Chapter 1 - Additional Notes Lecture

8/13/2019 Chapter 1 - Additional Notes Lecture

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EMT111CHAPTER 1

Introduction to Semiconductor

By

Pn. Aini Syuhada Md Zain


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Introduction to Semiconductor -

Chapter Outline :

1.8 Voltage Current Characteristic of a Diode

1.9 Diode Models

1.10 Testing a Diode


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-When a forward bias voltage isappliedcurrent called forwardcurrent,

-In this case with the voltageapplied is less than the barrier

potential so the diode for allpractical purposes is still in anon-conducting state. Current isvery small.

-Increase forward bias voltage

current also increase

FI

FIGURE 1-26 Forward-bias measurements

show general changes in VFand IFas VBIAS isincreased.

1.8 Voltage-Current Characteristic of a Diode

( V-I Characteristic for forward bias)


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-With the applied voltageexceeding the barrierpotential (0.7V), forward

current begins increasingrapidly.

-But the voltage across thediode increase only above0.7 V.

FIGURE 1-26 Forward-bias measurements

show general changes in VFand IFas VBIAS is

increased.




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-Plot the result ofmeasurement in Figure 1-26, you get the V-Icharacteristic curve for aforward bias diode

- Increase to the right

- increase upward

FFd IVr /

'

dynamic resistance rddecreases as you move up thecurve

FV

FI

VVF

7.0

zero

biasVV

F 7.0




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Reverse

Current

Breakdown

voltage

-not a normaloperation of pn

junction devices

-the value can be

vary for typical Si


( V-I Characteristic for Reverse bias)


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Combine-Forward bias

& Reverse bias CompleteV-Icharacteristic curve


( Complete V-I Characteristic curve)


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Forward biased

dioed :for a given value

of

For a given

Barrier potential

decrease as T

increase

Reverse currentbreakdown

small & can be

neglected

FIT ,

FV

FF

VI ,


( Temperature effect on the diode V-I Characteristic)


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Directional of current

cathodeanod

1.9 Diode Models

( Diode structure and symbol)


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DIODE

MODEL

The Ideal

Diode Model

The Complete

Diode Model

The Practical

Diode Model

1.9 Diode Models


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Assume

Forward

current, by

Ohms law

Ideal model of diode-simple switch:

Closed (on) switch -> FB

Open (off) switch -> RBVV

F 0

LIMIT

BIAS

FR

VI

BIASR

R

VV

AI

0

(1-2)

1.9 Diode Models

( The ideal Diode model)


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Adds the barrier potentialto the ideal switch model

is neglected

From figure (c):

The forward current [byapplying Kirchhoffs voltagelow to figure (a)]

Ohms Law

dr'

Equivalent to close

switch in series with a

small equivalent voltage

source equal to the barrier

potential 0.7V

Represent by

produced across the pnjunction

FV

Same as ideal diode

model

)(3.0

)(7.0

GeVV

SiVV

F

F

0LIMIT

RFBIAS VVV

LIMITFR RIV

LIMIT

LIMIT

FBIAS

F

R

VVI

BIASR

R

VV

AI

0

(1-3)

1.9 Diode Models ( The Practical Diode model)


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Complete model of diodeconsists:

Barrier potential

Dynamic resistance,

Internal reverse resistance,The forward voltage:

The forward current:

dr'

Rr'

acts as closed switch inseries with barrier

potential and small dr'

Rr'

acts as openswitch in parallel

with the large

'7.0dFF

rIVV

'

7.0

dLIMIT

BIAS

F

rR

VVI

(1-4)

(1-5)

1.9 Diode Models ( The Complete Diode model)


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10V

1.0k

5V

1.0k

(1) Determine the forward voltage and forward current

[forward b ias] for each of the diode model also find thevoltage across the limiting resistor in each cases.

Assumed rd = 10 at the determined value of forwardcurrent.

1.9 Diode Models ( Example)


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a) Ideal Model:

b) Practical Model:

(c) Complete model:

VARIV

mAV

R

VI

V

LIMITFR

BIAS

F

F

LIMIT

10)101)(1010(

10100010

0

33

VARIV

mAVV

R

VVI

VV

LIMITFR

LIMIT

FBIAS

F

F

LIMIT

3.9)101)(103.9(

3.91000

7.010)(7.0

33

VkmARIV

mVmAVrIVV

mAk

VV

rR

VVI

LIMITFR

dFF

dLIMIT

BIAS

F

LIMIT

21.9)1)(21.9(

792)10)(21.9(7.07.0

21.9101

7.0107.0

'

'

1.9 Diode Models ( Example)


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Diodes come in a variety of sizes and shapes. The design and structure is

determined by what type of circuit they will be used in.

1.9 Diode Models ( Typical Diodes)


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Testing a diode is quite simple, particularly if the multimeter

used has a diode check function. With the diode check functiona specific known voltage is applied from the meter across thediode.

K A A K

With the diode check

function a good diode willshow approximately .7 V or.3 V when forward biased.

When checking in reversebias the full applied testingvoltage will be seen on thedisplay.

1.10 Testing A Diodes ( By Digital multimeter)


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Defective Diode

1.10 Testing A Diodes ( By Digital multimeter)


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Select OHMs range

Good diode:

Forward-bias:

get low resistance reading (10 to 100ohm)

Reverse-bias:

get high reading (0 or infinity)

1.10 Testing A Diodes ( By Analog multimeterohmfunction )


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P-materials are doped with trivalent impurities

N-materials are doped with pentavalent impurities

Pand Ntype materials are joined together to form aPNjunction.

A diode is nothing more than a PNjunction.

At the junction a depletion region is formed. Thiscreates barrier which requires approximately .3 V for a

Germanium and .7 V for Silicon for conduction to takeplace.

Diodes, transistors, and integrated circuits areall made of semiconductor material.

Summary


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When reversed biased a diode can only withstandso much applied voltage. The voltage at which

avalanche current occurs is called reverse breakdownvoltage.

There are three ways of analyzing a diode. Theseare ideal, practical, and complex. Typically we use a

practical diode model.

A diode conducts when forward biased and does notconduct when reverse biased

Summary

Chapter 1 - Additional Notes Lecture

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