Semiconductors

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Chapter 16

Electronics FundamentalsCircuits, Devices and App lications - Floyd

Copyright 2007 Prentice-Hall


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Chapter 16



Summary

Semiconductors

Semiconductors are crystalline materials that are

characterized by specific energy bands for electrons.

The last energy band is the

conduction band, where electrons

are mobile.

Between the bands are gaps;

these gaps represent energiesthat electrons cannot posses.

Nucleus

First band

Second band

Valence band

Conduction band

Energy gap

Energy gap

Energy gap

Energy

The next to the last band is the

valence band, which is the energy

level associated with electrons

involved in bonding.


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Summary

Electron and hole current

At room temperature, some electrons have enough

energy to jump into the conduction band.

Valence band

Conduction band

Energy gap

Energy

After jumping the gap, these electrons are free to drift throughout

the material and form electron current when a voltage is applied.

Heatenergy

Electron-

hole pairFor every electron

in the conduction

band, a hole is leftbehind in the

valence band.


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Summary

Electron and hole current

The electrons in the conduction band and the holes in

the valence band are the charge carriers. In other words,

current in the conduction band is by electrons; current

in the valence band is by holes.

When an electron jumps to the conduction band, valence

electrons move from hole-to-hole in the valence band,

effectively creating hole current shown by gray arrows.

Si Si Si

Free

electron


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Summary

Impurities

By adding certain impurities to pure (intrinsic) silicon,

more holes or more electrons can be produced within

the crystal.

To increase the number of holes, trivalent

impurities are added, forming ap-type

semiconductor. These are elements to the

left of Si on the Periodic Table.

To increase the number of conduction

band electrons, pentavalent impurities

are added, forming an n-type

semiconductor. These are elements to

the right of Si on the Periodic Table. Si

B

Al

Ga

P

As

Sb

Ge

C

Sn

N

III IV V

In


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Summary

Thepnjunction diode

When apnjunction is formed, electrons in the n-material

diffuse across the junction and recombine with holes in

thep-material. This action continues until the voltage of

the barrier repels further diffusion. Further diffusion

across the barrier requires the application of a voltage.

Thepnjunction is basically a diode,

which is a device that allows currentin only one direction. A few typical

diodes are shown.


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Summary

Forward bias

When apnjunction is forward-biased, current is permitted.

The bias voltage pushes conduction-band electrons in the

n-region and holes in thep-region toward the junction

where they combine.

The barrier potential in the depletion

region must be overcome in order

for the external source to cause

current. For a silicon diode, this isabout 0.7 V.

p-region n-region

p n

+

R

VBIAS

The forward-bias causes the depletion region to be narrow.


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Summary

Reverse bias

When apnjunction is reverse-biased, the bias voltage

moves conduction-band electrons and holes away from the

junction, so current is prevented.

The diode effectively acts as an

insulator. A relatively few electrons

manage to diffuse across the

junction, creating only a tiny reversecurrent.

p-region n-region

p n

+VBIAS

R

The reverse-bias causes the depletion region to widen.


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Summary

Diode characteristics

The forward and reverse characteristics are shown on

a V-Icharacteristic curve.

In the forward bias region, current

increases dramatically after the

barrier potential (0.7 V for Si) is

reached. The voltage across the

diode remains approximately

equal to the barrier potential.

VR VF

IF

IR

Reversebias

Forward

bias

0.7 V

Barrierpotential

The reverse-biased diode

effectively acts as an insulator

until breakdown is reached.

VBR(breakdown)


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Summary

Diode models

The characteristic curve for a diode can be approximated

by various models of diode behavior. The model you will

use depends on your requirements.

The ideal model assumes the diode is

either an open or closed switch.

VR VF

IF

IR

Reversebias

Forward

bias

The complete model includes the

forward resistance of the diode.

The practical model includes the

barrier voltage in the approximation.0.7 V


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Summary

Half-wave Rectifier

Rectifiers are circuits that convert ac to dc. Special

diodes, called rectifier diodes, are designed to handle the

higher current requirements in these circuits.

The half-wave rectifier

converts ac to pulsating

dc by acting as a closed

switch during the

positive alteration.

The diode acts as an

open switch during the

negative alteration.

D

D

RL

RL

+ -

- +


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Summary

Full-wave Rectifier

The full-wave rectifier allows unidirectional current on

both alterations of the input. The center-tapped full-wave

rectifier uses two diodes and a center-tapped transformer.

F D1

D2RL

Vsec

2

Vsec2

The ac on each side of the center-tap is of the total secondary

voltage. Only one diode will be biased on at a time.


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Summary

Bridge Rectifier

The bridge rectifier is a type of full-wave circuit that uses

four diodes. The bridge rectifier does not require a

center-tapped transformer.

F

D1

D2RL

At any instant, two of the diodes are conducting and two are off.

D3

D4


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Summary

Peak inverse voltage

Diodes must be able to withstand a reverse voltage when

they are reverse biased. This is called the peak inverse

voltage (PIV). The PIV depends on the type of rectifier

circuit and the maximum secondary voltage.

For example, in a full-wave circuit, if one diode is conducting

(assuming 0 V drop), the other diode has the secondary voltage

across it as you can see from applying KVL around the green path.

0 V

Vsec

Notice that Vp(sec)= 2Vp(out)forthe full-wave circuit because

the output is referenced to the

center tap.


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Summary

Peak inverse voltage

For the bridge rectifier, KVL can be applied to a loop that

includes two of the diodes. Assume the top diode is

conducting (ideally, 0 V) and the lower diode is off. The

secondary voltage will appear across the non-conductingdiode in the loop.

0 V

Notice that Vp(sec)= Vp(out)for the bridge because the output is

across the entire secondary.

Vsec


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Summary

Power supplies

By adding a filter and regulator to the basic rectifier, a

basic power supply is formed.

7805

FD1

D2C1

D3

D4 C2

Typically, a large electrolytic capacitor is used as a filter before the

regulator, with a smaller one following the regulator to complete

filtering action.

1000 mF 1 mF

IC regulator


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Summary

Special-purpose diodes

Special purpose diodes include

Zener diodesused for establishing a reference voltage

Varactor diodesused as variable capacitors

Light-emitting diodesused in displays

Photodiodesused as light sensors


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Summary

Troubleshooting power supplies

Begin troubleshooting by analyzing the symptoms and how

it failed. Try to focus on the most likely causes of failure.

7805

FD1

D2C1

D3

D4 C21000 mF 1 mF

IC regulator

A power supply has no output, but was working until a

newly manufactured PC board was connected to it. (a) Analyze

possible failures. (b) Form a plan for troubleshooting.


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Summary


7805

FD1

D2C1

D3

D4 C21000 mF 1 mF

IC regulator

The supply had been working, so the problem is not

likely to be an incorrect part or wiring problem. The failure was

linked to the fact that a new PC board was connected to it,

which points to a possible overloading problem. If the load wastoo much for the supply, it is likely a fuse would have blown, or

a part would likely have overheated, accounting for the lack of

output.


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Summary


1.Disconnect power and check the fuse. If it is bad, replace it.

Before reapplying power, remove the load, open the power supply

case, and look for evidence of overheating (such as discolored

parts or boards). If no evidence of overheating proceed.

2.Check the new pc board (the load) for a short or overloading of

the power supply that would cause the fuse to blow. Look for

evidence of overheating.

3.Verify operation of the supply with measurements (see next

slide).

Based on the analysis, a sample plan is as

follows. (It can be modified as circumstances warrant.)


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Summary


Reapply power to the supply but with no load. If the output is

okay, put a resistive test load on the power supply and measure

the output to verify it is operational. If the output is correct, theproblem is probably with the new pc board. If not, you will need

to further refine the analysis and plan, looking for an internal

problem.

The analysis showed that a

likely cause of failure was due to an overload. For the

measurement step, it may be as simple as replacing the

fuse and confirming that the supply works. After

replacing the fuse:


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Majori ty carr ier

Minority carrier

PN junction

Diode

The most numerous charge carrier in a dopedsemiconductor material (either free electrons

or holes.

Selected Key Terms

The boundary between n-type andp-type

semiconductive materials.

An electronic device that permits current in

only one direction.

The least numerous charge carrier in a dopedsemiconductor material (either free electrons

or holes.


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Barrier

potential

Forward bias

Reverse bias

Full-wave

rectifier

A circuit that converts an alternating sine-

wave into a pulsating dc consisting of both

halves of a sine wave for each input cycle.

The condition in which a diode conducts

current.

The inherent voltage across the depletionregion of apnjunction diode.

Selected Key Terms

The condition in which a diode prevents

current.


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Bridge recti f ier

Zener diode

Varactor

Photodiode A diode whose reverse resistance changeswith incident light.

A type of diode that operates in reverse

breakdown (called zener breakdown) toprovide a voltage reference.

A type of full-wave rectifier consisting ofdiodes arranged in a four corner configuration.

Selected Key Terms

A diode used as a voltage-variable capacitor.


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Quiz

1. An energy level in a semiconductor crystal in which

electrons are mobile is called the

a. barrier potential.

b. energy band.

c. conduction band.

d. valence band.


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Quiz

2. A intrinsic silicon crystal is

a. a poor conductor of electricity.

b. an n-type of material.

c. ap-type of material.

d. an excellent conductor of electricity.


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Quiz

3. A small portion of the Periodic Table is shown. The

elements highlighted in yellow are

a. majority carriers.

b. minority carriers.

c. trivalent elements.

d. pentavalent elements.

Si

B

Al

Ga

P

As

Sb

Ge

C

Sn

N

III IV V

In


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Quiz

4. At room temperature, free electrons in ap-material

a. are the majority carrier.

b. are the minority carrier.

c. are in the valence band.

d. do not exist.


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Quiz

5. The breakdown voltage for a silicon diode is reached

when

a. the forward bias is 0.7 V.

b. the forward current is greater than 1 A.

c. the reverse bias is 0.7 V.

d. none of the above.


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Quiz

6. The circuit shown is a

a. half-wave rectifier.

b. full-wave rectifier.

c. bridge rectifier.

d. zener regulator.


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Quiz

7. PIV stands for

a. Positive Ion Value.

b. Programmable Input Varactor.

c. Peak Inverse Voltage.

d. Primary Input Voltage.


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Quiz

8. A type of diode used a a voltage-variable capacitor is a

a. varactor.

b. zener.

c. rectifier.

d. LED.


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Quiz

9. If one of the four diodes in a bridge rectifier is open, the

output will

a. be zero.

b. have as many pulses as normal.

c. have as many pulses as normal.

d. be unaffected.


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Quiz

10. When troubleshooting a power supply that has a

bridge rectifier, begin by

a. replacing the bridge rectifier.

b. replacing the transformer.

c. making measurements.

d. analyzing the symptoms and how it failed.


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Chapter 16

Electronics Fundamentals Copyright 2007 Prentice Hall

Quiz

Answers:

1. c

2. a

3. c

4. b

5. d

6. b

7. c

8. a

9. b

10. d

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