Chapter 5Field-Effect Transistors (FETs)2025/11/27
2025/11/27 Chapter 5 Field-Effect Transistors (FETs)
Content· Physical operation and current-voltagecharacteristicsDCanalysisBiasing in MOS amplifier circuit and basicconfiguration2025/11/27
2025/11/27 2 Content ⚫ Physical operation and current-voltage characteristics ⚫ DC analysis ⚫ Biasing in MOS amplifier circuit and basic configuration
Physical operation andcurrent -voltage characteristics2025/11/27?
2025/11/27 3 Physical operation and current -voltage characteristics
Introductionto FETFET: Field Effect TransistorThere are two types>MOSFET:metal-oxide-semiconductorFET> JFET:Junction FETMOSFET is also called the insulated-gate FET orIGFET.(inteD>Quite smallCore2Extremequad-core> Simple manufacturingprocess>Lowpowerconsumption> Widely used in VLSI circuits(>800 million on a single IC chip)2025/11/27
2025/11/27 4 ⚫ FET: Field Effect Transistor ⚫ There are two types ➢ MOSFET: metal-oxide-semiconductor FET ➢ JFET: Junction FET ⚫ MOSFET is also called the insulated-gate FET or IGFET. ➢ Quite small ➢ Simple manufacturing process ➢ Low power consumption ➢ Widely used in VLSI circuits(>800 million on a single IC chip) Introduction to FET
5DevicestructureofMOSFET (n-type)Gate(G)Source(S)Drain(D)OxideQ.9(Si02)MetalEHHHEntntChannelareap-typeSemiconductorSubstrate(Body) Body(B)For normal operation, it is needed to create aconducting channel between Source and Drain2025/11/27
2025/11/27 5 Device structure of MOSFET (n-type) p-type Semiconductor Substrate (Body) Body(B) n + n + Oxide (SiO2 ) Source(S) Gate(G) Drain(D) Metal ⚫ For normal operation, it is needed to create a conducting channel between Source and Drain Channel area
6Creating a channel for current flow> An n channel can be induced at+Gate electrodethe top of the substrate beneathUGS1Inducedthe gate by applying a positiveGSDOn-typeOxide(SiO2)channelvoltage to the gate> The channel is an inversionlaver> The value of VGs at which ap-type substrateDepletionregionsufficient number of mobileOBelectrons accumulate to form a一conducting channel is called thethreshold voltage (V)2025/11/27
2025/11/27 6 ➢An n channel can be induced at the top of the substrate beneath the gate by applying a positive voltage to the gate ➢The channel is an inversion layer ➢The value of VGS at which a sufficient number of mobile electrons accumulate to form a conducting channel is called the threshold voltage (Vt ) Creating a channel for current flow
Device structure of MOSFET (n-type)Source (S)Gate (G)Drain (D)>L = 0.1 to 3 μmOC0Oxide(SiO2)Metal>W=0.2to100 um(thickness=tax)>T.=2to50nmChannelregionp-type substrateMetalG(Body)DOxide (SiO,)urceBodytion(B)Cross-sectionview(pesubstrate(Body)ChannelregionBDrain region2025/11/27
2025/11/27 7 ➢L = 0.1 to 3 mm ➢W = 0.2 to 100 mm ➢Tox= 2 to 50 nm Device structure of MOSFET (n-type) Cross-section view
8ClassificationofFETAccording to the type of the channel, FETs canbe classified asMOSFETEnhancementtypeNchannel·Depletiontype·EnhancementtypePchannel.DepletiontypeJFET.PchannelNchannel2025/11/27
2025/11/27 8 ⚫ According to the type of the channel, FETs can be classified as ➢ MOSFET ▪ N channel ▪ P channel ➢ JFET ▪ P channel ▪ N channel Classification of FET •Enhancement type •Depletion type •Enhancement type •Depletion type
Drain current under small voltage Vps> An NMOS transistor with Vcs > V, and with a small Vpsapplied.- The channel depth is uniform and the device acts as aresistance.+HUGSUps (small)> The channel conductance isGQoproportional to effective voltage,or excess gate voltage, (vgs_ V)> Drain current is proportional toInducedn-channel(VGs - V) and Vpsp-type substrateB2025/11/27
2025/11/27 9 Drain current under small voltage vDS ➢An NMOS transistor with vGS > Vt and with a small vDS applied. − The channel depth is uniform and the device acts as a resistance. ➢The channel conductance is proportional to effective voltage, or excess gate voltage, (vGS – Vt ) . ➢Drain current is proportional to (vGS – Vt ) and vDS
10Drain current under small voltage Vpsip (mA)VGS=V,+2V0.4UGS=V,+1.5V0.3UGS=V,+I V0.2UGS=V,+0.5V0.1UGS≤V50Ups(m))01001502002025/11/27
2025/11/27 10 Drain current under small voltage vDS