Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MALVINO & BATES SEVENTH EDITION Electronic PRINCIPLES

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MOSFETs Chapter 14

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Topics Covered in Chapter 14 The depletion-mode MOSFET D-MOSFET curves Depletion-mode MOSFET amplifiers The enhancement-mode MOSFET The ohmic region

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Topics Covered in Chapter 14 (Continued) Digital switching CMOS Power FETs MOSFET amplifiers MOSFET testing

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. D-MOSFET curves Similar to JFET curves with depletion- mode operation D-MOSFETs can also operate in the enhancement mode with drain current greater than I DSS

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Drain curves

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Depletion-mode MOSFET amplifiers Mainly used as RF amplifiers Good high frequency response Low noise High input impedance Dual-gate MOSFETs can be used for AGC circuits

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Depletion-mode MOSFET The D-MOSFET has a source, gate, and drain The gate is insulated from the channel Exhibits very high input resistance Limited use, mainly RF circuits

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. D-MOSFET structure

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Depletion-mode MOSFET n Source Gate Drain V DD V GG p n Source Gate Drain V DD V GG p Since the gate is insulated, this device can also be operated in the enhancement mode. Metal oxide insulator (depletion mode)(enhancement mode)

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MOSFETs Current flows through a narrow channel between the gate and substrate. SiO 2 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.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. n Source Gate Drain V DD p n n-channel E-MOSFET G S D V GG Gate bias enhances the channel and turns the device on.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Source Gate Drain V DD p n p-channel E-MOSFET G S D V GG Gate bias enhances the channel and turns the device on. p

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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. p-channel E-MOSFET

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. V DS IDID V GS = +15 V V GS = +10 V V GS = +5 V V GS(th) n-channel E-MOSFET drain curves Ohmic region Constant current region

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. n-channel E-MOSFET transconductance curve V GS(th) V GS(on) I D(sat) V GS IDID Active Ohmic

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gate breakdown The SiO 2 insulating layer is very thin. It is easily destroyed by excessive gate- source voltage. V GS(max) ratings are typically in tens of volts. Circuit transients and static discharges can cause damage. Some devices have built-in gate protection such as a zener diode in parallel with the gate and source.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. I D(on) V GS = V GS(on) Drain-source on resistance V DS(on) Q test R DS(on) = V DS(on) I D(on) IDID V DS

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The ohmic region Since the E-MOSFET is primarily a switching device, it typically operates between cutoff and saturation When biased in the ohmic region, the E- MOSFET acts like a small resistance

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. I D(on) V GS = V GS(on) Biasing in the ohmic region V DD Q test I D(sat) RDRD +V DD V GS Q I D(sat) < I D(on) when V GS = V GS(on) ensures saturation IDID V DS

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Active-load switching One MOSFET acts like a large resistor and the other MOSFET acts like a switch

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Passive and active loads RDRD +V DD v in v out +V DD v in v out Q2Q2 Q1Q1 Passive loadActive load (for Q1, V GS = V DS )...

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. V DS IDID +15 V +10 V +5 V V GS = V DS produces a two-terminal curve 5 V 10 V 15 V V GS.... Ohmic

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +V DD v in v out Q2Q2 Q1Q1 It is desirable that R DSQ2(on) << R DQ1 Active loading in a digital inverter 0 V +V DD V DS(active) I D(active) R DQ1 = 0 V +V DD (The ideal output swings from 0 volts to +V DD )

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CMOS Uses two complementary MOSFETs One MOSFET conducts and the other shuts off The CMOS inverter is a basic digital circuit Exhibits very low power consumption

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +V DD v in v out Q 1 (p-channel) Complementary MOS (CMOS) inverter Q 2 (n-channel) P D(static)  0 0 V +V DD 0 V +V DD..

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CMOS inverter input-output graph V DD v in v out V DD 2 2 Crossover point P D(dynamic)  0

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. High-power EMOS Uses 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 since there are no minority carriers

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. dc-to-ac converter v ac 0 V +V GS(on) Power FET An application includes a UPS for a PC system

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. dc-to-dc converter v dc 0 V +V GS(on) Power FET

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. E-MOSFET amplifiers V GS must be greater than V GS (th) Drain-feedback bias is similar to collector-feedback bias

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MOSFET testing Ohmmeter and DMM testing are not effective The use of a curve tracer is a good method of testing Simple substitution will also determine a faulty device