ElectronicsElectronics

Principles & ApplicationsPrinciples & ApplicationsFifth EditionFifth Edition

Chapter 5Junction Transistors

©1999 Glencoe/McGraw-Hill

Charles A. Schuler

• Amplification• Transistors• Characteristic Curves• Transistor Testing• Other Transistor Types

INTRODUCTION

Amplifier Out

InGain =

In

Out

N

P

N

NPN Transistor Structure

The collector is lightly doped. C

The base is thin and is lightly doped.

B

The emitter is heavily doped. E

The C-B junction is reverse biased.

N

P

N

NPN Transistor Bias

C

B

E

No current flows.

The B-E junction is forward biased.

N

P

N

NPN Transistor Bias

C

B

E

Current flows.

When both junctionsare biased....

N

P

N

NPN Transistor Bias

C

B

E

Current flowseverywhere.

Most of the emitter carriersdiffuse through the thin base

region since they are attractedby the collector.

Note that IB is smallerthan IE or IC.

IC

IB

IE

N

P

N C

B

E

Although IB is smallerit controls IE and IC.

IC

IB

IE

Note: when the switch opens, all

currents go to zero.

Gain is something smallcontrolling something large

(IB is small).

Transistor structure and bias quiz

The heaviest doping is found in the___________ region. emitter

The thinnest of all three regions is calledthe ____________. base

The collector-base junction is ___________biased. reverse

The base-emitter junction is ____________biased. forward

The majority of the emitter carriers flow tothe ___________. collector

N

P

C

B

E

IC = 99 mA

IB = 1 mA

IE = 100 mA

= IC

IB

The current gain frombase to collector

is called

99 mA

1 mA= 99

N

P

C

B

E

IC = 99 mA

IB = 1 mA

IE = 100 mA

IE = IB + IC

99 mA= 1 mA +

= 100 mA

Kirchhoff’scurrent law:

C

B

E

IC = 99 mA

IB = 1 mA

IE = 100 mA

In a PNP transistor,holes flow from

emitter to collector.

Notice the PNPbias voltages.

Transistor currents quiz

is the ratio of collector current to ______current. base

The sum of the base and collector currentsis the __________ current. emitter

In NPN transistors, the flow from emitter tocollector is composed of _______. electrons

In PNP transistors, the flow from emitter tocollector is composed of _______. holes

Both NPN and PNP transistors show__________ gain. current

Emitter

NPN schematic symbol

Base

Collector

Memory aid: NPNmeans Not Pointing iN.

Collector

Base

Emitter

PNP schematic symbol

IB

IC

VCE

B

C

E

This circuit is used tocollect IC versus

VCE data forseveral values of IB.

0 2 4 6 8 10 12 14 16 18

2468

101214

VCE in Volts

IC in mA

When graphed, the data provide anNPN collector family of curves.

20 A

0 A

100 A

80 A

60 A

40 A

0 2 4 6 8 10 12 14 16 18

2468

101214

VCE in Volts

IC in mA

20 A

0 A

100 A

80 A

60 A

40 A

= IC

IB

= 15040 A

6 mA

100 A

14 mA= 140 This type of gain

is called dc or hFE.

0 2 4 6 8 10 12 14 16 18

2468

101214

VCE in Volts

IC in mA

20 A

0 A

100 A

80 A

60 A

40 A

ac = Another type of gainis called ac or hfe.

IC

IB

= 12520 A

2.5 mA

0 2 4 6 8 10 12 14 16 18

2468

101214

VCE in Volts

IC in mA

20 A

0 A

100 A

80 A

60 A

40 A

IBWith these values of IB:

The C-E model is a resistor.

C

E

0 2 4 6 8 10 12 14 16 18

2468

101214

VCE in Volts

IC in mA

20 A

0 A

100 A

80 A

60 A

40 A

IB When IB >> 100 A

VCE 0

The model is a closed switch.

0 2 4 6 8 10 12 14 16 18

2468

101214

VCE in Volts

IC in mA

20 A

0 A

100 A

80 A

60 A

40 A

IB When IB = 0

IC = 0

The model is an open switch.

Transistor operating conditions quiz

When IB is large and VCE 0, the transistoracts as a ___________ switch. closed

When IB = 0 and IC = 0, the transistoracts as an ___________ switch. open

When IB > 0 and VCE > 0, the transistoracts as a ___________. resistor

Two current gain measures are dc and__________. ac

The symbol hfe is the same as _________.ac

0

E-B junction is forward biased by the ohmmeter.

V

mA

NPN

E

BC

0

The C-E resistance is very high.

V

mA

NPN

E

BC

0

The meter reading is < 100 kdue to gain.

V

mA

NPN

E

BC

100 k

Current OutCurrent In CurrentAmplifier

The BJT isa currentamplifier.

Current OutVoltage In VoltageAmplifier

The JFET isa voltage

controlledamplifier.

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.

DrainSource

Drain

Source

Gate

Gate

P

N-channel

P-type substrate

A negative gate voltagecan push the carriers from

the channel and turn the JFET off.

0VDS in Volts

ID in mA

-4 V

-5 V

0 V

-1 V

-2 V

-3 VVGS

N-channel JFET drain family of characteristic curves

This is known as a depletion-mode device.

n

Source

Gate

Drain

VDD

p

n

It’s possible to make enhancementtype field effect transistors as well.

G

S

D

VGG

Gate bias enhances the channel and turns the device on.

Metaloxide

insulator

N-channelMOSFET

0VDS in Volts

ID in mA

1 V

0 V

5 V

4 V

3 V

2 VVGS

Enhancement mode MOSFET drain family of characteristic curves

Drain

Source

Gate

Base 2

Base 1

Emitter

The unijunction transistor fires when its emitter voltage reaches VP.

VP

Emitter current

Em

itte

r vo

ltag

e

Then, the emitter voltagedrops due to its negativeresistance characteristic.

The UJT is not useful as an amplifier.It is used in timing and control applications.

Other transistor types quiz

BJTs are __________ -controlledamplifiers. current

FETs are __________ -controlledamplifiers. voltage

JFETs operate in the _________ mode.depletion

MOSFETs operate in the __________mode. enhancement

UJTs are not useful as __________.amplifiers

REVIEW

• Amplification• Transistors• Characteristic Curves• Transistor Testing• Other Transistor Types