University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell

CMOS Transistors and Gates

University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 2

Simple electronicsOhm’s Law - V = IR

voltage (V) equals current (I) times resistance (R)

Hydraulic AnalogyCharge liquidCurrent flow rateVoltage water pressureResistance related to length and radius of pipe (kL/r4)

Hydraulic pictures fromhttp://hyperphysics.phy-astr.gsu.edu

University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 3

Hydraulic analogyVoltage water pressure

Current flow rate

Volume flow rate in m3/sec, etc. Current flow rate in coulombs/sec = amps

University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 4

Hydraulic analogy

Resistance related to length and radius of pipe (kL/r4)

Ground reservoir

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Hydraulic analogyResistances in series

Resistances in parallel

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CMOS Transistors

Need circuits to represent 2 discrete values1,0 for binary representationsTrue, False for Boolean logic

Let high voltage (Vdd) represent 1, or true

Let low voltage (0 volts or gnd) represent 0, or falseIf we have some switches to control whether or not these voltages can propagate through a circuit, we can build a computer with them

Note, the earliest digital computers were electromechanical, made out of relays, so this is hardly a new idea

Our switches will be CMOS transistors

University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 7

1 (Vdd)

Two kinds of transistorsN-type

1 (Vdd)

P-type

1 (Vdd)

1 (Vdd)source

draingate

s

dg

s

dg

s

dg

University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 8

0 (gnd)

Two kinds of transistorsN-type

0 (gnd)

P-type

0 (gnd)

0 (gnd)

s

dg

s

dg

s

dg

source

draing

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How they work as switches

N-type

n+ n+

p-type body

W

L

tox

SiO2 gate oxide(good insulator, εox = 3.9)

polysilicongatε

+-

Vgs = 0

n+ n+

+-

Vgd

p-type body

b

g

s d

When gate is not at higher voltage than source• no excess electrons in channel under gate• so no current can flow• switch is open

QuickTime™ and a decompressor

are needed to see this picture.

University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 10

How they work as switches

N-type

When Vgs > Vth ,the threshold voltage• excess electrons attracted into channel• current flows and switch is closed• drain voltage cannot be more than source voltage = Vg-Vth

• this is at most Vdd-Vth

• Vdd-Vth is still considered a 1, but a weak 1• if source voltage is 0, then drain voltage is too, so 0 still strong

+-

Vgs > Vt

n+ n+

+-

Vgd = Vgs

+-

Vgs > Vt

n+ n+

+-

Vgs > Vgd > Vt

Vds = 0

0 < Vds < Vgs-Vt

p-type body

p-type body

b

g

s d

b

g

s d Ids

CMOS transistor pictures fromUT ECE VLSI course slides

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CMOS circuit rules

Never create a path from Vdd to gnd

Don’t pass weak valuesN-type transistors pass weak 1’s (Vdd - Vth)

N-type transistors pass strong 0’s (gnd)Use N-type transistors only to pass 0’s (n to negative)Conversely for P-type transistors

Pass weak 0’s (Vth), strong 1’s (Vdd)

Use P-type transistors only to pass 1’s (p to positive)

Never leave a wire undrivenMake sure there’s always a path to Vdd or gnd

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Example CMOS gate - inverter

In Out0 11 0

Truth table Circuit

Note how all 3 design rules are obeyedCircuit amplifies weak input 1 or 0