lec3Mos - Introduction to CMOS VLSI Design CMOS Transistor...

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Introduction to CMOS VLSI Design CMOS Transistor Theory
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MOS devices Slide 2 CMOS VLSI Design Outline Introduction MOS Capacitor nMOS I-V Characteristics pMOS I-V Characteristics Gate and Diffusion Capacitance Pass Transistors RC Delay Models
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MOS devices Slide 3 CMOS VLSI Design 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 ( V/ t) -> t = (C/I) V Capacitance and current determine speed Also explore what a “degraded level” really means
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MOS devices Slide 4 CMOS VLSI Design MOS Capacitor Gate and body form MOS capacitor 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
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MOS devices Slide 5 CMOS VLSI Design Terminal Voltages 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 Source and drain are symmetric diffusion terminals By convention, source is terminal at lower voltage – Hence V ds 0 nMOS body is grounded. First assume source is 0 too. Three regions of operation Cutoff Linear Saturation V g V s V d V gd V gs V ds + - + - + -
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MOS devices Slide 6 CMOS VLSI Design nMOS Cutoff No channel I ds = 0 + - V gs = 0 n+ n+ + - V gd p-type body b g s d
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MOS devices Slide 7 CMOS VLSI Design nMOS Linear Channel forms Current flows from d to s e - from s to d I ds increases with V ds Similar to linear resistor + - V gs > V t n+ n+ + - V gd = V gs + - V gs > V t n+ n+ + - V gs > V gd > V t V ds = 0 0 < V ds < V gs -V t p-type body p-type body b g s d b g s d I ds
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MOS devices Slide 8 CMOS VLSI Design nMOS Saturation Channel pinches off I ds independent of V ds We say current saturates 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
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MOS devices Slide 9 CMOS VLSI Design I-V Characteristics In Linear region, I ds depends on How much charge is in the channel? How fast is the charge moving?
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MOS devices Slide 10 CMOS VLSI Design Channel Charge MOS structure looks like parallel plate capacitor while operating in inversion Gate – oxide – channel 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)
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This note was uploaded on 10/13/2009 for the course ECE 482 taught by Professor Adnanaziz during the Spring '07 term at University of Texas at Austin.

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lec3Mos - Introduction to CMOS VLSI Design CMOS Transistor...

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