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ELE1110C Tutorial 11Luk Chun Pong29-11-2006Outline:JFET and MOSFETBJT, JFET and MOSFET

BJTJFETMOSFETBaseCollectorEmitterDrainDrainBaseSourceSourceGateGate

BaseCollectorEmitter

GateDrainSource

SourceDrainGateBodyBodyBJTJFETMOSJETFunctionVoltageControlledCurrent sourceVoltage controlled Resistor, Voltage controlled current sourceVoltage controlled Resistor, Voltage controlled current sourceOperationVBE controls IC

VGS controls ID for Large VDSVGS controls ID for Large VDSAdvantageHigh Gain (gm)

Extremely High Input Impedance

Extremely High Input Impedance

TypePNP, NPNn-channel, p-channeln-channel, p-channel

BJT, JFET and MOSFETPart 1: JFETJunction Field Effect TransistorExtremely large input impedanceVery small gate currentAdvantageSmaller gain (gm) than bipolar transistorDisadvantageMore difficult to analyzeFor BJT, VBE ~ 0.6V (~0.5 0.8)For JFET, VGS vary over a wide rangeJFETs are operate at depletion-mode they're "on" unless a gate voltage is applied

Two types of JFET

N-channel JFETP-channel JFETDifferenceCase 1: VDS is small,Operation of JFET (n-channel)VG=0PNVS=0VDVT increase depletion => reduce IDIncrease VDS => increase IDAnd so ID remains constant -> current sourceVG= -2VVS=0VD = 5VPN-channelMore reverse biased:-2 5 = -7VLarge VDS saturation region (constant ID, voltage controlled current source)

Larger depletion region

Use VGS to control the current ID (Voltage controlled current sourceVDS increases, ID remains constantLinear region,VDS increases, ID increases(Voltage controlled current source)

IDSSVPSaturation current IDSS = I (drain-source saturation)Depends on FETVGS = 0, ID = MAXID vs VGS (saturation region)

Problem Set 8

Part 2: MOSFETMetal Oxide Semiconductor Field Effect TransistorTwo modes: enhancement mode and depletion mode 4 terminal devices: source, drain, gate, and bodyDifferent between Enhancement and depletion modeFor n-channel

1. Enhancement:increase VGS, increase the current

2. Depletion: increase the VGS, decrease the current

pnnmetalinsulatorgatesourcedrain- - - - - -- - - - - - -ve charge, conducting channelbodyStructure of MOSFET (N-channel)NMOS and PMOS

Operation of MOSFET (Enhancement mode)1. Cut-off regionVGS < VTH (Threshold Voltage), No conduction

2. Triode or linear region VGS > VTH and VDS < VGS VTH

3. Saturation regionVGS > VTH and VDS > VGS VTH

VDSIDVGS VTH = VDSConstant ID for wide range of VDS, but increase VGS increase IDID increase as VDS increases or VGS increasesNMOS inverter and problems

large static power dissipation

Large Time constantSlow response!C(Complementary)MOS Inverter

NMOSPMOSAdvantage:- Zero Resistance , small RC, fast switching- No static power consumption

Disadvantage:Both MOSFETs are partially conducting during state changing

Charging up the stray capacitor of next stage consumes energy

Total energy dissipation per second =

Problem set 8 Q2