Lecture2_3

Lecture2_3 - CMOS VLSI For Computer Engineering Lecture 2-...

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CMOS VLSI For Computer Engineering Lecture 2- Device Theory CMOS VLSI for Computer Engineering 1 Parts adapted from Lecture notes by Mary Jane Irwin www.cse.psu.edu/~cg477 , J. Rabaey et al. http://bwrc.eecs.berkeley.edu/IcBook and David Harris http://http://www3.hmc.edu/~harris/cmosvlsi/4e January 16, 2012
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CMOS VLSI for Computer Engineering 2 Outline Introduction Diodes MOS Capacitor nMOS I-V Characteristics pMOS I-V Characteristics Device Capacitance Body Effect Short Channel effects January 16, 2012
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CMOS VLSI for Computer Engineering The Diode n p p n B A SiO 2 Al A B Al A B Cross-section of pn-junction in an IC process One-dimensional representation diode symbol Mostly occurring as parasitic element in Digital ICs
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CMOS VLSI for Computer Engineering Depletion Region hole diffusion electron diffusion p n hole drift electron drift Charge Density Distance x + - Electrical x Field x Potential V ξ ρ W 2 -W 1 ψ 0 (a) Current flow. (b) Charge density. (c) Electric field. (d) Electrostatic potential.
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CMOS VLSI for Computer Engineering Diode Current
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CMOS VLSI for Computer Engineering Forward Bias x p n0 n p0 -W 1 W 2 0 p n (W 2 ) n-region p-region L p diffusion Typically avoided in Digital ICs
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CMOS VLSI for Computer Engineering Reverse Bias x p n0 n p0 -W 1 W 2 0 n-region p-region diffusion The Dominant Operation Mode
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CMOS VLSI for Computer Engineering Models for Manual Analysis V D I D = I S (e V D / φ T – 1) + V D + + V Don I D (a) Ideal diode model (b) First-order diode model
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CMOS VLSI for Computer Engineering Junction Capacitance
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CMOS VLSI for Computer Engineering Linearizing the Junction Capacitance Replace non-linear capacitance by large-signal equivalent linear capacitance which displaces  equal charge  over voltage swing of interest
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CMOS VLSI for Computer Engineering Diffusion Capacitance
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CMOS VLSI for Computer Engineering Secondary Effects –25.0 –15.0 –5.0 5.0 V D (V) –0.1 I D (A) 0.1 0 0 Avalanche Breakdown
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CMOS VLSI for Computer Engineering Diode Model I D R S C D + - V D
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CMOS VLSI for Computer Engineering 14 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 January 16, 2012
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CMOS VLSI for Computer Engineering 15 polysilicon gate (a) silicon dioxide insulator p-type body + - V g < 0 MOS Capacitor Gate and body form MOS capacitor Operating modes Accumulation Depletion Inversion (b) + - 0 < V g < V t depletion region (c) + - V g > V t depletion region inversion region
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CMOS VLSI for Computer Engineering Threshold Voltage (without body effect) Point at which the surface potential is sufficiently positive so that the response to gate charge is compensated by mobile electrons (for NMOS) rather than dopants 16 Potential energy due to work function difference between gate and body ~Surface potential at the point of inversion = ~potential due to uncovered dopant atoms
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Lecture2_3 - CMOS VLSI For Computer Engineering Lecture 2-...

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