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MALVINO

SIXTH EDITION. MALVINO. Electronic. PRINCIPLES. MOSFETs. Chapter 14. Metal oxide insulator. Drain. n. V DD. Gate. p. V GG. Source. (depletion mode). (enhancement mode). Depletion-mode MOSFET. Drain. n. V DD. Gate. p. V GG. Source.

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MALVINO

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  1. SIXTH EDITION MALVINO Electronic PRINCIPLES

  2. MOSFETs Chapter 14

  3. Metal oxide insulator Drain n VDD Gate p VGG Source (depletion mode) (enhancement mode) Depletion-mode MOSFET Drain n VDD Gate p VGG Source Since the gate is insulated, this device can also be operated in the enhancement mode.

  4. MOSFETs • Current flows through a narrow channel between the gate and substrate. • SiO2 insulates the gate from the channel. • Depletion mode forces the carriers from the channel. • Enhancement mode attracts carriers into the channel. • E-MOSFETs are normally-off devices.

  5. D G S VGG n-channel E-MOSFET Drain n VDD Gate p n Source Gate bias enhances the channel and turns the device on.

  6. n-channel E-MOSFET • The p-substrate extends all the way to the silicon dioxide. • No n-channel exists between the source and drain. • This transistor is normally off when the gate voltage is zero. • A positive gate voltage attracts electrons into the p-region to create an n-type inversion layer and turns the device on.

  7. VGG p-channel E-MOSFET Drain p D VDD Gate n G p S Source Gate bias enhances the channel and turns the device on.

  8. p-channel E-MOSFET • The n-substrate extends all the way to the silicon dioxide. • No p-channel exists between the source and drain. • This transistor is normally off when the gate voltage is zero. • A negative gate voltage attracts holes into the n-region to create an p-type inversion layer and turns the device on.

  9. n-channel E-MOSFET drain curves +15 V Ohmic region Constant current region ID +10 V +5 V VGS(th) VDS

  10. n-channel E-MOSFET transconductance curve ID Ohmic ID(sat) Active VGS VGS(th) VGS(on)

  11. Gate breakdown • The SiO2 insulating layer is very thin. • It is easily destroyed by excessive gate-source voltage. • VGS(max) ratings are typically in tens of volts. • Circuit transients and static discharges can cause damage. • Some devices have built-in gate protection.

  12. VDS(on) ID(on) Drain-source on resistance VGS = VGS(on) Qtest ID(on) RDS(on) = VDS(on)

  13. Biasing in the ohmic region VGS = VGS(on) +VDD Qtest ID(on) ID(sat) RD Q VGS VDD ID(sat) < ID(on) when VGS = VGS(on) ensures saturation

  14. Passive and active loads +VDD +VDD RD Q1 vout vout Q2 vin vin Passive load Active load (for Q1,VGS = VDS)

  15. VGS = VDS produces a two-terminal curve +15 V +10 V ID VGS +5 V 5 V 15 V 10 V VDS

  16. VDS(active) ID(active) +VDD +VDD 0 V 0 V Active loading in a digital inverter +VDD RDQ1 = Q1 vout Q2 vin It’s desirable that RDSQ2(on) << RDQ1. (The ideal output swings from 0 volts to +VDD.)

  17. +VDD +VDD 0 V 0 V Complementary MOS (CMOS) inverter +VDD Q1 (p-channel) vin vout Q2 (n-channel) PD(static)@ 0

  18. VDD VDD 2 2 CMOS inverter input-output graph vout VDD Crossover point PD(dynamic)> 0 vin VDD

  19. High-power EMOS • Use different channel geometries to extend ratings • Brand names such as VMOS, TMOS and hexFET • No thermal runaway • Can operate in parallel without current hogging • Faster switching due to no minority carriers

  20. dc-to-ac converter vac Power FET +VGS(on) 0 V

  21. dc-to-dc converter vdc Power FET +VGS(on) 0 V

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