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MOSFET Structure Source Gate Drain Gate Oxide Field Oxide n+ p-Si L Bulk (Substrate) • Importance for LSI/VLSI – Low fabrication cost – Small size – Low power consumption • Applications – Microprocessors – Memories – Power Devices • Basic Properties – – – – – Unipolar device Very high input impedance Capable of power gain 3/4 terminal device, G, S, D, B Two possible device types: enhancement mode; depletion mode – Two possible channel types: n-channel; p-channel Symbols D D B G S p Channel MOSFET B G S n Channel MOSFET Current-Voltage Characteristic IDS B C D A VDS Channel Formation S p-Si VG n-Channel B D +V DS Analysis: Low VDS (A) I DS Q n TR Qn Channel Charge TR Channel Transit Time L vd vd Drift Velocity Qn CV CO (VGS VT )WL I DS n CO (VGS VT )WL VDS L2 I DS n W C (V V )V L O GS T DS L2 TR nVDS Intermediate VDS (B) Source Channel Drain VT VG VG-channel VDS VDS/2 Increased VDS S p-Si VG n-Channel B D +V DS Analysis: Intermediate VDS VDS Qn CO (VG VT )WL 2 First Order Approximation Gate to Channel Voltage = VGS-VDS/2 V W I DS n CO VG VT DS VDS L 2 VDS2 W n CO (VG VT )VDS L 2 Extra term! Large VDS: Saturation (C) Source Channel VG-channel VG Drain VDS VT Pinch-off Analysis: Saturation (C) Pinch-off VDS ( sat ) VG VT Substitute for VDS(sat) in equation for IDS to get IDS(sat) VDS2 W I DS n CO (VGS VT )VDS L 2 (VGS VDS ) 2 W 2 I DS ( sat ) n CO (VGS VT ) L 2 n W 2 CO VGS VT 2 L constant Avalanche and Punch-Through (D) • For very large VDS, IDS increases rapidly due to drain junction avalanche. • Can give rise to parasitic bipolar action. • In short channel transistors, the drain depletion region may reach the source depletion region giving rise to ‘Punch Through’.