64.1Creating a channel for current flow> An n channel can be induced atthe top of the substrate beneath+Gate electrodeUGSthe gate by applying a positiveInducedGSDOn-typevoltage to the gateOxide(SiO2)channel> The channel is an inversionlaver> The value of VGs at which ap-type substratesufficient number of mobileDepletionregionOBelectrons accumulate to form aconducting channel is called thethreshold voltage (V)2025/11/27
4.1 Creating a channel for current flow 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 )
74.1Device structure of MOSFETMaindevicedimensionsfeature sizechannel lengthLSource (S)Gate (G)Drain (D)CO特征尺寸Oxide(SiO2)Metalchannel width(thickness=ta)> oxide thickness Tox (2ChannelnFregionto 50 nm)p-type substrateMetalG(Body)DOxide (SiO)SourceBodynegion(B)Cross-sectionviewp-type substrate(Body)ChannelregionBDrainregion2025/11/27
2025/11/27 7 ➢channel length L ➢channel width ➢oxide thickness Tox ( 2 to 50 nm) 4.1 Device structure of MOSFET Cross-section view feature size 特征尺寸 Main device dimensions
84.1ClassificationofFETAccordingtothetypeofthechannel,FETscanbeclassifiedasMOSFETEnhancementtypeNchannel·Depletiontype·EnhancementtypePchannel·DepletiontypeJFETPchannelNchannel2025/11/27
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 4.1 Classification of FET •Enhancement type •Depletion type •Enhancement type •Depletion type
94.1Drain current @ 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+UGSUps (small)> The channel conductance isGOTproportional to effective voltage,or excess gate voltage, (vgs_ V)> Drain current is proportional toInducedn-channel(VGs- V) and Vpsp-type substrateB二2025/11/27
4.1 Drain current @ small voltage vDS 2025/11/27 9 ➢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
104.1Drain currentsmall voltage 'psip (mA)VGS=V,+2V0.4UGS=V,+1.5V0.3UGS=V,+1 V0.2UGS=V,+0.5V0.1UGS≤V50Ups(m))01001502002025/11/27
4.1 Drain current @ small voltage vDS 2025/11/27 10