Seminar project on semiconductor & diode, Study Guides, Projects, Research of Physics

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WELCOME

PROJECT REPORT ON

SEMI-CONDUCTOR

$

DIODES

BY-HIMANGSHU BAISHYA

B.Sc. 3

RD SEM……

INTRODUCTION-

In semi-conductor the forbidden energy gap is

very small(=1eV). Its valance band is full, but

conduction band is empty. At absolute zero

temperature, it behave like an insulator. At room

temperature some electrons in valance band

acquire sufficient energy to jump to conduction

band. The conductivity of semi-conductor

increases with rise of temperature.

TYPES OF SEMI-CONDUCTOR-

When an element of group-V, such as phosphorous, arsenic or

antimony having 5 valance e- is added to Ge or Si, four of the five

valance e- of P (or As or Sb) are shared in covalent bonding, while

the 5

th e- is comparatively free to move. Each impurity atom in this way donates a free e- to the

semiconductor. Such impurity e- are called donors. They form a donor energy level just below the conduction band.

As the energy gap between donor energy level and conduction band is very small(0.45eV), these e- can easily

jump to conduction band even at room temperature. Hence the semi-conductor containing donor type impurity is

called n-type semi-conductor.

P-N JUNCTION-

When a p-type and an n-type crystals are combined, the

combination is called a p-n junction. In a p-n junction, due to

higher concentration of e- on n-side and holes on p-side,

diffusion of e- towards p-side and holes towards n-side take

place. When a hole shifts, it leaves behind a negative

immobile ion. Similarly, when an e- shifts, it leaves behind

an immobile positive ion. This sets a potential difference

across the junction and hence, an electric field directed

from n-side to p-side. The region on either side of the

junction, which becomes depleted (free) of mobile charge

carries, is called “depletion region”. Its width is of the order

of 10-6 cm. The potential difference developed across the

BIASING OF P-N JUNCTION-

When positive terminal of the battery is connected p-side and

negative terminal to n-side, the diode is in forward biased.

This type of biasing permits easy flow of current across the

junction. Due to applied voltage, holes in p-region are

repealed by the positive terminal and e- in n-region are

repelled by the negative terminal of the battery. As a result

both the holes and e- move towards the junction. They enter

the depletion region and recombine. Consequently the width

of the depletion layer is reduced and the height of the

potential barrier is lower. As a result, a large current flows

through the junction. It is called forward current. The

TYPES OF JUNCTION

DIODES-



ZENER DIODE



PHOTO DIODE



SOLAR CELL



LIGHT EMITTING DIODE (LED)

WORKING PRINCIPLE OF JUNCTION DIODES-

ZENER DIODE-

Zener diode is a specially designed p-n junction diode used to

operate in the reverse breakdown voltage region continuously. It

is named after its inventor C. Zener. It is heavily doped Ge or Si

p-n junction diode with high power rating. Due to this, the

depletion region formed is very thin (<10-6m) and the electric

field of the junction is extremely high (5x106 V/m). In reverse

bias, Zener diode has a sharp breakdown voltage Vz, called

Zener voltage. When the applied reverse voltage V is less than

zener voltage (V<Vz), the zener diode does not conduct any

current and is said to be in “OFF” state. When V>=Vz, it conducts

large current for insignificant charge in reverse bias voltage and

The magnitude of this photo current depends on the

intensity of incident light. More the intensity, larger is

reverse current. The total reverse current through the photo

diode is then given by Ir = Id + Io, where Io is the additional

reverse current which flows through the photo diode due to

incident light. It is called photo conducting current. The

variation of reverse current Ir through photo diode, with the

variation of reverse bias voltage Vr across it is called the VI

characteristics of photo diode.

USES:-

Photo diodes are used in-

Light operated switch.

Photo detectors to detect radiations.

Demodulation of optical signals.

SOLAR CELL-

A solar cell is a p-n junction diode, which work on photo voltaic

effect. It converts solar energy into electrical energy.

A typical solar cell consist of a n-type semi-conductor crystal

with Ni plated charge collecting electrode over which is a thin

layer of p-type semi-conductor.

When exposed to light, the e- holes are generated in the

depletion layer. Due to junction field e- in p-side slide down to

n-side, while holes in n-side move towards p-side. As a result

the top metal contact acts as a positive electrode and the

bottom metal contact as negative electrode.

LIGHT EMITTING DIODE (LED)-

It is a p-n junction diode in which, at forward bias state, hole-e-

pairs recombine and the energy is released in form of light.

The principle on which LED works is spontaneous emission of

radiation. When a p-n junction diode is forward biased, free e-

from n-region cross the barrier and enter the p-region. They

combine with the holes. As free e- lies in conduction band and

holes lies in valance band, the e- fall from higher to lower

energy level and energy is released in form of radiation.

The frequency of emitted radiation in LED depends upon the

band gap energy of the semi-conductor used.

In Gallium Arsenic (Ga,As), energy appears in infra-red region,

in gallium Arsenic Phosphide (Ga As P), it appears in visible

region

while in Gallium Phosphate (Ga P), it appears as green light.

The intensity of emitted radiation in LED depends upon the

forward current flowing through the LED. More the current

more the hole-electron pair recombination and hence, more

the intensity of emitted radiation.

Uses :-

LED’S are used in-

Display of watches, calculator etc.

Short distance optical fibre communication.

Decorative items.

Remotes of electronic device.

Burglar alarm.

Signal lamps.