EMT111EMT111CHAPTER 1CHAPTER 1

Introduction to Introduction to SemiconductorSemiconductor

ByByPn. ‘Aini Syuhada Md ZainPn. ‘Aini Syuhada Md Zain

Introduction to Semiconductor -Introduction to Semiconductor -Chapter Outline :Chapter Outline :

1.81.8 Voltage Current Characteristic of a Voltage Current Characteristic of a

DiodeDiode 1.91.9 Diode ModelsDiode Models 1.101.10 Testing a DiodeTesting a Diode

-When a forward bias voltage is applied – current called forward current,

-In this case with the voltage applied is less than the barrier potential so the diode for all practical purposes is still in a non-conducting state. Current is very small.

-Increase forward bias voltage – current also increase

FI

FIGURE 1-26FIGURE 1-26 Forward-bias measurements Forward-bias measurements show general changes in Vshow general changes in VFF and I and IFF as V as VBIAS BIAS is is

increased.increased.

1.81.8 Voltage-Current Characteristic of Voltage-Current Characteristic of a Diodea Diode ( V-I Characteristic for forward bias)( V-I Characteristic for forward bias)

-With the applied voltage exceeding the barrier potential (0.7V), forward current begins increasing rapidly.

-But the voltage across the diode increase only above 0.7 V.

FIGURE 1-26FIGURE 1-26 Forward-bias measurements Forward-bias measurements show general changes in Vshow general changes in VFF and I and IFF as V as VBIAS BIAS is is

increased.increased.

1.81.8 Voltage-Current Characteristic of Voltage-Current Characteristic of a Diodea Diode ( V-I Characteristic for forward bias)( V-I Characteristic for forward bias)

-Plot the result of measurement in Figure 1-26, you get the V-I characteristic curve for a forward bias diode

- Increase to the right

- increase upward

FFd IVr /'

dynamic resistance dynamic resistance r’r’dd decreases as you move up the decreases as you move up the

curvecurve

FV

FI

VVF 7.0

zerobias

VVF 7.0

1.81.8 Voltage-Current Characteristic of Voltage-Current Characteristic of a Diodea Diode ( V-I Characteristic for forward bias)( V-I Characteristic for forward bias)

Reverse Current

Breakdown voltage-not a normal operation of pn junction devices- the value can be vary for typical Si

1.81.8 Voltage-Current Characteristic of Voltage-Current Characteristic of a Diodea Diode ( V-I Characteristic for Reverse bias)( V-I Characteristic for Reverse bias)

Combine-Forward bias& Reverse bias CompleteV-I characteristic curve

1.81.8 Voltage-Current Characteristic of Voltage-Current Characteristic of a Diodea Diode ( Complete V-I Characteristic curve)( Complete V-I Characteristic curve)

• Forward biased dioed : for a given value of

• For a given

• Barrier potential decrease as T increase

• Reverse current breakdown – small & can be neglected

FIT ,

FV

FF VI ,

1.81.8 Voltage-Current Characteristic of Voltage-Current Characteristic of a Diodea Diode ( Temperature effect on the diode V-I ( Temperature effect on the diode V-I Characteristic)Characteristic)

Directional of current

cathodeanod

1.91.9 Diode Models Diode Models ( Diode structure and symbol)( Diode structure and symbol)

DIODE MODEL

The Ideal Diode Model

The Complete Diode Model

The Practical Diode Model

1.91.9 Diode Models Diode Models

•Assume •Forward current, by Ohm’s law

Ideal model of diode- simple switch:

•Closed (on) switch -> FB

•Open (off) switch -> RB

VVF 0

LIMIT

BIASF R

VI

BIASR

R

VV

AI

0

(1-2)

1.91.9 Diode Models Diode Models ( The ideal Diode model)( The ideal Diode model)

•Adds the barrier potential to the ideal switch model

• ‘ is neglected

•From figure (c):

The forward current [by applying Kirchhoff’s voltage low to figure (a)]

Ohm’s 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 pn junction

FV

•Same as ideal diode model

)(3.0

)(7.0

GeVV

SiVV

F

F

0LIMITRFBIAS VVV

LIMITFR RIVLIMIT

LIMIT

FBIASF R

VVI

BIASR

R

VV

AI

0

(1-3)

1.91.9 Diode Models Diode Models ( The Practical Diode ( The Practical Diode model)model)

Complete model of diode consists:

•Barrier potential

•Dynamic resistance,

•Internal reverse resistance,

•The forward voltage:

•The forward current:

dr '

Rr '•acts as closed switch in series with barrier potential and small

dr 'Rr '

•acts as open switch in parallel with the large

'7.0 dFF rIVV

'

7.0

dLIMIT

BIASF rR

VVI

(1-4)

(1-5)

1.91.9 Diode Models Diode Models ( The Complete Diode ( The Complete Diode model)model)

10V10V

1.0kΩ1.0kΩ

5V5V

1.0kΩ1.0kΩ

(1) Determine the forward voltage and forward current [forward bias] for each of the diode model also find the voltage across the limiting resistor in each cases. Assumed rd’ = 10 at the determined value of forward current.

1.91.9 Diode Models Diode Models ( Example)( Example)

a)a) Ideal ModelIdeal Model::

b)b) Practical ModelPractical Model::

(c) (c) Complete model:Complete model:

VARIV

mAV

R

VI

V

LIMITFR

BIASF

F

LIMIT10)101)(1010(

101000

10

0

33

VARIV

mAVV

R

VVI

VV

LIMITFR

LIMIT

FBIASF

F

LIMIT3.9)101)(103.9(

3.91000

7.010)(

7.0

33

VkmARIV

mVmAVrIVV

mAk

VV

rR

VVI

LIMITFR

dFF

dLIMIT

BIASF

LIMIT21.9)1)(21.9(

792)10)(21.9(7.07.0

21.9101

7.0107.0

'

'

1.91.9 Diode Models Diode Models ( Example)( Example)

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

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

1.91.9 Diode Models Diode Models ( Typical Diodes)( Typical Diodes)

Testing a diode is quite simple, particularly if the multimeter used has a diode check function. With the diode check function a specific known voltage is applied from the meter across the diode.

K A A K

With the diode check function a good diode will show approximately .7 V or .3 V when forward biased.

When checking in reverse bias the full applied testing voltage will be seen on the display.

1.101.10 Testing A Diodes Testing A Diodes ( By Digital ( By Digital multimeter)multimeter)

Defective Diode

1.101.10 Testing A Diodes Testing A Diodes ( By Digital ( By Digital multimeter)multimeter)

Select OHMs range

Good diode:

Forward-bias: get low resistance reading (10 to 100 ohm)

Reverse-bias: get high reading (0 or infinity)

1.101.10 Testing A Diodes Testing A Diodes ( By Analog multimeter ( By Analog multimeter – ohm – ohm function ) function )

P-materials are doped with trivalent impurities

N-materials are doped with pentavalent impurities P and N type materials are joined together to form a PN junction.

A diode is nothing more than a PN junction.

At the junction a depletion region is formed. This creates barrier which requires approximately .3 V for a Germanium and .7 V for Silicon for conduction to take place.

Diodes, transistors, and integrated circuits are all made of semiconductor material.

SummarySummary

When reversed biased a diode can only withstand so much applied voltage. The voltage at which avalanche current occurs is called reverse breakdown voltage.

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

A diode conducts when forward biased and does not conduct when reverse biased

SummarySummary