Chapter 5Field Effect Transistors

(FET)

Course Learning Outcome

At the end of the lesson, the students shall be able to;1) Define the type of MOSFET2) Describe their operation principles3) Analyze the I-V characteristic4) Analyze MOSFET biasing circuit

Lesson Outline

Introduction to FET/MOSFET Structure of E&D-MOSFET Basic operation of MOSFET The IV characteristic of MOSFET Biasing circuit

1. MOSFET is one type of field-effect transistor.

Field Effect Transistors (FET)

1. FETs are unipolar devices because they operate only with one type of charge carrier.

2. FET is a voltage-controlled device.

3. FET mainly has two types:

a) Junction Field-Effect Transistor (JFET) b) Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

MOSFET

2. MOSFET has no pn junction structure.

3. MOSFET has two types:

a) Enhancement MOSFET b) Depletion MOSFET

DrainSource

Drain

Source

Gate

Gate

Structure of anN-channel JFET

P-type substrate

P

N-channel

The channel has carriers so it conducts from source to drain.

Junction Field-Effect Transistor (JFET)

DrainSource

Drain

Source

Gate

Gate

P

N-channel

P-type substrate

A negative gate voltage can push the carriers from the channel and turn the JFET off.

Junction Field-Effect Transistor (JFET)

0VDS (Volts)

ID (mA)

-4 V

-5 V

0 V

-1 V

-2 V

-3 VVGS

N-channel JFET I-V characteristic curves.

This is known as a depletion-mode device.

JFET Transfer Characteristic

Structure and Symbol for Depletion MOSFET

Structure

n-channel depletion-type MOSFET

Structure and Symbol for Enhancement MOSFET

Symbol

n-channel enhancement-type MOSFET

SEM image of MOSFET

Cross-section

1. The D-MOSFET can be operated in depletion or enhancement modes.

2. To be operated in depletion mode, the gate is made more negative effectively narrowing the channel or depleting the channel of electrons.

3. The E-MOSFET can operate in only the enhancement mode.

4. To be operated in the enhancement mode the gate is made more positive, attracting more electrons into the channel for better current flow.

Basic Operation for MOSFET

D-MOSFET

Source (S)

Gate (G)

Drain (D)

VDDp G

S

D

VGG

Gate bias enhances the channel and turns the device on.

n-channel E-MOSFET

n

n

Basic Operation for Enhancement MOSFET

Source (S)

Gate (G)

Drain (D)

VDDp G

S

D

VGG

n-channel D-MOSFET

n

n

Basic Operation for Depletion MOSFET

Change in channel and depletion region with increasing level of VDS for a fixed value of VGS.

MOSFET characteristics

Calculating ID with given parameters in the

enhancement mode and depletion mode is the same.

Example : Drain and transfer characteristics for an n- channel depletion-type MOSFET

Drain (D)

Gate (G)

Source (S)

-+

- + D-MOSFETs can operate in both depletion and enhancement modes.

Example : Drain and transfer characteristics for an n- channel enhancement-type MOSFET

Drain (D)

Gate (G)

Source (S)

E-MOSFETs can operate in enhancement mode.

TGSDS VVVsat

V6

V2V8

satDSV

6 V

Equation for E-MOSFET

1. The equation for the E-MOSFET transfer characteristic curve is

2TGSD VVKI

where K is the constant depends on the particular MOSFET.

Transfer Characteristics

Examplen-channel enhancement-type MOSFET with and23 AV105.0 K V4TV

23 V4105.0 GSD VI

mA5.0

V4V5105.0 23

DI

mA8

V4V8105.0 23

DI

Quiz

1. D-MOSFETs can operate in both _____________ and ______________modes.depletion enhancement

2. E-MOSFETs can only operate in the _____________ mode. enhancement

3. D-MOSFETs have a ________________ between the drain and source.physical channel

5. In a MOSFET, the process of removing or depleting the channel of charge carriers and thus decreasing the channel conductivity is called ____________.

6. In a MOSFET, the process of creating a channel or increasing the conductivity of the channel by the addition of charge carriers is called ______________.

depletion

enhancement

4. E-MOSFETs have an _______________ between the drain and source.induced channel

7. In a MOSFET, the gate is ____________ from the channel.insulated

MOSFET Biasing

1. The 3 ways to bias a MOSFET are

a) zero-bias (D-MOSFET).

b) voltage-divider bias (E-MOSFET, D-MOSFET).

c) drain-feedback bias (E-MOSFET).

Zero-bias

MOSFET Biasing

2. For D-MOSFET, zero biasing has no applied bias voltage, VGS to the gate.

3. For D-MOSFET, input voltage, VGS swings it into depletion and enhancement mode.

4. For E-MOSFETs, zero biasing cannot be used.

5. For E-MOSFETs, voltage-divider bias must be used to set the VGS greater than the threshold voltage, VT .

2TGSD VVKI

6. For E-MOSFETs, drain-feedback bias has no voltage drop across RG making VGS = VDS.

D

DSDDD R

VVI

Example: Drain-feedback bias The MOSFET has a VT = 3 V, determine the amount of drain current, ID

The meter indicates VGS = 8.5 V

V5.8 GSDS VV

mA38.1

k7.4

V5.8V15

D

DSDDD R

VVI

DC Load Line and Q-Point

n-channel E-MOSFET Drain-feedback bias

Example:

DC Load Line and Q-Point

Voltage-divider biasn-channel E-MOSFET

SUMMARY

For the network of figure below, the levels of VDQ and IDQ are specified. Determine the required values of RD and RS.

Exercise