ElectronicsElectronics

Principles & ApplicationsPrinciples & ApplicationsFifth EditionFifth Edition

Chapter 3Junction Diodes

©1999 Glencoe/McGraw-Hill

Charles A. Schuler

• The PN Junction• Characteristic Curves of Diodes• Diode Lead Identification• Diode Types and Applications

INTRODUCTION

P

The P-side of a junction diode is doped with acceptor atoms.

N

The N-side of a junction diode is doped with donor atoms.

Schematic Symbol

CathodeAnode

junction

A silicon crystal

Zero Bias

Depletion region

The electrons near the junction cross over and fill the holes near the junction.

Having no carriers, the depletion region is an insulator.

Forward Bias

The carriers move toward the junctionand collapse the depletion region.

The diode is on.

Reverse Bias

The carriers move away from the junction.

The depletion region is reestablishedand the diode is off.

mA

0 0.5 1.0 1.50

25

50

75

100

125

150

175

200

Volts

10 ohms

5 ohms

20 ohms

Resistor volt-ampere characteristic curves

For

war

d c

urr

ent

in m

A

0 0.5 1.0 1.50

25

50

75

100

125

150

175

200

Forward bias in volts

Silicon diode volt-ampere characteristic curve

knee

Linearity

• The volt-ampere characteristic curve for a resistor is a straight line (linear).

• A diode has a non-linear characteristic curve.

• The barrier potential produces a knee in the diode curve.

• The knee voltage is around 0.6 to 0.7 volts for silicon diodes.

For

war

d c

urr

ent

in m

A

0 0.5 1.0 1.50

25

50

75

100

125

150

175

200

Forward bias in volts

The effect of temperature on the diode curve

-50 oC

25 oC

100 oC

0200400600

20

40

60

80

100

120

140

Reverse bias in Volts

Reversecurrentin mA

Silicon diode reverse bias characteristic curve

breakdown

Cathode lead

Anode lead

Anode lead

0

V

mA

Cathode lead

The diode is forward biased by the ohmmeter.

Cathode lead

Anode lead

0

The diode is reverse biased by the ohmmeter.

V

mA

Silicon diode ohmmeter testing

• Low resistance in both directions: the diode is shorted.

• High resistance in both directions: the diode is open.

• Relatively low resistance in the reverse direction: the diode is leaky.

• The ratio of reverse resistance to forward resistance is > 1000: the diode is good.

Diode quiz

At zero bias, diodes show a depletion regionwhich acts as an________. insulator

The depletion region is collapsed by applying_________ bias. forward

The depletion region is made wider by applying________ bias. reverse

A forward-biased diode has its anode________ with respect to its cathode. positive

Diode forward voltage drop decreases astemperature ________. increases

0246

20

40

60

80

100

120

140

Reverse bias in Volts

Reversecurrentin mA

A zener diode is designed to break down andconduct backwards at lower voltages.

0246

20

40

60

80

100

120

140

Reverse bias in Volts

Reversecurrentin mAI

V

V

The voltage across a conductingzener is relatively constant.

Unregulated SupplyLoad

The load is in parallel with the zener and will see a relatively constant voltage as long as the zener is conducting.

Using a zener diode as a voltage regulator

VIN VOUT

When VIN < 1.2 VPP

The diodes do not conduct.

This circuit is called a clipper or limiter.

VOUT is not clipped

VIN VOUT

When VIN > 1.2 VPP

+ 0.6 V

- 0.6 V

VOUT is clipped

The diodes conduct.

VIN VOUT

When VIN > 1.2 VPP

This circuit is called a clamp or dc restorer.

0.6 VVOUT is clamped.

dc

C is charged.

C

2VOUT(dc) =

VPP - 0.6 V

The inductive kickcan cause damage.

CEMF

The coil dischargesthrough the diode and

there is no arc.

Transient suppression diodes can be used with inductive loads.

CEMF

LED

As the electrons cross the junction, they lose energy in the form of photons.

Powersupply

VS

RS

The typical voltage drop for most LEDs is from 1.5 to 2.5 V.

LED

IS =VS - VD

RS

LED circuit

VD

Powersupply

VS

RS

Photodiodes are reverse biased and conduct in the presence of light.

Photodiode

Input

Output

Input

Output

Optocoupler

Step-index multimode fiber

Input pulse Output pulse

The combined shorter and longer path lengthsact to stretch the output pulse.

Step-index multimode fiber

Input

Output

Due to pulse stretching, high speed data transmission is not possible.

Input

Output

Single mode fiber

A single path means no pulse stretching andhigh speed data transmission is possible.

L C ZERO BIAS

C is maximum and fR is minimum.

Tuning diode

L C REVERSE BIAS

C is less and fR increases.

Tuning diode

L C MAX. REVERSE BIAS

C is minimum and fR is maximum.

Tuning diode

Diode applications quiz

A circuit used to control the amplitude of asignal is the ________. clipper

A circuit used to add a dc component to asignal is the ________. clamp

A device containing an LED and a photo-diode is the ________. optocoupler

A tuning diode shows less capacitance asreverse bias ________. increases

The device that is often used to regulatevoltage is the ________ diode. zener

REVIEW

• The PN Junction• Characteristic Curves of Diodes• Diode Lead Identification• Diode Types and Applications