Analytical Performance Evaluation Of AlInGaN/GaN metal

2014 INTERNATIONAL CONFERENCE ON COMPUTATION OF POWER, ENERGY, INFORMATION AND COMMUNICATION (ICCPEIC)
Analytical Performance Evaluation Of
AlInGaN/GaN metal Insulator Semiconductor
Hetero structure Field Effect Transistor And Its
Comparison With Conventional MISHFET’s
(AlGaN/GaN)For Sheet Carrier Concentration
1
Senthilraja.N,2Anbuselvan.N,3Dr.Mohankumar.N
1
PG-Scholar(VLSI),2Asst.Proff.,3HOD,
SKP Engineering college,
Tiruvannamalai, India.
[email protected].
Abstract:-A High Electron Mobility Transistor (HEMT)
model is proposed in this paper, we will spotlight on
quaternary materials in addition to that MISHEMT
structure is integrated in order to analyze the number of
electron focus under the channel. It’s one of the gifted
candidate for future microwave and millimeter power
devices.The quaternary materials are very perceptive
towards the development of polarization charges at the
heterostructure interface and results into high Two –
dimensional electron gas sheet charge density 2DEG
( ). The amendment of various structural parameters
such as Al and In mole fraction of AlxlnyGa1-x-yN from
0.40 to 0.60, thickness of AlInGaN cap layer, doping
concentrations give rise to a noteworthy………… change
in the 2DEG transport and its detail discussion are
presented in this paper. The 2DEG density with the
variation of AlxlnyGa1-x-yN thickness and Al and In mole
fraction (x)(y) are discussed.
I.
INTRODUCTION
The Gallium nitride (GaN) is most promising wide bandgap semiconductor for use in high power microwave
devices.It has functioned at 320 °C, and higher values are
well within theoretical limits. By combining four
devices,20W has been developed at X-band.GaN metal
insulator semiconductor High Electron Mobility
Transistor(MISHEMT) are unique in that the Two
dimensional electron gas(2DEG) is supported not by
intentional doping, but instead by polarization charge
developed at the interface between the bulk GaN region
and the AlInGaN epitaxial layer.The polarization charge
is
composed
of
two
parts:spontaneous
and
piezoelectric.This
behaviour
is
unlike
other
semiconductors.High Electron Mobility Transistors have
emerged as a promising candidate for microwave(f > 1
GHz) power amplification,with applications ranging from
satellite links to wrireless communications,from highways
to electronic warfare. Also,they have a potential for low
frequency(f < 100 MHz) high voltage(up to 1 kV)
switching power control.Until recently .power HEMT’s
were primarily based on AlGaAs/GaAs/InGaAs and
related epitaxial films grown on GaAs or InP substrates.In
late 1990’s AlGaN/GaN,AlGaN/InGaN HEMT’s grown
on sapphire, HEMTs grown on sapphire, insulating 4H
SiC, conducting SiC, and even bulk GaN have
demonstrated much larger output powers and have
become promising contenders for a variety of high power
amplification and switching applications. Moreover, the
use of wide band gap semiconductors in power amplifiers
not only increases the output power, but also extends the
temperature tolerance and the radiation hardness of the
circuits. The latter is corroborated by recently
demonstrated operation of GaN-based HFETs at 750ºC. In
this chapter, we review the various aspects of these
devices, namely history, current status, technology,
characteristics, modeling and circuits.The HEMT is also
known as MODFET (Modulation-doped FET), TEGFET
(Two-dimensional Electron Gas FET), SDHT (Selectively
Doped Heterostructure Transistor) or simply, HFET
(Heterojunction FET). The unique feature of the HEMT is
channel formation from carriers accumulated along a
grossly asymmetric heterojunction, i.e. a junction between
a heavily doped high band gap and a lightly doped low
band gap region.In HEMTs based on GaN substrates, this
carrier accumulation is mainly due to polarization charges
developed along the heterojunction in the high band gap
AlGaN side. This is in contrast to the situation in other
HEMTs, such as those on GaAs or InP substrates. Here,
the accumulation is a result of carrier diffusion from the
heavily doped to the lightly doped region, the diffusion
being enhanced significantly by the band gap difference
between the two regions. Whatever the physical origin of
carrier accumulation, the accumulated carriers might have
high mobility due to their separation from their heavily
doped source region, and their location in the low-doped
region where impurity scattering is absent. The
development of MOCVD and MBE technologies made
heterojunctions practical. Although substantial research
on GaN growth was initiated, the technological spin-offs
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SENTHILRAJA.N et al.: ANALYTICAL PERFORMANCE EVALUATION OF ALINGAN/GAN METAL INSULATOR SEMICONDUCTOR
came late because of lack of ideal substrates. The
enhanced mobility effect was first demonstrated in
AlGaAs/GaAs heterojunctions , and applied to
demonstrate a HEMT . Electron mobility enhancement at
Al- GaN/GaN heterojunction was first reported . Later,
this enhancement was attributed to the 2D nature of the
electrons, based on observations of mobility increase with
lowering of the temperature down to 77 K and
Shubnikov-de-Haas oscillations. The potential of
AlInGaN/GaN MISHEMTs for microwave electronics
was demonstrated. With advancements in material quality,
heterostructure design, and ohmic contact formation, the
excellent power capability of these HEMTs. Recently,
some AlGaN/GaN structures useful for several hundred
volts power switching have been proposed.
II.MODELING OF AlInGaN/GaN MISHEMT’s
2.1AlInGaN/GaN MISHEMT’s:
Compound Semiconductors from the group III-V
achieved significant progress in Optoelectronic devices.
GaN-based material has unique properties and becomes an
attractive candidate for future microwave and nanoscale
power devices. AlxlnyGa1-x-yN /GaN MISHEMTs have some
unique features that make it outperform upon other AlxGa1xN/GaN metal insulator semiconductor heterostructures such
as high 2DEG sheet Carrier density, Carrier mobility and
transconductance (gm). Depending on the alloy combination
of the materials increase the Sheet Charge density (ns) and
mobility. The higher the mole fraction of Al and In
concentration, the lower the probability of the electron-wave
penetration because the band-discontinuity between the
conduction band of GaN and that of higher Al and In mole
fraction of barrier AlInGaN, i.e. effective barrier height is
larger ΔEc,eff .The GaN wurtzite HCP structure with nonCentro-symmetric property is the main source for the
formation of two-dimensional Electron gas (2DEG) at the
AlxlnyGa1-x-yN/GaN interface, mainly due to Piezoelectric and
Spontaneous polarization as it does not require any external
force like electric field or deformation, resulting in high
Sheet carrier density in the order of 10 13 cm-2. In this work,
we have modeled the Sheet carrier density ns, with respect to
gate voltage vgs , Capacitance cg , permittivity  , thickness
and also explored the DC characteristics, transconductance
and frequency analysis model for AlxlnyGa1-x-yN /GaN
MISHEMTS. The effects of Piezoelectric and Spontaneous
polarization along-with Channel length modulation
parameters are considered in this model and explained. All
the results are validated using experimental data.
Fig.2.1. Schematic diagram of quaternary based AlxlnyGa1-x-yN /GaN
HEMT’s with gate length Lg, AlInGaN barrier and. direct wide band
gap semiconductor having very high polarization effect and is the main
source of 2DEG density (ns), formed at the interface.
2.2 BAND DIAGRAM OF AlInGaN/GaN MISHEMTS:
Compared to the thick AlN discussed previously,
the role of thin AlN is different. When AlN is thick than a
critical thickness (for the 2DEG formation), the AlN layer
itself is the major contributor to the formation of 2DEG
and the addition of the GaN (or AlGaN) cap on top of it
just decreases the 2DEG density. However, when the AlN
is thin (~1nm in this case) and below the critical
thickness, it cannot form the 2DEG directly, instead the
AlGaN layer on top of it is the major contributor. The role
of the thin AlN layer to the properties of 2DEG can be
describe as a larger effective Ec (Ec,eff) than the Ec in a
standard AlGaN/GaN HEMT, which is discussed in detail
in the following.
Simulation was performed to study the effects on
the charge and mobility when a thin (~1nm) AlN layer
was inserted. The band diagram of an AlGaN/AlN/GaN
heterostructure simulated by 1D Poisson Solver is shown
in Fig.1.9 (a). As a comparison, the band diagram of the
standard AlGaN/GaN HEMT is also displayed in Fig.1.9
(b). A very obvious change observed after the insertion of
the thin AlN layer is that the energy band has a sharp
peak.
Fig 2.2 Band diagrams of the heterostructures. AlInGaN/GaN
MISHEMT;
The 2DEG density in the AlInGaN/GaN structure can be
obtained by:
n sh  ε 0 ε s /q(d t  ε s t in / ε in)[(q N d t in d d / ε 0 ε in) 
q N d d 2 / ε 0 ε s (1  2 d s / d d)  V geff  Φ b (m)  Δ E c  E f
d
Recall,
n sh  ε 0 ε s /q(d t  ε s t in / ε in)[(q N d t in d d / ε 0 ε in) 
q N d d 2 / ε 0 ε s (1  2 d s / d d)  V geff  Φ b (m)  Δ E c  E f
d
Equation (3) is very similar to equation (4), except ΔE’c,eff
is used in (2.6), instead of ΔEc,AlGaN (tAlN is very thin and
can be ignored). Therefore, the behavior of 2DEG in the
new structure can be attributed to the larger ΔE’c,eff (or
ΔEc,eff) which is caused by the insertion of the thin AlN
layer.Because the AlN layer is very thin, only ~1nm, the
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2014 INTERNATIONAL CONFERENCE ON COMPUTATION OF POWER, ENERGY, INFORMATION AND COMMUNICATION (ICCPEIC)
effect of ΔE’c,eff to the 2DEG density is still limited,
compared with the term ΔAlGaNtAlGaN when the AlGaN is
thick . As a comparison, the standard AlGaN/GaN HEMT
structures had 250the charge density was still a strong function of Al
composition, similar to standard AlGaN/GaN HEMTs.
When the Al composition was varied from 0.27 to 0.45, the
2DEG density increased from 1.45x1013cm-2 to 2.5x1013cm2
.
molefractions
Al048In0.18Ga034N/GaN
&
Al0.57In0.23Ga0.20N/GaN MISHEMT devices and the
corresponding electron concentration under the channel
have found increase in number.the previously analysed
paper having the value is given by the fig 3
2.3.Device calculations:
Induced
polarization
in
AlxlnyGa1-x-yN
/GaN
heterostructure:
AlGaN/AlN/GaN heterojunction possesses high electron
concentration due to its strong polarization effect and high
breakdown due to the larger band gap. A high sheet carrier
density is induced in the channel even if the barrier layer is
un-doped due to two types of polarization i.e., Piezoelectric
and Spontaneous. Piezoelectric polarization due to the crystal
lattice mismatch between the layers can be calculated by
Vegard’s interpolation formula and is given by:
P sp(Al x In y Ga zN )  X. P sp (AlN)  Y. P sp (InN)  Z. P sp (GaN) 
(2.3)
b AlGaN (X)(Z)  b InGaN (Y)(Z)  b AlInN (X)(Y)
and an equilibrium lattice constant a(x), the basal strain
field 𝜂1(x) for the alloy matched to a GaN substrate
defined as
η1 (X)  (a GaN  a(X))/a(X)
Spontaneous polarization due to lack of inversion symmetry
and high electro-negativity of Nitrogen atom and is given in
terms of Al-mole fraction(x) as And the total polarization
σint induced at the heterostructure is the sum of Ppz and Psp of
the two layers forming the heterostructure and is given by
the expression the linear interpolation of dielectric constant
is given by
 ( x)  10.4  0.3 X
The threshold voltage of the HEMT [4] given by
V th  Φ b (m)  Δ E c  q N d d d2 /2 ε 0 ε s (1  2 d s / d d) 
(q N d t in d d / ε 0 ε in)  [σ pz (m)(d t  ε s t in / ε in / ε 0 ε s]  k1
here, φeff(x) is the effective height of the Schottky barrier,
ΔEc,eff is the Conduction band offset between AlN spacer
and GaN. Nd is the doping concentration in the AlGaN
barrier, σint is the total induced polarization charge at the
interface.
RESULTS AND DISCUSSIONS:
In this model, we have discussed AlInGaN/GaN
MISHEMT devices with two different molefractions have
calibrated with experimental data to validate our model.
The model and experimental comparisons [10-12] are
done for quaternary based AlxlnyGa1-x-yN/GaN, having the
FIG 3.variation of Ns iwth
FIG2.3. AlxlnyGa1-x-yN /GaN HEMT’s.
In Fig 2.3 The plot for Fig.4 shows the variation of ns
with barrier thickness using equation (2.2). From this plot,
we can say that, the decrease in ns is either based on the
reduced barrier thickness dd, the reduced interface
polarization charge σint. Higher the Al mole fraction will
lead to higher sheet carrier density. The variation of the
gate to source voltage with respect to sheet carrier density
shows excellent agreement with the experimental data
[10] as shown in Fig.4 using equation(4) as well as in
equation(5) both suit well for calculation of ns.
Fig.3 Variation of Sheet carrier density with respect to Al
and In mole fraction and thickness of barrier layer
AlInGaN for AlxlnyGa1-x-yN /GaN HEMT’S.The variation
of ns shows better match for calculating performance of
drain current model.
The sheet carrier density unified model for Ef and ns
applicable for a full range of voltages [6] is given by
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SENTHILRAJA.N et al.: ANALYTICAL PERFORMANCE EVALUATION OF ALINGAN/GAN METAL INSULATOR SEMICONDUCTOR
n sh  ε 0 ε s /q(d t  ε s t in / ε in)[(q N d t in d d / ε 0 ε in) 
q N d d 2 / ε 0 ε s (1  2 d s / d d)  V geff  Φ b (m)  Δ E c  E f
d
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Fig.4 Variation of Sheet carrier density with respect to
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Fig5. Variation of Sheet carrier density with respect to
various values of gate-to-source voltage with d=15nm and
Voff = -7.6V. (Al 0.57In 0.23Ga 0.20N/)Data plot ns in log
scale for experimental data
CONCLUSION
14. Analytical performance evaluation of AlGaN/GaN MISHEMT and
its comparison with conventional HFET’s for high power microwave
applications.
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A two-dimensional physics-based model has been
calculated for AlInGaN/GaN MISHEMT. At the initial
step, the effect of Piezoelectric and Spontaneous and the
polarization
dependent
threshold
voltages
are
discussed.and the results have been analyzed with the
experimental datas. The device shows the very good
candidate for future microwave and nanoscale power
devices.
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