Electronics II Lecture 5(b): Metal-Oxide Si FET MOSFET · Summary The metal oxide semiconductor FET uses an insulated gate to isolate the gate from the channel. Two types are the

Summary

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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

Page 1-21

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

VGG


+

–

–

n2 n

p substrate

E-MOSFETID

SiO Induced

+ – +

+ – –

n + n

–

Source

The MOSFET


Page 2-21

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

RD RD

mode, dependingthe gate voltage.

onDD DD


+

–

–

+

–

+

+ –

–

–

D-MOSFET

n n

– + + –– + + –– + p

+V + – p

+V

– + + –– + + –

VGG n VGG n

operating in D-mode operating in E-mode

The MOSFET


Page 3-21

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


Page 4-21

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.

D

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)


I

0 VGS(th) +VGS

The MOSFET


Page 5-21

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.

D

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)


0

I

I DSS

–VGS

The MOSFET


Page 6-21

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.


GR1

+VDD +VDD

RD RRD

R2

Voltage-divider bias Drain-feedback bias

MOSFET Biasing


Page 7-21

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

0


VGS =

+VDD +VDD

RD RD

C

input

RG RG

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

MOSFET Biasing


Page 8-21

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.


Page 9-21

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.


Page 10-21

Quiz


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


Page 11-21

Quiz


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)


Page 12-21

Quiz


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

a. VDSID

b. VGS

c. VS

d. Vth

VDS


Page 13-21

Quiz

m VGS

b. m V

Dc. m VDS

d. m V


4. Transconductance can be expressed as

a. gID

gIG

DS

gI

gIG

GS


Page 14-21

Quiz


5. JFETs cannot be biased using

a. self bias

b. voltage-divider bias

c. zero bias

d. current-source bias


Page 15-21

Quiz

56 kC


E

+15 V

RC1

Q1

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

1

d. undefined 1.0 µF2N3904 C2

Vs = R2 10 µF R3

1.0 kHz 6.2 k 2N5458 100 k ff VGG


Page 16-21

Quiz

Q1d. resistor


E

+15 V

R1 3.9 k Vout

1

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


Page 17-21

Quiz


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

Quiz


ID

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


Page 19-21

Quiz


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


Page 20-21

Quiz


Answers:

1. b 6. a

2. d 7. d

3. b 8. b

4. a 9. a

5. c 10. a


Page 21-21

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Electronics II Lecture 5(b): Metal-Oxide Si FET MOSFET · Summary The metal oxide semiconductor FET uses an insulated gate to isolate the gate from the channel. Two types are the