p-n juction

8/11/2019 p-n juction

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9/10/14

2013 Copyright by David Owino

Objectives

1.Define the p-n junction2.Describe the formation and biasing of the p-n

junction

3.Define semi-conductor diode and sketch its circuit symbol

4.Mention at least two types of diodes

5.Sketch current-voltage characteristics for a diode


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P n junction

When an intrinsic s.c simultaneously dopedby trivalent and pentavalent atomssuch that

one half becomes n-type and the other p-type

s.c a region is formedThe boundary or interface separating a p and

n type material is called p-n junction

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3

N-

type

P-

type

Junction


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Cont

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Formation of p n junction

The free s and holes near the junctiondiffuses across it

s enter the p-zone as holes move into the n-

zone as shownp-type n-type

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- -

-

-

--

+ +

++

+

++

+

+

+ +

+++ +

+

+

- --

-

--

-

--

-

-

Depletion layer


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ont

Re-combination of mobile chargecarriers takes place on either side of the

junction

This depletes mobile charge carrierswithin the region

The region remain with uncovered fixed

ions on either side of the junctionThis region occupied by the uncovered

fixed ions is called depletion layer9/10/14


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The uncovered ions set up a potentialdifference/barrier.

The barrier in turn sets up a field that

stops further diffusion of mobile charges.The depletion layer has very high

resistance to the movement of charge

carriers.

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CONT

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O T

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Biasing the p n junction

Biasing means connecting /applying p.dacross the p-n junction

Biasing can either increaseor reduce

the potential barrierThere are two ways of biasing a p-n

junction to e.m.f of the source, namely:

Forward biasReverse bias

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Forward bias

P-type is connected to +ve & N-type to theve terminal of an external cell/battery.

Holes in the p-region are repelled away by

the +ve terminal towards the junction.

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P-

type

N-

type

VB

VEVB-barrier potential

VE-applied potential

holes

s


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A Forward Biased p n junction

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ont

At the same time, theve terminalrepels the free s from n-region towards

the junction.

The potential barrier/depletion layerdisappears-is reduced considerably.

Thus, forward resistance is lowered &

hence charge carriers cross the junctionwith ease.

The crossing over of charge carriers

constitutes a very large forward current9/10/14


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O T

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ont

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Reverse bias

P-type is connected to -ve & N-type to the+ve terminal of an external cell/battery.

Holes in the n-region are attracted by theve

terminal of the battery.

Electrons are also attracted to the +ve

terminal.

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P-

type

N-

type

VB

VE

holes

s


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A Reverse Biased p n junction

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ont

The thickness of depletion layer increasesdue to high concentration of fixed +ve & -ve

ions.

Increase in depletion layer increases the

resistance of the p-n junction.

Since no majority charge carriers diffuse

across the junction, no current flows thro the

circuit.NB:

P-n junction in reverse bias behaves like an

open switch that blocks the current flow.9/10/14


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O T

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Semi conductor diode

It is also known as p-n junction diode.This is a device that offers high resistance

when reverse biased & low resistance when

forward biased.

Or

Is a one-way conducting device consisting of

a p-n junction & having anode & cathode at its

terminals.Anode is +ve (p-type)

cathode isve ( n-type)

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2013 C i ht b D id O i


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Diode characteristicsIs a graph that shows the

relationship btn current & voltage

across a diode.There are two types of diode xtics,

namely:

a)Forward xticb)Reverse xtic

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2013 C i ht b D id O i


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ontThe xtics can be investigated by the circuitbelow.

With the help of variable resistor, different

values of V and I are recorded, then;

The values are used to plot a graph of I vsV

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v

m

A

-+


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Forward xticVo-threshold V/cut inV

Vois the V required

to overcome barrier

potential or to start theconduction.

When the p.b is

overcomed, the I

increases rapidly.The curve is non-

linear showing that

diode is non-ohmic.9/10/14 2013 Copyright by David Owino 25

VVo

I


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Reverse xtic

VbInvestigated byreversing the terminals of

the set-up.

When reverse V is zero,

a small leakage (due tominority c.c) flows.

As V is increased, there

is no change in Iuntil Vb

when appreciable Iflows.

At Vb(breakdown V) the

diode is damaged.

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-V

-I



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Combined xtic

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-Reverse biasI is very small

while reverse

bias V is very

high.

-Forward bias

I is very large

while theforward bias

V is very

small.



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A diode in reverse bias has very high resistance.

A damaged diode conducts irrespective ofbiasing.

The diode that operate at Vbis called zener

diode.Vb- breakdown V is the voltage at which a diode

conducts current in its reverse mode.

It is used for voltage control.

Its circuit symbol is shown.

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ont

Other types of diodes include:a)Light Emitting diode (LED)Its circuit symbol is

a)Laser diodes

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leds

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leds

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leds

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EXAMPLE 1

The fig. shows two ways of biasing a P.Njunction.

a.In which circuit will current flow? (1mk)

-ya.Explain your answer in (a) above. (1mk)

-forward biased; depletion is reduced

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EXAMPLE 2

Study the fig. and use it to answer the

questions that follow.

a)If L1 and L2are identical bulbs, explain

what happens when:a)Only S1is closed. (2mks)

D1is reverse biased; L1and L2light with

equal brightness because they share the9/10/14

0 3 C py g y O

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EXAMPLE 3

The fig. shows a circle with two diodes Pand

Qand a cell.

Explain the observation which would be made

if Sis closed.(2mks)P is forward biased while Q is reverse biased; A1

reads/deflects while A2does not.

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py g y

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py g y

REFERENCES

1.simple-semiconductors.com/8.html2.Abbort, A.F. (1979). Ordinary level Physics. London

: Heinemann Educational Boooks

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