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COMP541

Transistors and all that…

a brief overview

Montek Singh

Sep 8, 2014

Transistors as switches At an abstract level, transistors are merely

switches3-ported voltage-controlled switch

n-type: conduct when control input is 1p-type: conduct when control input is 0

2

g

s

d

g = 0

s

d

g = 1

s

d

g

d

s

d

s

d

s

nMOS

pMOS

OFF ON

ON OFF

Silicon as a semiconductor Transistors are built from silicon Pure Si itself does not conduct well Impurities are added to make it conducting

As provides free electrons n-typeB provides free “holes” p-type

Figure 1.26 Silicon lattice and dopant atoms

MOS Transistors MOS = Metal-oxide semiconductor 3 terminals

gate: the voltage here controls whether current flowssource and drain: are what the current flows between

Figure 1.29 nMOS and pMOS transistors

nMOS Transistors Gate = 0

OFF = disconnectno current flows

between source & drain

Gate = 1ON= connect

current can flow between source & drain

positive gate voltage draws in electrons to form a channel

Figure 1.30 nMOS transistor operation

pMOS Transistors Just the opposite

Gate = 1 disconnectGate = 0 connect

Summary:

6

g

s

d

g = 0

s

d

g = 1

s

d

g

d

s

d

s

d

s

nMOS

pMOS

OFF ON

ON OFF

CMOS Topologies There is actually more to it than

connect/disconnectnMOS: pass good 0’s, but bad 1’s

so connect source to GNDpMOS: pass good 1’s, but bad 0’s

so connect source to VDD

Typically use them incomplementary fashion:nMOS network at bottom

pulls output value down to 0pMOS network at top

pulls output value up to 1only one of the two networks must conduct at a time!

or smoke may be produced if neither network conducts output will be floating 7

pMOSpull-upnetwork

outputinputs

nMOSpull-downnetwork

Inverter

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VDD

A Y

GND

N1

P1

NOT

Y = A

A Y0 11 0

A Y

A P1 N1 Y

0 ON OFF 1

1 OFF ON 0

NAND

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A

B

Y

N2

N1

P2 P1

NAND

Y = AB

A B Y0 0 10 1 11 0 11 1 0

AB

Y

A B P1 P2 N1 N2 Y

0 0 ON ON OFF OFF 1

0 1 ON OFF OFF ON 1

1 0 OFF ON ON OFF 1

1 1 OFF OFF ON ON 0

3-input NOR Gate?

10

B

CY

A

2-input AND Gate?

11

AB

Y

Transmission Gates Transmission gate is a switch:

nMOS pass 1’s poorlypMOS pass 0’s poorlyTransmission gate is a better switch

passes both 0 and 1 wellWhen EN = 1, the switch is ON:

A is connected to BWhen EN = 0, the switch is OFF:

A is not connected to B

IMPORTANT: Transmission gates are not driverswill NOT remove input noise to produce clean(er)

outputsimply connect A and B together (current could even flow

backward!)

use very carefully!

A B

EN

EN

Logic using Transmission Gates Typically combine two (or more) transmission

gates Together form an actual logic gate whose output is

always driven 0 or 1Exactly one transmission gate drives the output;

all remaining transmission gates float their outputs

Example: XORwhen C = 0, TG0 conducts

F = Awhen C = 1, TG1 conducts

F = A’ therefore:

F = A xor C

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TG0

TG1

Tristate buffer and tristate inverter When enabled: sends input to output When disabled: output is floating (‘Z’) Implementation:

Tristate buffer using only a pass gate If on: output input If off: output is floating

Tristate inverter Top half and bottom half are not fully

complementary Either both conduct: output NOT(input)

– will act as a driver! Or both off: output is floating

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E A Y0 0 Z0 1 Z1 0 01 1 1

A

E

Y

A Y

EN

EN

Power Consumption Power = Energy consumed per unit time

Dynamic power consumptionStatic power consumption

Dynamic Power Consumption Energy consumed due to switching activity:

All wires and transistor gates have capacitanceEnergy required to charge a capacitance, C, to VDD is

CVDD2

Circuit running at frequency f: transistors switch (from 1 to 0 or vice versa) at that frequency

Capacitor is charged f/2 times per second (discharging from 1 to 0 is free)

Pdynamic = ½CVDD2f

Static Power Consumption Power consumed when no gates are switching

Caused by the quiescent supply current, IDD (also called the leakage current)

Pstatic = IDDVDD

Power Consumption Example Estimate the power consumption of a wireless

handheld computerVDD = 1.2 VC = 20 nF f = 1 GHz IDD = 20 mA

P = ½CVDD2f + IDDVDD

= ½(20 nF)(1.2 V)2(1 GHz) + (20 mA)(1.2 V) = 14.4 W