Spansion Embedded NAND NAND Flash Basics

Spansion Embedded NAND
NAND Flash Basics
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© 2012 Spansion, Inc.
CONFIDENTIAL
NOR vs. NAND Flash – Structure
BL
SSL
WL0
WL1
WL2
Control Gate
WL13
WL14
WL15
DSL
Floating Gate
Drain
Source
Source
Drain
Bit line Contact (DRC)
Source
DSL (Drain Sel. Line)
WL1
WL (Control Gate)
WL2
Floating Gate
WL3
WL4
SSL (Source Sel. Line)
Source
Bit line
Cell Source Line
NAND Cell
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NOR Cell
CONFIDENTIAL
Drain
NAND Flash Organization
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CONFIDENTIAL
Concept of NAND Cell
1) NAND Flash Cell
2) MOS Transistor
Control gate
Gate
Floating gate
Source
P-well
Drain
VS
Source
P-well
Drain
NAND flash as a solid-state memory device
Each memory cell represent its “zero” or “one” state, which is identified by means of amount
of trapped electrons in the floating gate.
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CONFIDENTIAL
NAND Operation - Read
0V
0V
Control gate
Control gate
Floating gate
Floating gate
Drain
Source
Drain
Source
Current
P-well
Current
P-well
Programmed Cell, bit data= “Zero”
Erased Cell, bit data= “One”
Read Operation in a programmed cell
Read Operation in an erased cell
How to judge whether a bit data is ‘0’ or ‘1’ from a NAND cell
Control gate is biased as judging voltage(0V) and then
If current can’t flow from Drain to Source, the NAND cell is programmed (bit data ‘1’).
If current can flow from Drain to Source, the NAND cell is erased (bit data ‘0’).
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© 2012 Spansion, Inc.
CONFIDENTIAL
NAND Operation - Program
18V
Number of
bits
Control gate
Erase cell
Floating gate
‘1’
Drain
Source
Program
cell
P-well
0V
‘0’
Vth (Threshold Voltage)
Program Operation, bit data= “Zero”
Vth Shift by Program Operation
NAND Cell Program Operation
• In order to program the cell transistor, a high voltage (e.g. 18V) is applied to
its control gate and a ground voltage is applied to its drain and substrate.
• Under this bias condition, electrons in the substrate are injected into the
floating gate by F-N tunneling effect.
• The threshold voltage of programmed cell shifts into positive direction (e.g.
1V or higher).
• This state is defined as data “zero” and a flash memory cell having the data
“zero” a state is called an “off-cell”.
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© 2012 Spansion, Inc.
CONFIDENTIAL
NAND Operation - Erase
0V
Number of
bits
Control gate
Erase cell
Floating gate
‘1’
Drain
Source
P-well
20V
Program
cell
‘0’
Vth (Threshold Voltage)
Erase Operation, bit data= “One”
Vth Shift by Erase Operation
NAND Flash Cell Erase Operation
•
A cell transistor erase is carried out by applying a ground voltage to
its control gate and a voltage (e.g. 20V) to its substrate.
• This large voltage difference sets up a strong electric field between
the floating gate and the substrate such that electrons on the floating
gate are discharged into the substrate.
• A threshold voltage of erased cell transistor shifts into a negative
direction (e.g. -3V). This state is defined as data “one” and the state
is called an “on-cell”.
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© 2012 Spansion, Inc.
CONFIDENTIAL
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CONFIDENTIAL

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