332 479 concepts in vlsi design lecture 6 cmos transistor theory
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332:479 Concepts in VLSI Design Lecture 6 CMOS Transistor Theory. David Harris and Michael Bushnell Harvey Mudd College and Rutgers University Spring 2004. Outline. Introduction MOS Capacitor n MOS I-V Characteristics p MOS I-V Characteristics Gate and Diffusion Capacitance

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332 479 concepts in vlsi design lecture 6 cmos transistor theory

332:479 Concepts in VLSIDesignLecture 6 CMOS Transistor Theory

David Harris and Michael Bushnell

Harvey Mudd College and Rutgers University

Spring 2004


Outline
Outline

  • Introduction

  • MOS Capacitor

  • nMOS I-V Characteristics

  • pMOS I-V Characteristics

  • Gate and Diffusion Capacitance

  • Pass Transistors

  • RC Delay Models

  • Adjustments for non-ideal 2nd-order effects

  • Small-signal MOSFET model

Material from: CMOS VLSI Design,

by Weste and Harris, Addison-Wesley, 2005

Concepts in VLSI Des. Lec. 6


Introduction
Introduction

  • So far, we have treated transistors as ideal switches

  • An ON transistor passes a finite amount of current

    • Depends on terminal voltages

    • Derive current-voltage (I-V) relationships

  • Transistor gate, source, drain all have capacitance

    • I = C (DV/Dt) -> Dt = (C/I) DV

    • Capacitance and current determine speed

  • Also explore what a “degraded level” really means

Concepts in VLSI Des. Lec. 6


Mos characteristics
MOS Characteristics

  • MOS – majority carrier device

  • Carriers: e-- in nMOS, holes in pMOS

  • Vt– channel threshold voltage (cuts off for voltages < Vt)

Concepts in VLSI Des. Lec. 6


N mos enhancement transistor
nMOS Enhancement Transistor

  • Moderately doped p type Si substrate

  • 2 Heavily doped n+ regions

Concepts in VLSI Des. Lec. 6


I vs v plots
I vs. V Plots

  • Enhancement and depletion transistors

    • CMOS – uses only enhancement transistors

    • nMOS – uses both

Concepts in VLSI Des. Lec. 6


Materials and dopants
Materials and Dopants

  • SiO2 – low loss, high dielectric strength

    • High gate fields are possible

  • n type impurities: P, As, Sb

  • p type impurities: B, Al, Ga, In

Concepts in VLSI Des. Lec. 6


Bipolar vs mos
Bipolar vs. MOS

  • Bipolar – p-n junction – metallurgical

  • MOS

    • Inversion layer / substrate junction field-induced

    • Voltage-controlled switch, conducts when VgsVt

    • e-- swept along channel when Vds > 0 by horizontal component ofE

    • Pinch-off – conduction by e–- drift mechanism caused by positive drain voltage

    • Pinched-off channel voltage:Vgs – Vt(saturated)

    • Reverse-biased p-n junction insulates from the substrate

Concepts in VLSI Des. Lec. 6


Jfet vs fet transistors
JFET vs. FET Transistors

  • Junction FET (JFET) – channel is deep in semiconductor

  • MOSFET – For given Vds & Vgs,Ids controlled by:

    • Distance between source & drain L

    • Channel width W

    • Vt

    • Gate oxide thickness tox

    • e gate oxide

    • Carrier mobility m

Concepts in VLSI Des. Lec. 6


Jfet transistor
JFET Transistor

Concepts in VLSI Des. Lec. 6


Mos capacitor
MOS Capacitor

  • Gate and body form MOS capacitor

  • Operating modes

    • Accumulation

    • Depletion

    • Inversion

Concepts in VLSI Des. Lec. 6


Terminal voltages
Terminal Voltages

  • Mode of operation depends on Vg, Vd, Vs

    • Vgs = Vg – Vs

    • Vgd = Vg – Vd

    • Vds = Vd – Vs = Vgs - Vgd

  • Source and drain are symmetric diffusion terminals

    • By convention, source is terminal at lower voltage

    • Hence Vds 0

  • nMOS body is grounded. First assume source is 0 too.

  • Three regions of operation

    • Cutoff

    • Linear

    • Saturation

Concepts in VLSI Des. Lec. 6


N mos cutoff
nMOS Cutoff

  • No channel

  • Ids = 0

Concepts in VLSI Des. Lec. 6


N mos linear
nMOS Linear

  • Channel forms

  • Current flows from d to s

    • e- from s to d

  • Ids increases with Vds

  • Similar to linear resistor

  • At drain end of channel, only difference between gate & drain voltages effective for channel creation

Concepts in VLSI Des. Lec. 6


N mos saturation
nMOS Saturation

  • Channel pinches off

  • Ids independent of Vds

  • We say current saturates

  • Similar to current source

Concepts in VLSI Des. Lec. 6


I v characteristics
I-V Characteristics

  • In Linear region, Ids depends on

    • How much charge is in the channel?

    • How fast is the charge moving?

Concepts in VLSI Des. Lec. 6


Channel charge
Channel Charge

  • MOS structure looks like parallel plate capacitor while operating in inversion

    • Gate – oxide – channel

  • Qchannel =

Concepts in VLSI Des. Lec. 6


Channel charge1
Channel Charge

  • MOS structure looks like parallel plate capacitor while operating in inversion

    • Gate – oxide – channel

  • Qchannel = CV

  • C =

Concepts in VLSI Des. Lec. 6


Channel charge2
Channel Charge

  • MOS structure looks like parallel plate capacitor while operating in inversion

    • Gate – oxide – channel

  • Qchannel = CV

  • C = Cg = eoxWL/tox = CoxWL

  • V =

Cox = eox / tox

Concepts in VLSI Des. Lec. 6


Channel charge3
Channel Charge

  • MOS structure looks like parallel plate capacitor while operating in inversion

    • Gate – oxide – channel

  • Qchannel = CV

  • C = Cg = eoxWL/tox = CoxWL

  • V = Vgc – Vt = (Vgs – Vds/2) – Vt

Cox = eox / tox

Concepts in VLSI Des. Lec. 6


Carrier velocity
Carrier velocity

  • Charge is carried by e-

  • Carrier velocity v proportional to lateral E-field between source and drain

  • v =

Concepts in VLSI Des. Lec. 6


Carrier velocity1
Carrier Velocity

  • Charge is carried by e-

  • Carrier velocity v proportional to lateral E-field between source and drain

  • v = mE m called mobility

  • E =

Concepts in VLSI Des. Lec. 6


Carrier velocity2
Carrier Velocity

  • Charge is carried by e-

  • Carrier velocity v proportional to lateral E-field between source and drain

  • v = mE m called mobility

  • E = Vds/L

  • Time for carrier to cross channel:

    • t =

Concepts in VLSI Des. Lec. 6


Carrier velocity3
Carrier Velocity

  • Charge is carried by e-

  • Carrier velocity v proportional to lateral E-field between source and drain

  • v = mE m called mobility

  • E = Vds/L

  • Time for carrier to cross channel:

    • t = L / v

Concepts in VLSI Des. Lec. 6


N mos linear i v
nMOS Linear I-V

  • Now we know

    • How much charge Qchannel is in the channel

    • How much time t each carrier takes to cross

Concepts in VLSI Des. Lec. 6


N mos linear i v1
nMOS Linear I-V

  • Now we know

    • How much charge Qchannel is in the channel

    • How much time t each carrier takes to cross

Concepts in VLSI Des. Lec. 6


N mos linear i v2
nMOS Linear I-V

  • Now we know

    • How much charge Qchannel is in the channel

    • How much time t each carrier takes to cross

Concepts in VLSI Des. Lec. 6


N mos saturation i v
nMOS Saturation I-V

  • If Vgd < Vt, channel pinches off near drain

    • When Vds > Vdsat = Vgs – Vt

  • Now drain voltage no longer increases current

Concepts in VLSI Des. Lec. 6


N mos saturation i v1
nMOS Saturation I-V

  • If Vgd < Vt, channel pinches off near drain

    • When Vds > Vdsat = Vgs – Vt

  • Now drain voltage no longer increases current

Concepts in VLSI Des. Lec. 6


N mos saturation i v2
nMOS Saturation I-V

  • If Vgd < Vt, channel pinches off near drain

    • When Vds > Vdsat = Vgs – Vt

  • Now drain voltage no longer increases current

Concepts in VLSI Des. Lec. 6


N mos i v summary
nMOS I-V Summary

  • Shockley 1st order transistor models

Concepts in VLSI Des. Lec. 6


Example
Example

  • We will be using a 0.6 mm process for your project

    • From AMI Semiconductor

    • tox = 100 Å

    • m = 350 cm2/V*s

    • Vt = 0.7 V

  • Plot Ids vs. Vds

    • Vgs = 0, 1, 2, 3, 4, 5

    • Use W/L = 4/2 l

Concepts in VLSI Des. Lec. 6


P mos i v
pMOS I-V

  • All dopings and voltages are inverted for pMOS

  • Mobility mp is determined by holes

    • Typically 2-3x lower than that of electrons mn

    • 120 cm2/V*s in AMI 0.6 mm process

  • Thus pMOS must be wider to provide same current

    • In this class, assume mn / mp = 2

Concepts in VLSI Des. Lec. 6


Summary
Summary

  • Current Characteristics of MOSFET

  • Calculation of VtandImportant 2nd-Order Effects

  • Small-Signal MOSFET Model

  • Models in this lecture

    • For pedagogical purposes only

    • Obsolete for deep-submicron technology

    • Real transistor parameter differences:

      • Much higher transistor current leakage

      • Body effect less significant than predicted

      • Vt is lower than predicted

Concepts in VLSI Des. Lec. 6


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