Summary Electronics II Lecture 5(b): Metal-Oxide Si FET MOSFET A/Lectr. Khalid Shakir Dept. Of Electrical Engineering College of Engineering Maysan University Page 1-21
Summary The MOSFET The metal oxide semiconductor FET uses an insulated gate to isolate the gate from the channel. Two types are the enhancement mode (E-MOSFET) and the depletion mode (D-MOSFET). An E-MOSFET has no channel until it is induced by a voltage applied to the gate, so it operates only in enhancement mode. An n- channel type is illustrated here; a positive gate voltage induces the channel. Gate SiO Drain 2 n p substrate E-MOSFET V GG Induced channel I D + n + n Source n R D + + + V DD Page 2-21
Summary The MOSFET The D-MOSFET has a channel that can is controlled by the gate voltage. For an n-channel type, a negative voltage depletes the channel; and a positive voltage enhances the channel. A D-MOSFET can operate in either mode, depending on the gate voltage. n R D D-MOSFET + + + + + + + + + p V DD + p + + + + + V GG n V GG n + n R D V DD operating in D-mode operating in E-mode Page 3-21
Summary The MOSFET MOSFET symbols are shown. Notice the broken representing the E-MOSFET that has an induced The n channel has an inward pointing arrow. E-MOSFETs D-MOSFETs D D D D line channel. G G G G S S S S n channel p channel n channel p channel Page 4-21
Summary The MOSFET The transfer curve for a MOSFET is has the same parabolic shape as the JFET but the position is shifted along the x-axis. The transfer curve for an n-channel E-MOSFET is entirely in the first quadrant as shown. I The curve starts at V GS(th), which is a nonzero voltage that is required to have channel conduction. The equation for the drain current is I D K V GS V GS(th) D 0 V GS(th) +V GS Page 5-21
Summary The MOSFET Recall that the D-MOSFET can be operated in either mode. For the n-channel device illustrated, operation to the left of the y-axis means it is in depletion mode; operation to the right means is in enhancement mode. As with the JFET, I D is zero at V GS(off). When VGS is 0, the drain current is IDSS, which for this device is not the maximum current. The equation for drain current is I D I DSS I D I DSS V GS(off) V GS V GS(off) 0 Page 6-21
Summary MOSFET Biasing E-MOSFETs can be biased using bias methods like the BJT methods studied earlier. Voltage-divider bias and drainfeedback bias are illustrated for n-channel devices. +V DD +V DD R 1 R D R R D G R 2 Voltage-divider bias Drain-feedback bias Page 7-21
Summary MOSFET Biasing The simplest way to bias a D-MOSFET is with zero bias. This works because the device can operate in either depletion or enhancement mode, so the gate can go above or below 0 V. +V DD +V DD R D R D V G = 0 V I DSS ac R G V GS = 0 input C R G Zero bias, which can only be used for the D-MOSFET Page 8-21
Selected Key Terms JFET Junction field-effect transistor; one of two major types of field-effect transistors. Drain One of three terminals of a FET analogous to the collector of a BJT. Source One of three terminals of a FET analogous to the emitter of a BJT. Gate One of three terminals of a FET analogous to the base of a BJT. Page 9-21
Selected Key Terms Transconductance The ratio of a change in drain current to a (g m ) change in gate-to-source voltage in a FET. MOSFET Metal oxide semiconductor field effect transistor; one of two major types of FETs; sometimes called IGFET. Depeletion In a MOSFET, the process of removing or depleting the channel of charge carriers and thus decreasing the channel conductivity. Enhancement In a MOSFET, the process of creating a channel or increasing the conductivity of the channel by the addition of charge carriers. Page 10-21
1. If an n-channel JFET has a positive drain voltage and the gate-source voltage is zero, the drain current will be a. zero b. I DSS c. I GSS d. none of the above Page 11-21
2. For a JFET, two voltages with the same magnitude but opposite signs are a. V D and V p b. V D and V S c. V GS(th) and V cutoff d. V p and V GS(off) Page 12-21
3. A set of characteristic curves for a JFET are shown. The blue lines represent different values of a. V DS I D b. V GS c. V S d. V th V DS Page 13-21
4. Transconductance can be expressed as a. g g m b. m g c. m g d. m I D V GS I G V I D VI DS G V DS GS Page 14-21
5. JFETs cannot be biased using a. self bias b. voltage-divider bias c. zero bias d. current-source bias Page 15-21
6. The JFET operating point in the circuit shown is a. at the origin b. at I sat V CC +15 V c. at V R CC R 3.9 C k V 1 out 1 d. undefined 1.0 µf C 56 k Q 1 2N3904 C 2 V s = R 2 10 µf R 3 400 mv pp 39 k R Q 2 E 1.0 khz 6.2 k 2N5458 100 k ff V GG Page 16-21
7. The JFET in this circuit acts like a(n) a. voltage source b. amplifier V CC +15 V c. capacitor R C d. resistor C 1 1.0 µf R 1 3.9 k V out 56 k Q 1 2N3904 C 2 V s = R 2 10 µf R 3 400 mv pp 39 k R Q 2 E 1.0 khz 6.2 k 2N5458 100 k ff V GG Page 17-21
8. The symbol for a p-channel E-MOSFET is D D D D G G G G S S S S a. b. c. d. Page 18-21
9. The transfer curve shown is for an n-channel a. E-MOSFET b. D-MOSFET I D c. JFET d. all of the above 0 V GS(th) +V GS Page 19-21
10. A type of FET that can use the same bias method as a BJT is a(n) a. E-MOSFET b. D-MOSFET c. JFET d. all of the above Page 20-21
Answers: 1. b 6. a 2. d 7. d 3. b 8. b 4. a 9. a 5. c 10. a Page 21-21