
(NaOH) sonication to provide a more hydrophillic surface and avoid the need for a plasma ash.
The below cleaning routine is generally found to provide good surface preparation although exact
timings will depend upon the power and temperature of the sonic bath. To achieve hot solutions we
generally fill the sonic bath with boiling water from a kettle which results in a bath temperature of
around 70 to 80 degrees.






5 mins sonication in hot 1% Hellmanex III solution
2x boiling water "dump rinse"
5 mins sonication in warm IPA
2x cold water dump rinse
Optional 5 mins sonication in hot 10% NaOH and 2x cold water dump rinse
Storage in cold DI water.
Once placed in DI water the substrates are stable for at least 24 hours (and probably much more).
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Appendix II
PMMA gate dielectric
For top-gate devices the choice of dielectric material is a critical consideration as it must have low
leakage and also have an orthogonal solvent to the semiconductor layers below.
Solution processed gate dielectrics are therefore an area of active research with many new polymers
being developed with the desired properties. However, of the commercially available polymers we
have found that PMMA gives the best combination of performance, simplicity and compatibility.
However, the molecular weight and solvent used has a large impact on performance.
We have found that PMMA with Mw=120K gives good performance when dissolved in butanone.
Molecular weights significantly higher than this can cause solubility issues and uneven films while
significantly lower can cause pin-holes and solvent incompatibility with solution process gates. While
there are many non-polar solvents that will dissolve PMMA very well, these often also dissolve
polymeric semiconductor layers. We have therefore found that butanone gives the right
combination of solubility for the PMMA with a moderate boiling point ideal for film forming
properties and with little effect on most polymeric semiconductors such as P3HT, PBTTT and PCDTBT
(it will however still dissolve small molecule materials such as TIPS-pentacene and PCBM).
Process routine for PMMA gate dielectric







PMMA with Mw=120,000 bought from Sigma Aldrich
Dissolved in butanone at 100 mg/ml using stirbar and hotplate set to 80 C
Cooled for 10 mins before filtering with 0.45 um PVDF syringe filter
Spin coat using a static dispense of 100 ul and fully coat the substrate before spinning
Spin speed of 1000 RPM for 30 seconds (fastest acceleration).
Bake at 105 C for 5 mins to fully evaporate solvent and take the PMMA to the glass
transition temperature to reduce leakage.
Solutions stable for at least several weeks.
The thickness of the resulting layer should be around 1200 nm and we generally obtain a film
uniformity of better than 10%.
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Appendix III
PEDOT:PSS Gate Routine
The use of a solution processed gate enables rapid fabrication without the use of costly and slow
vacuum deposition equipment. However, any solution processed gate also needs to be solvent
compatible with the gate-insulator (and preferably organic semiconductor as well). It should also
have the right wetting and film forming properties to enable good even substrate coverage.
We have found that high conductivity formulations of PEDOT:PSS with additives to control the
wetting and drying process give good performance with the below recipe working well.
Solution components:
Component
PH1000 PEDOT:PSS
N-Methyl-2-pyrrolidone
Triton X-100
Isopropyl Alcohol
Order to add
1
2
3
4
% by weight
42.5
2.5
1
54 %
Fabrication Routine:




Mix the above components in the given order using a magnetic stir-bar but not heated
(PEDOT:PSS will phase separate if heated above 40 C).
Spin coat using a 50 ul dynamic dispense at 4000 RPM
Spin for ~2 mins until fully dry
Solutions stable for several days but resistivity increased over time.
Using the Ossila ITO OFET substrates this should result in a resistance between the two cathode
connection terminals of around 250 Ω.
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Appendix IV
OTS Preparation
Getting a good OTS preparation on a substrate surface can increase mobilities by orders of
magnitude, however getting a good monolayer can be hard to achieve.
In general OTS will polymerise over time when exposed to ambient conditions resulting and "stringy"
white aggregates in the bottle. As such, we generally store OTS under inert atmosphere (N2
glovebox).
However, in order to get a good coating on a substrate, a small amount of water is beneficial for the
cross-linking process. For this reason (and also the amount of solvents used) we generally prepare
OTS substrates under ambient conditions. The choice of solvent is also critical and we have found
cyclohexane to work well using the following procedure.
OTS preparation procedure:







OTS stored under inert atmosphere (N2 glovebox).
Small amount of OTS mixed 1:9 with anhydrous cyclohexane in inert conditions (glovebox).
Clean substrates placed in a beaker with lid and immersed in HPLC grade cyclohexane under
ambient conditions.
OTS:cyclohexane mixture removed from glovebox and small amount added to beaker to
produce overall concentration of 1 mMol.
Left under ambient conditions for 20 mins.
2x dump rinses in cyclohexane
Baked on hotplate under ambient conditions at 150 C
The above recipe should result in a water contact angle of around 110 °C or greater.
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Appendix V
Note on transient time.
If the capacitance and resistivity component of the measurement apparatus and of the DUM (CDUM
and Rm) are different from zero, a constant voltage V0 applied at time t=0 will not instantaneously
transfer to the DUM, but it will instead be governed by the following equation,
,
Eq A1
where VDUM(t) is the voltage at the DUM at a time t, with R = Rm + RDUM and C = Cm + CDUM total7
resistance and capacitance of the measurement apparatus and DUM. RC has the unit of measure of
time and is usually indicated as the time constant τ of the system (τ = RC). If a voltage V0 is applied at
t=0, VDUM(τ)≈0.63 V0.
Therefore, for each acquired data point Delay must be 9/10 time larger than the time constant RC.
For Ossila FACT1, with two meter-long BNC cable, typical value of resistance and capacitance are R =
1.4 Ω and C = 250 pF, which gives negligible time constant τ.
The time constant τ is sometimes more conveniently expresses in term of the cutting off frequency
fc,
.
In addition to the device time constant, the dielectric capacitance charging time and, more
importantly, the transistor ON/OFF frequency. While the former time constant is described by EqA1,
with fc ranging 1250 KHz for SiO2 to 9 KHz for Al2O3, the latter is given by
.
Eq
A2
In Eq A2, μ is the mobility, VDS the drain voltage and L is the channel length.
7
Here, we assume that the resistance and capacitance elements of both DUM and measurement apparatus
are in series and parallel, respectively.
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Appendix V
Measurement Flowchart
CC: IDS, i(VDS, i) and gate leakage IGS, i(VDS, i) with i =1, 2,...N for j =1, 2...M gate voltage sweep.
TC: IDS, k(VGS, k) with k =1, 2,...L for linear and saturation regime VDS,r, r= 1, 2.
Initialisation
Multiplexer
CC?
NO
Yes
CC: Apply VGS,j
j =0
TC: Apply VDS,r
r =0
j = j+1
Wait t=Gate Delay
Wait t=Drain Delay
Apply VDS,i
i = i+1
Wait t=Drain Delay
Acquire IDS,i (VDS,i)
NO
Apply VGS.k
k =0
k =k+1
j =0
Acquire IGS,i (VDS,i)
i > N?
Wait t=Gate Delay
Acquire IDS,k (VGS,k)
NO
k > L?
Yes
NO
j > M?
r = 2?
Yes
Yes
Log CC data
Log TC data
Yes
TC?
Yes
Next FET?
device
Next FET?
NO device
Yes
Yes
NO
STOP
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