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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 switches 3-ported voltage-controlled switch n-type: conduct when control input is 1 p-type: conduct when control input is 0.

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Presentation Transcript
transistors as switches
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
silicon as a semiconductor
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

mos transistors
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

nmos transistors
nMOS Transistors
  • Gate = 0
    • OFF = disconnect
      • 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

pmos transistors
pMOS Transistors
  • Just the opposite
    • Gate = 1  disconnect
    • Gate = 0  connect
  • Summary:
cmos topologies
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 incomplementary 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
transmission gates
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:
      • Ais 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!
logic using transmission gates
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

tristate buffer and tristate inverter
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 fullycomplementary
      • Either both conduct: output  NOT(input)
        • will act as a driver!
      • Or both off: output is floating
power consumption
Power Consumption
  • Power = Energy consumed per unit time
    • Dynamic power consumption
    • Static power consumption
dynamic power consumption
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

static power consumption
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
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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