Topic 1 Diode

7/27/2019 Topic 1 Diode

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Malaysian Institute of Aviation Technology

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ELECTRONIC FUNDAMENTAL

(MECHANICAL)AGD 20102

Course Outline
http://../Notes%20By%20week/MQA%20AGD%2020102%20Electronics.dochttp://../Notes%20By%20week/MQA%20AGD%2020102%20Electronics.dochttp://../Notes%20By%20week/MQA%20AGD%2020102%20Electronics.doc


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Chapter 1 DiodesObjectives

After studying the material in this chapter, you should be able to:

1. Describe basic atomic structure of semiconductor material and doping process.

1. Identify the terminals of a pn-junction diode, given the schematic symbol for the

component.

1. Analyze the schematic diagram of a simple diode circuit to determine:

Whether the diode is conducting The direction of current through any conducting diode.

1. List the main parameters of the pn-junction diode, and explain how each limitsthe use of the component.


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Conductor, Insulator & Semiconductor

Conductor- material that allows electrons to flow,

e.g.: gold, silver, copper and aluminum

Insulator- material that restricts current flow,e.g. : wood, plastic

Semiconductor- material that is betweenconductorand insulator in its ability to conduct electrical current,e.g.:. germanium or silicon


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Introduction to Semiconductor

Material

The valence shell (outermost shell) determines the electrical and chemicalproperties of an atom:

Conductors : Three or less electrons in the valence shell

Semiconductor : Four electrons in the valence shell

Insulator : Five or more electrons in the valence shell


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Energy diagrams for the threetypes of solids


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Energy gap The difference between the

energy levels of any two orbital shells.

Conduction band The band outside the

valence shell.


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The atomics structure of silicon and germanium:


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Covalent Bonding

A means of holding atoms together by

sharing valence electrons.

The center atom (at right) is

electrically stable because its

covalent bond is complete.

Anintrinsic (pure) silicon is

one that has no impurities .


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Doping

Doping is the process ofadding impurity

elements to intrinsic (pure) semiconductors toincrease and control conductivity within the

material.

By adding impurities, n-type and p-typeextrinsic semiconductive material can be

produced


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Trivalent element element that has three valenceelectrons

p-type material A semiconductor that has addedtrivalent impurities.

Pentavalent element One that has five valenceelectrons

n-type material A semiconductor that has addedpentavalent impurities.

Elements In Doping Process


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N-type Material

A semiconductor that has addedpentavalent impurities, eg:arsenic,phosphorus

The pentavalent atom (As) has afifth valence electron that is not apart of the covalent bond.

Relatively little energy is required to force

the excess electron into the conduction band.

Electrons are called majority carrierand holes are minority

carrier.


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P-type Material

A semiconductor that hasadded trivalent impurities, eg:aluminium,indium,

The bond requires one more valence

electron than the trivalent atom (Al)

is capable of providing.

The electron shortage results in the bond

having a valence-band hole.

Converse of n-type material, holes are called majority carriers

and electrons are minority carriers.


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The PN Junction

At the instant of the pn junction formation, free

electrons near the junction in the n-regionbegin to

diffuse across the junction into p-region where they

combine with holes near the junction.


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For every electron that diffuses across the junction and

combines with hole, a positive charge is left in the n region

and negative charge is created in thep region, forming abarrier potential.

This action continues until the voltage of the barrier repels

further diffusion.


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DIODE STRUCTURE

Semiconductor diode is formed by joining the

p-type and n-type material with conductivecontacts and wire leads connected to each

region.


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Diode

A two-electrode (two-terminal) device

that acts as a one-way conductor.

The n region is called the cathode andp region is called anode.

Symbol for diode


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Examples of diode


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BiasA potential applied to a pn junction to

obtain a desired mode of operation.

Forward bias A potential used to reduce the

resistance of a pn junction.

Reverse bias A potential used to increase theresistance of a pnjunction.


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Forward Voltage

Forward voltage (VF) is the voltage across a

forward biasedpnjunction.

VF0.7 V (for silicon)

VF0.3 V (for germanium)


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Forward Biased pn- junction Diodes The component is fwdbiasedso that the cathode is more

negative than the anode.

The voltage across the diode exceeds its barrier potential.

The diode conducts fully when VF is approximately 0.7V(for silicon) or0.3 V (for germanium).

The value of IF depends on the circuit voltage andresistance values.


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Reverse Biased pn-Junction Diodes The component is reverse biasedso that the cathode

is more positive than the anode.

The voltage across the diode is approximately equalto the applied voltage (V).

The diode current is approximately 0 A (as indicated

by the ammeter).


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Diode Characteristics Curve


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VF and Circuit Analysis

m

R

VI

VV

S

T

SR

3.4V0.7

V0.7


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For the diode below determine VD, VR and ID

7.V0.7SRVV

R

VS

T3.3

V0.7

7.V0.7SRVV

VD = 0.7V


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Repeat Example with the diode reverse. Thus the equivalent

circuit is

Find the value of VD

VD = E = 8 V


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If the diode is biased with the voltage source less thanVD, the diode also acting like open circuit

Diode circuit Equivalent circuit


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Determine Vo, I1, ID1and ID2 for the parallel diode below

Since the source voltage is greater than the diode then the current flow andthe voltage across diode is 0.7 V, thus Vo is 0.7 V

The current is

k

VV

R

VE

R

VDR .2

33.0

)7.010(1


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Continue.

Since diodes are similarthus the current will

be the same , then

IIDD

0.12

1

21


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Parameters of thePN-Junction Diode
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Diode Characteristics Curve


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Peak Reverse Voltage (VRRM )

VRRM is the maximum reverse

voltage that wont force a pn

junction to conduct.

When VR > VRRM , diode reverse

current (IR) increases rapidly as

the depletion layer breaks down.

VRRM is a diode parameter that

can be found on the component

spec sheet.


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Forward Current and PowerDissipation

Average Forward Current (I0 ) Themaximum

allowable value of dc forward current for a diode. Forward Power Dissipation ( PD(max) ) The

maximum possible power dissipation of the forward-biased diode.

I0

and PD(max)

are diode parameters that are listed onthe component spec sheet.

F

D

V

PI

(ma0


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Component Substitution

A substitute diode can be used in a circuit if:

Its VRRM rating is at least 20% greater than the

maximum reverse voltage produced by the circuit.

Its I0 rating is at least 20% greater than the

average (dc) value of IF generated by the circuit.

Its PD(max) rating is at least 20% greater than thevalue of PF required by the circuit.


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Diode Reverse Current (IR )The low-level current through a reverse biasedpnjunction, made up of:

Reverse saturation current (IS) A current caused

by thermal activity in a reverse biased diode.

Surface leakage current (ISL) A current along the

surface of a reverse-biased diode.


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Temperature Effects on IF and VF When temperature increases:

IF increases (at a specified value of VF)

VF decreases (at a specified value of IF)


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Temperature Effects on IR IR increases with increases in temperature.


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Diode Resistance Test


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Meter Diode Test Function


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Summary

Difference between conductor, insulator &

semiconductor? What is doping process?

Majority carrier for P & N material?

Identify forward & reverse bias? What is the value of diode resistance

when fwd bias?


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Diode ApplicationsObjectives

After studying the material in this chapter, you should be

able to:1. Identify different types of rectifier

2. Calculate the the peak and dc (average) load voltageand current values for half and full wave rectifier.

3. Describe of the operation of full wave rectifier.

4. Discuss the effects that filtering has on the output ofa rectifier.

5. Describe basic operation of rectifier of three phase


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Half-wave Rectifiers


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Half-wave Rectifiers Half-wave rectifier A diode placed in series

between a transformer (or ac line input) and its

load.


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Positive Half-wave Rectifiers

This circuit converts an

ac input to a series ofpositive pulses.


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Average Load Voltage and Current

Average voltage (Vave) The dc equivalent of a voltage

waveform.

Average current (Iave) The dc equivalent of a current

waveform.

For the output from a half-wave rectifier:

pkVVave

pI

ave io ff

1 32


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Negative Half-wave Rectifiers

This circuit converts anac input to a series ofnegative pulses.


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Peak Inverse Voltage (PIV)Peak inverse voltage (PIV) The maximum diode

reverse bias produced by a given circuit.

For the diode in a half-wave rectifier:

(pkPIV SV


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Half-wave Rectifier With CapacitiveFilter

)pk(2PIV SV

Initial Charging of the capacitor The capacitor discharging


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Half-wave rectifier with and without filtering


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Full-wave Rectifier


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Full-wave RectifierA full-wave rectifier allows

unidirectional current

to the load during the entireinput cycle


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Full-wave Rectifier OperationDiodes conduct during

alternate half cycles of

the input signal.

VL(pk) is approximately

half the value of VS(pk).

The circuit produces

two positive half-cycles

for each input cycle.


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Average Load Voltage and Current Average voltage (Vave) The dc equivalent of a voltage

waveform.

Average current (Iave) The dc equivalent of a currentwaveform.

For the output from a full-wave rectifier:

pk2VVave

pk2IIave io ff 2

1 2 3


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Peak Inverse Voltage (PIV) Peak inverse voltage (PIV) The maximum diode

reverse bias produced by a given circuit.

For the diode in a full-wave rectifier:

.0PIV(pk)SV


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Full-Wave Bridge Rectifiers The most commonly used because:

It does not require the use of a center-tapped

transformer.

It can be coupled directly to the ac power line.

It produces a higher dc output than a full-wavecenter-tapped rectifier (nearly twice)


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Bridge Rectifier Operation Conduction alternates between two diode pairs.


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Calculating load voltage and currentrelationships

V7.0PIV

2

2

V4.1

S(pk)

ave

ave

)(pk

ave

(pk))(pk

V

ff

R

V

I

VV

VV

io

L

L

SL


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Three Phase Half Wave Rectifier

Output ripple frequency = 3 times the supply frequency


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Three Phase Full Wave Rectifier

Output ripple frequency = 6 times the supply frequency

w

x

y


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Other Applications of Diode

Clipping or limiting used to clip off portions of signal voltages (above or below certain

levels).

Clamping adds a DC voltage to an ac voltage. Also known as DC restorer

Change the reference level of waveform without reducing its amplitude(move up or down)

Voltage Multiplier use clamping action to increase peak rectified voltages without

increasing input transformers rating.

typically used in a High Energy Ignition Unit (HEIU)


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Summary Half-wave Rectifiers

Full-wave Rectifier Bridge Rectifiers

Three Phase Rectifiers


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Thyristors, Optoelectronic &Special Devices


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Objectives

After studying the material in this chapter, you should be able to explain of the

following devices:

1. SCR2. Triac

3. Diac

4. LED

5. Photodiode

6. Zener Diode

7. Schottky Diode8. Varactor Diode

9. Varistor Diode


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Thyristors A semiconductor that used forswitching

purposes

Replaced solenoid or relay for controlling the

load current to motors.

Advantage: no moving parts, so prevent from

wear, corrosion & arching. Two common type : SCR (silicon controlled

rectifier) and TRIAC(triode AC semiconductor)


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Silicon-Controlled Rectifiers (SCRs) Silicon-controlled rectifier (SCR) A three-terminal

device.

The third terminal, called the gate, provides an additional

method fortriggering the device.

Allow current to flow once the certain level of gate signal is

achieved.


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SCR Triggering

A positive gate pulse

triggers the SCR into

conduction. Once conduction

begins, the gate input

signal has no effect

on the device.

The SCR is driven intocutoffby anode

current interruption or

forced commutation.


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SCR Operating Curve

Holding

current


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Graphs of SCR Inputs & Outputs
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SCR can be turn off by:

The device is reverse biased

The supply removed

The voltage across the device is reduced

so that the current falls below its holding

value

l i i f i i h l


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Triacs Triac A bidirectional thyristor whose forward and

reversecharacteristics are identical to the forward

characteristics of the SCR. Also referred to as triodes and bidirectional triode thyristors.

M l i I tit t f A i ti T h l


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Triac Construction The primary conducting terminals are referred to as main

terminal 1 (MT1) and main terminal 2 (MT2).

The triac is essentially complementary SCRs connected inparallel.


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M l i I tit t f A i ti T h l


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Triac Triggering

Mala sian Instit te of A iation Technolog


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Controlling Triac Triggering



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Diacs Basically a TRIAC without a gate terminal

Designed to conduct at specific voltage , and

normally used as a triggerfor another device such as

Triac.

Compensated ofnonsymmetrical triggercharacteristic

of Triac.



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Diac Operating Curves



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Thyristor Applications

Ideal for switching warning circuits on aircraft.

Example: Engine warning light of excessive

temperature in turbine engine will illuminate untilpilot interrupts the light circuit.

Controlling large amount of current flow tomotor, heater or lighting circuit.



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Optoelectronic



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Light Emitters and Detectors

Light emitter

Optoelectronic devices that produce light.

The LED is an example of a light emitter.

Light detector

Optoelectronic devices that respond to light.

Example : photodiodes



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Light Light Electromagnetic energy that falls within a specific

range of frequencies.



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Wavelength Wavelength () The physical length of one cycle of a

transmitted electromagnetic wave.

f

c



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Light Intensity Light intensity The amount of light per unit area received

by a given photodetector. Also called irradiance.

Light intensity decreases as the distance between the light

emitter and detector increases



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Light Emitting Diode (LED) Commonly used forindicator lamps and readout

display in a wide variety of equipment.

Most common application : seven segmentdisplay.

Seven segment display component



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Light Emitting Diode (LED)

Made of semiconductor compound gallium

arsenide phosphide. Emit light when fwd biased and light emitteddirect proportional to current flow.

Colorobtained depending on the composition

and impurity content of the compound



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Photodiodes Photodiode A diode

whose reverse

conduction is light-

intensity controlled.

Light current (IL) The

reverse current with an

active light input present.

Dark current (ID) Thereverse current with no

active light input present.



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Photodiode Ratings Wavelength of peak spectral response (S) The

wavelength that causes the strongest response in a

photodetector.

Sensitivity The response of a photodetector to a

specified light intensity, measured in mA/mW/cm2.

Spectral responseA measure of a photodetectors

response to a chance in input wavelength.



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Photodiode Application

Detect changes in light intensity

Example : As ambient light detectorin thecockpit to automatically adjust the

brilliance ofinstrument readouts for

changing light conditions.



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Special Devices



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Zener Diode Will conduct electricity under certain voltage

conditions.

Ideal for use in voltage regulator circuit. Designed to operate at or above its breakdown

voltage.

CathodeAnode

Symbol of zener diode



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Schottky Diode

Semiconductor diode with a low forward voltagedrop and a very fast switching action.

Capable of operating at frequency up to 20Ghz.

Lower power loss across the device in forwardbiased.

Most important application in digital computer.



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Varactor A varactor diode is a variable-capacitance diode.

The capacitance of the junction changes with theamount of reverse voltage.

As reverse bias voltage increases, depletionregion widens, capacitance become smaller.

Generally used in electronic tuning circuits andcommunication systems.

Symbol of Varactor



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Varistor Metal Oxide Varistor(MOV) is a voltage dependent

resistor.

Have an electrical behavior similar to back to backzener diodes

High voltage resistor breakdown and becomeconductor (constant current diode)

Used for transient voltage suppression, voltagestabilization and switch contact protection



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Summary Thyristors

Optoelectronic

Special Devices