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COMP541 Transistors and all that… a brief overview

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1 COMP541 Transistors and all that… a brief overview
Montek Singh Sep 8, 2014

2 Transistors as switches
At an abstract level, transistors are merely switches 3-ported voltage-controlled switch n-type: conduct when control input is 1 p-type: conduct when control input is 0

3 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-type B provides free “holes”  p-type Figure 1.26 Silicon lattice and dopant atoms

4 MOS Transistors MOS = Metal-oxide semiconductor 3 terminals
gate: the voltage here controls whether current flows source and drain: are what the current flows between Figure 1.29 nMOS and pMOS transistors

5 nMOS Transistors Gate = 0 Gate = 1 OFF = disconnect ON= connect
no current flows between source & drain Gate = 1 ON= connect current can flow between source & drain positive gate voltage draws in electrons to form a channel Figure 1.30 nMOS transistor operation

6 pMOS Transistors Just the opposite Summary: Gate = 1  disconnect
Gate = 0  connect Summary:

7 CMOS Topologies There is actually more to it than connect/disconnect
nMOS: pass good 0’s, but bad 1’s so connect source to GND pMOS: pass good 1’s, but bad 0’s so connect source to VDD Typically use them in complementary fashion: nMOS network at bottom pulls output value down to 0 pMOS network at top pulls output value up to 1 only one of the two networks must conduct at a time! or smoke may be produced if neither network conducts  output will be floating

8 Inverter A P1 N1 Y ON OFF 1

9 NAND A B P1 P2 N1 N2 Y ON OFF 1

10 3-input NOR Gate?

11 2-input AND Gate?

12 Transmission Gates Transmission gate is a switch:
nMOS pass 1’s poorly pMOS pass 0’s poorly Transmission gate is a better switch passes both 0 and 1 well When EN = 1, the switch is ON: A is connected to B When EN = 0, the switch is OFF: A is not connected to B IMPORTANT: Transmission gates are not drivers will NOT remove input noise to produce clean(er) output simply connect A and B together (current could even flow backward!) use very carefully!

13 Logic using Transmission Gates
Typically combine two (or more) transmission gates Together form an actual logic gate whose output is always driven 0 or 1 Exactly one transmission gate drives the output; all remaining transmission gates float their outputs Example: XOR when C = 0, TG0 conducts F = A when C = 1, TG1 conducts F = A’ therefore: F = A xor C TG0 TG1

14 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

15 Power Consumption Power = Energy consumed per unit time
Dynamic power consumption Static power consumption

16 Dynamic Power Consumption
Energy consumed due to switching activity: All wires and transistor gates have capacitance Energy 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

17 Static Power Consumption
Power consumed when no gates are switching Caused by the quiescent supply current, IDD (also called the leakage current) Pstatic = IDDVDD

18 Power Consumption Example
Estimate the power consumption of a wireless handheld computer VDD = 1.2 V C = 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

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