Fund. of. Electronics Ch02 Diodes

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Fundamentals Of Electronic Circuits

Chapter 2:

Semiconductors, PN Junction and

o es

Associate Prof. Dr. Soliman Mahmoud

Electrical and Computer Engineering Department

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Chapter(2)

,Diodes

Objectives and outline:

1. Conductivit of Solids

2. Semiconductors

. pn unc on4. Diodes

5. Diode applications

Fund. Of Electronic

Circuits


Electronics and Electrical Engineering Department

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1. Conductivit of Solids

Conductivit is the abilit of a material to carr

an electrical current.

,

1. Good conductor: very low resistance (metals)

. oor con uc or or nsu a or : very gresistance (ceramics, oxides)

3.Semiconductor: somewhere in between

(silicon, germanium, gallium arsenide)

Fund. Of Electronic

Circuits



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2. Semiconductors

valence of four. This means an isolateda om o e ma er a as our e ec rons n

its valence orbit.

Conductors have 1 valence electrons,

electrons and Insulators have 8 valence

.

Fund. Of Electronic

Circuits



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2.1 Silicon atom

used semiconductor.

An isolated atom has

pro ons anelectrons as shown:

Valence orbit with 4 electrons

Fund. Of Electronic

Circuits



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2.2 intrinsic ure semiconductors

properties:

No impurities only covalent

bonds.

All bonds complete at 0K.

Part of electrons from covalentbonds is released at higher

.

valence bonds model

Fund. Of Electronic

Circuits



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2.3 Do in of semiconductors

One wa to increase conductivit of a semiconductoris by doping.

Doping This means adding impurity atoms to an intrinsic crystal to alter its

electrical conductivity. A doped semiconductor is called anextrinsic semiconductor.

some materials form an excess of electrons and produce an n-type semiconductor

some materials form an excess of holes and roduce a -t esemiconductor

both n-type and p-type materials have much greater conductivitythan ure semiconductors

this is extrinsic conduction.

Fund. Of Electronic

Circuits



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n-t e semiconductors

,

pentavalent atoms are added to the pure.

Pentavalent atoms include 5 electrons in

the valence orbit( ex: Phosphorus atoms).

extra electron to the silicon crystal, they

are o en re erre o as onor mpur es.

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Circuits



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n-t e semiconductors contd.

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Circuits



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-t e semiconductors

By using a trivalent impurity, one whose

atoms have onl 3 valence electrons(ex:Boron).

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Circuits



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p-type semiconductors (contd.)

By using a trivalent impurity, one whose atoms haveonly 3 valence electrons (ex:Boron).

Fund. Of Electronic

Circuits



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3. pn Junction

- -joined, this forms apnjunction

the ma orit char e carriers on

each side diffuse across the

junction where they combine withan remove e c arge carr ers o

the opposite polarity.

,

are few free charge carriers and wehave a depletion layer(also called

a space-c arge ayer.

The diffusion of positive charge in one

direction and negative charge in theo er pro uces a c arge m a ance

this results in a potential barrieracross the junction.

Fund. Of Electronic

Circuits



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3.1 Isolated n JunctionPotential barrier the barrier opposes the flow

o ma or y c arge carr ersand only a small number

have enough energy to.

This generates a smalldiffusion current.

flow of minoritycarriers andany that come close to it willbe swept over

This generates a smalldrift current.

for an isolated junctionese wo curren s musbalance each other and thenet current is zero.

Fund. Of ElectronicCircuits



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3.2 Forward bias n JunctionsForward bias

if the p-type side is madepositive with respect to then-type side the height of the

barrier is reduced

have sufficient energy tosurmount it

increases while the driftcurrent remains the same

there is thus a net current flowacross the junction whichincreases with the applied

voltage.




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3.3 Reverse bias n Junction

Reverse bias -

with respect to then-type side the height of thebarrier is increased

the number of majority chargecarriers that have sufficient energyto surmount it rapidly decreases

e us on curren ere orevanishes while the drift currentremains the same

leakage current caused by the(approximately constant) drift

current the leakage current is usually

negligible (a few nA).




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3.4 n Junction Currents

pnjunction current is given approximately by

= 1expT

s

VII

, Te is the electronic charge, Vis the applied voltage, k is

Boltzmanns constant T is the absolute tem erature

and (Greek lettereta) is a constant in the range 1 to

2 determined by the junction material for most purposes we can assume =1.




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3.4 n Junction Currents contd.

Thus,

1expeV

II s

at room temperature e/kT~ 40 V1

If V> + 0.1 V,eV

If V< 0.1 V,

s skT

IS is the reverse saturation current.

( ) ss

III = 10




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4. Diodes

n ea o e passes e ec r c y n one rec on

but not in the other.


Associate Prof. Dr. Soliman MahmoudElectronics and Electrical Engineering Department

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4. Diodes contd.

One a lication of diodes is in rectification

the example below shows a half-wave rectifier.

In practice, no real diode has ideal characteristics

diodes.



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4.1 Semiconductor diodes



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4.1 Silicon diodes

-

device



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4.2 Diode e uivalent circuits

Sometimes we re resent a diode b an

equivalent circuit. Models have different levels of

so histication.



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4.3 Diode circuit anal sis

The non-linear behaviour of diodes makes anal sisdifficult consider this simple circuit.

Applying Kirchoffs voltage law VVE RD+=IRVD+=

From the diode equation

expS D

=

these twosimultaneous



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equations.


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4.4 Diode circuit anal sis contd.

One approach is through the use of a load line.

VVE RD+=

D



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4.4 Diode circuit anal sis contd.

circuits

however, this is rarely done, since if an equivalentcircuit is used, the circuit can normally be analysed

directly, without resorting to a graphical method.



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4.4 Diode circuit anal sis

VEI

ON= ON

rR

VEI

+

=



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4.5 Zener breakdown diode: Symbol

Model for the zener diode.Circuit symbol for a

zener diode.



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5.Diode circuitsHalf-wave

(AC DC)

peak output

voltage is equal to

e pea npu

voltage minus the

of the diode

used to produce a



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5. Diode circuits contd.

u -wave rec er

use of a dioder ge re uces

the time for which

to maintain theoutput voltage

and thus reduce

the ripple voltage.



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Fund. of. Electronics Ch02 Diodes

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