ECE 431 Digital Circuit Design Chapter 3 MOS Transistor (MOSFET) (slides 2: key Notes) Lecture given by Qiliang Li 1.

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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

Q

C

QV 0

0 2

)|2||2|(0 FSBFTT VVV ox

SiA

C

Nq

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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)( yQW

dydR

In

Resistance of tiny part of the channel dy :

Voltage drop across dy : dyyQW

IdRIdV

In

DDC

)(

DSDS V

CTCGSn

V

CIn

L

D dVVVVWdVyQWdyI0

0

00

)()(

])(2[2

20 DSDSTGS

oxnD VVVV

L

WCI

16

L

WCk oxn

])(2[2

20 DSDSTGSD VVVV

kI

Saturation? Find the maximum

0

DS

D

V

I

]2)(2[ 0 DSTGSDS

D VVVkV

I

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(111

L

L

LLLL

)1(1

DSVL

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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

DDI

VC

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

)]12

1()12

1[(2

|2|21

0 j

dD

j

dSjFASi

oxT x

x

x

x

L

xNq

CV

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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