LECTURE 3 CMOS TRANSISTOR THEORY

# LECTURE 3 CMOS TRANSISTOR THEORY - Introduction to CMOS...

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Introduction to CMOS VLSI Design Lecture 3: CMOS Transistor Theory David Harris Harvey Mudd College Spring 2004

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3: CMOS Transistor Theory Slide 2 CMOS VLSI Design Outline q Introduction q MOS Capacitor q nMOS I-V Characteristics q pMOS I-V Characteristics q Gate and Diffusion Capacitance q Pass Transistors q RC Delay Models
3: CMOS Transistor Theory Slide 3 CMOS VLSI Design Introduction q So far, we have treated transistors as ideal switches q An ON transistor passes a finite amount of current – Depends on terminal voltages – Derive current-voltage (I-V) relationships q Transistor gate, source, drain all have capacitance – I = C ( V/ t) -> t = (C/I) V – Capacitance and current determine speed q Also explore what a “degraded level” really means

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3: CMOS Transistor Theory Slide 4 CMOS VLSI Design MOS Capacitor q Gate and body form MOS capacitor q Operating modes – Accumulation – Depletion – Inversion polysilicon gate (a) silicon dioxide insulator p-type body + - V g < 0 (b) + - 0 < V g < V t depletion region (c) + - V g > V t depletion region inversion region
3: CMOS Transistor Theory Slide 5 CMOS VLSI Design Terminal Voltages q Mode of operation depends on V g , V d , V s – V gs = V g – V s – V gd = V g – V d – V ds = V d – V s = V gs - V gd q Source and drain are symmetric diffusion terminals – By convention, source is terminal at lower voltage – Hence V ds 0 q nMOS body is grounded. First assume source is 0 too. q Three regions of operation Cutoff Linear Saturation V g V s V d V gd V gs V ds + - + - + -

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3: CMOS Transistor Theory Slide 6 CMOS VLSI Design nMOS Cutoff q No channel q I ds = 0 + - V gs = 0 n+ n+ + - V gd p-type body b g s d
3: CMOS Transistor Theory Slide 7 CMOS VLSI Design nMOS Linear q Channel forms q Current flows from d to s – e - from s to d q I ds increases with V ds q Similar to linear resistor + - V gs > V t n+ n+ + - V gd = V + - V > V t n+ n+ + - V gs > V gd > V t V ds = 0 0 < V < V gs -V t p-type body p-type body b g s d b g s d I ds

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3: CMOS Transistor Theory Slide 8 CMOS VLSI Design nMOS Saturation q Channel pinches off q I ds independent of V ds q We say current saturates q Similar to current source + - V gs > V t n+ n+ + - V gd < V t V ds > V gs -V t p-type body b g s d I ds
3: CMOS Transistor Theory Slide 9 CMOS VLSI Design I-V Characteristics q In Linear region, I ds depends on – How much charge is in the channel? – How fast is the charge moving?

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3: CMOS Transistor Theory Slide 10 CMOS VLSI Design Channel Charge q MOS structure looks like parallel plate capacitor while operating in inversion – Gate – oxide – channel q Q channel = n+ n+ p-type body + V gd gate + + source - V gs - drain V ds channel - V g V s V d C g n+ n+ p-type body L t ox SiO 2 gate oxide (good insulator, ε ox = 3.9)
3: CMOS Transistor Theory Slide 11 CMOS VLSI Design

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## This note was uploaded on 03/19/2008 for the course EE 577B taught by Professor Bhatti during the Spring '08 term at USC.

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LECTURE 3 CMOS TRANSISTOR THEORY - Introduction to CMOS...

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