Electronics II Lecture 5(b): Metal-Oxide Si FET MOSFET · Summary The metal oxide semiconductor FET...

Summary

Electronics IILecture 5(b): Metal-Oxide Si FET MOSFET

A/Lectr. Khalid ShakirDept. Of Electrical Engineering

College of EngineeringMaysan University

Copyright @2013 by Dept. of Electrical Engineering, Electronics II- Lecture 5(b)/1st Semester 013/014College of Engineering, Maysan University

Summary

The metal oxide semiconductor FET uses an insulated gateto isolate the gate from the channel. Two types are theenhancement mode (E-MOSFET) and the depletion mode(D-MOSFET).An E-MOSFET has nochannel until it is inducedby a voltage applied to thegate, so it operates only inenhancement mode. An n-channel type is illustratedhere; a positive gate voltageinduces the channel.

RDDrain

channel

+Gate VDD

p substrate

E-MOSFETID

SiO Induced

+ – +

+ – –

Source

The MOSFET

Summary

The D-MOSFET has a channel that can is controlled by thegate voltage. For an n-channel type, a negative voltagedepletes the channel; and a positive voltage enhances thechannel.

A D-MOSFET canoperate in either

mode, dependingthe gate voltage.

onDD DD

D-MOSFET

– + + –– + + –– + p

+V + – p

– + + –– + + –

VGG n VGG n

operating in D-mode operating in E-mode

The MOSFET

Summary

MOSFET symbols are shown. Notice the brokenan inducedarrow.

linechannel.representing the E-MOSFET that has

The n channel has an inward pointing

E-MOSFETs D-MOSFETs

D D D D

G G G G

S S S S

n channel p channel n channel p channel

The MOSFET

Summary

The transfershape as theThe transfer

curve forJFET butcurve for

a MOSFET isthe position is

has the same parabolicshifted along the x-axis.

an n-channel E-MOSFET is entirely inthe first quadrant as shown.

The curve starts at VGS(th), which is anonzero voltage that is required to havechannel conduction. Thethe drain current is

equation for

ID K VGS VGS(th)

0 VGS(th) +VGS

The MOSFET

Summary

Recall that the D-MOSFET can be operated in either mode.For the n-channel device illustrated, operation to the left ofthe y-axis means it is in depletion mode; operation to theright means is in enhancement mode.

As with the JFET, ID is zero at VGS(off).When VGS is 0, the drain current isIDSS, which for this device is not themaximum current. The equation fordrain current is

I ID DSS VGS(off)VGS(off)

–VGS

The MOSFET

Summary

E-MOSFETs can be biased using bias methods like the BJTmethods studied earlier. Voltage-divider bias and drain-feedback bias are illustrated for n-channel devices.

+VDD +VDD

RD RRD

Voltage-divider bias Drain-feedback bias

MOSFET Biasing

Summary

The simplest way to bias a D-MOSFET is with zero bias. Thisworks because the device can operate in either depletion or

V.enhancement mode, so the gate can go above or below 0

= 0 V IDSS acVG

+VDD +VDD

Zero bias, which can only be used for the D-MOSFET

MOSFET Biasing

Selected Key Terms

JFET Junction field-effect transistor; one of twomajor types of field-effect transistors.

Drain One of three terminals of a FET analogous tothe collector of a BJT.

Source One of three terminals of a FET analogous tothe emitter of a BJT.

Gate One of three terminals of a FET analogous tothe base of a BJT.

Selected Key Terms

Transconductance The ratio of a change in drain current to a(gm) change in gate-to-source voltage in a FET.

MOSFET Metal oxide semiconductor field effecttransistor; one of two major types of FETs;sometimes called IGFET.

Depeletion In a MOSFET, the process of removing ordepleting the channel of charge carriers andthus decreasing the channel conductivity.

Enhancement In a MOSFET, the process of creating achannel or increasing the conductivity of thechannel by the addition of charge carriers.

1. If an n-channel JFET has a positive drain voltage and thegate-source voltage is zero, the drain current will be

a. zero

b. IDSS

c. IGSS

d. none of the above

2. For a JFET, two voltages with the same magnitude butopposite signs are

a. VD and Vp

b. VD and VS

c. VGS(th) and Vcutoff

d. Vp and VGS(off)

3. A set of characteristic curves for a JFET are shown. Theblue lines represent different values of

a. VDSID

b. VGS

d. Vth

b. m V

Dc. m VDS

d. m V

4. Transconductance can be expressed as

a. gID

5. JFETs cannot be biased using

a. self bias

b. voltage-divider bias

c. zero bias

d. current-source bias

400 mV pp 39 k R Q2

6. The JFET operating point in the circuit shown is

a. at the origin

b. at IsatVCC

c. at VCC R 3.9 k Vout

d. undefined 1.0 µF2N3904 C2

Vs = R2 10 µF R3

1.0 kHz 6.2 k 2N5458 100 k ff VGG

Q1d. resistor

R1 3.9 k Vout

2N3904 C

400 mV pp 39 k R Q2

7. The JFET in this circuit acts like a(n)

a. voltage source

b. amplifier VCC

c. capacitor RC

C 56 k

1.0 µF 2

Vs = R2 10 µF R3

1.0 kHz 6.2 k 2N5458 100 k ff VGG

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.

9. The transfer curve shown is for an n-channel

a. E-MOSFET

b. D-MOSFET

c. JFET

d. all of the above

0 VGS(th) +VGS

10. A type of FET that can use the same bias method as aBJT is a(n)

a. E-MOSFET

b. D-MOSFET

c. JFET

d. all of the above

Answers:

1. b 6. a

2. d 7. d

3. b 8. b

4. a 9. a

5. c 10. a

Electronics II Lecture 5(b): Metal-Oxide Si FET MOSFET · Summary The metal oxide semiconductor FET...

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