ECE 431 Digital Circuit Design Chapter 3 MOS Transistor (MOSFET)
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ECE 431 Digital Circuit Design Chapter 3 MOS Transistor (MOSFET) (slides 2: key Notes) Lecture given by Qiliang Li 1
description
ECE 431 Digital Circuit Design Chapter 3 MOS Transistor (MOSFET). (slides 2: key Notes) Lecture given by Qiliang Li. Introduction. Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is the fundamental building block for modern digital integrated circuit and analog circuit. - PowerPoint PPT Presentation
Transcript of ECE 431 Digital Circuit Design Chapter 3 MOS Transistor (MOSFET)
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ECE 431 Digital Circuit Design
Chapter 3 MOS Transistor (MOSFET)
(slides 2: key Notes)Lecture given by Qiliang Li
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Introduction
• Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is the fundamental building block for modern digital integrated circuit and analog circuit.
MOS structure I-V Characteristics of MOSFET MOSFET scaling and short-channel effect MOSFET capacitance
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3.1-3.3 MOS structure
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Energy band diagrams of the components of MOS structure
SMFB
FpCS
FpFmMS
V
EEqq
EEq
)(
Flat-band voltage
qVFB
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Energy band diagram of a combined MOS structure
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Example 1 for MOS structure
The electron affinity of silicon is 4.15eVFor a p-Si: , gate metal is Al: , what is VFB?An:
eVq Fp 2.0eVq M 1.4
eVeVeVq S 9.475.015.4
eVeVeVqq SM 8.09.41.4
VV SMFB 8.0 4.1eV 4.15eV
0.55eV
0.2eV
Al Si
Ec
EiEFp
Ev
VFB
M
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MOS structure: Inversion
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MOSFET structure
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MOSFET with Bias
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Start of Strong Inversion. MOSFET starts to turn on.
ox
ox
ox
BFGCT C
QCQV 0
0 2
)|2||2|(0 FSBFTT VVV ox
SiA
CNq
2
Threshold VoltageBody-effect coefficient
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|2|2
|2|2
0
0
FSiDB
FSiAB
NqQ
NqQ
Depletion region charge at VSB = 0
for p-Si Negative b/c hole is depleted
for n-Si Positive b/c electron is depleted
Oxide fixed charge is positive Qox > 0
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Find VT0 and VT
N-channel MOSFET
Substrate Na=1E16 cm-3
Poly-Si gate Nd=2E20 cm-3
Tox=50 nm
Oxide interface fixed charge density Nox=4E10 cm-3
Example 2
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3.4 MOSFET I-V Characteristics
])([)( 0TCGSoxI VyVVCyQ Why it is negative?
n channel
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)( yQWdydR
In
Resistance of tiny part of the channel dy :
Voltage drop across dy : dyyQW
IdRIdVIn
DDC
)(
DSDS V
CTCGSn
V
CIn
L
D dVVVVWdVyQWdyI0
000
)()(
])(2[2
20 DSDSTGS
oxnD VVVV
LWCI
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LWCk oxn
])(2[2
20 DSDSTGSD VVVVkI
Saturation? Find the maximum
0
DS
D
VI
]2)(2[ 0 DSTGSDS
D VVVkVI
0TGSDSat VVV
Example 3
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Channel length modulation effect
Channel length modulation effect:The effective channel length during saturation is significantly shorter than before saturation.
LLL '
)1(111LL
LLLL
)1(1DSV
L
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Example 4
from Eq 3.23
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Full Scaling (Constant-Field Scaling)Quantity Before Scaling Full Scaling
Channel Length L 1/S
Channel Width W 1/S
Gate oxide thickness tox 1/S
Junction depth xj 1/S
Power supply voltage VDD 1/S
Threshold voltage VT0 1/S
Doping densities NA and ND S
Oxide capacitance Cox S
Drain current ID 1/S
Power dissipation P 1/S2
Power density P/area 1
Time delay tdelay
Capacitance ?Cox W L -> 1/S
DDIVC
Power = IDD VDD
3.5 MOSFET Scaling and Small-Geometry Effects
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Short Channel Effects
VT0 decreases
Why?How much?
Mobility degradation
Oxide breakdown becauseof hot carrier effect
00)(0 TTchannelshortT VVV
)]121()121[(2
|2|210
j
dD
j
dSjFASi
oxT x
xxx
Lx
NqC
V
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Example 5
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VT decreases significantly
Example 5 continue…
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3.6: MOSFET Capacitance
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Oxide Capacitance
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Junction Capacitance
