CMOS Transistors and Boolean Logic Gates

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University of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell CMOS Transistors and Boolean Logic Gates

Transcript of CMOS Transistors and Boolean Logic Gates

Page 1: CMOS Transistors and Boolean Logic Gates

University of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell

CMOS Transistors and BooleanLogic Gates

Page 2: CMOS Transistors and Boolean Logic Gates

University of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 2

CMOS Transistors

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

Let high voltage (Vdd) represent 1, or trueLet low voltage (0 volts or gnd) represent 0, or falseIf we have some switches to control whether or not thesevoltages can propagate through a circuit, we can build acomputer with them

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

Our switches will be CMOS transistors

Page 3: CMOS Transistors and Boolean Logic Gates

University of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 3

1 (Vdd)

Two kinds of transistorsN-type

1 (Vdd)

P-type

1 (Vdd)

1 (Vdd)source

draingate

s

dg

s

dg

s

dg

Page 4: CMOS Transistors and Boolean Logic Gates

University of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 4

0 (gnd)

Two kinds of transistorsN-type

0 (gnd)

P-type

0 (gnd)

0 (gnd)

s

dg

s

dg

s

dg

source

draing

Page 5: CMOS Transistors and Boolean Logic Gates

University of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 5

How they work as switches

N-type

n+ n+

p-type body

W

L

tox

SiO2 gate oxide

(good insulator, !ox

= 3.9)

polysilicon

gate

+-

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

Page 6: CMOS Transistors and Boolean Logic Gates

University of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 6

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 dIds

CMOS transistor pictures fromUT ECE VLSI course slides

Page 7: CMOS Transistors and Boolean Logic Gates

University of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 7

CMOS circuit rules

Never create a path from Vdd to gndDon’t pass weak values

N-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

Page 8: CMOS Transistors and Boolean Logic Gates

University of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 8

Example CMOS gate - inverter

0110

OutIn

Truth table Circuit

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