Semi and di

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PHYSICS PROJECT

Roll no 71

Semiconductors and

Diodes

Prepared By

Nishant Kulkarni

M H A T O B A T U K A R A M B A L W A D K A R J U N I O R C O L L E G E


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Table of Contents

Acknowledgements 3

Semiconductors..............................................................................4

Types Of Semiconductors ............................................................. 5

N type Semiconductor 6

P Type Semiconductors ......................................................... 7

Diodes........................................................................................... 8

Zener Diode 9

Photo Diode 10

Light Emitting Diode 11

Conclusion 12

References.......................................................................... 13


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I would like to thank my physics teacher for giving me this

project. Semiconductors and diodes play an important part of

our lives. I would like to finally thank my parents for

supporting me to do this project.


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There are basically two types of semiconductors- P type And N type.

By controlling the amount of impurities added to the semiconductor material it is

possible to control its conductivity. These impurities are called donors or

acceptorsdepending on whether they produce electrons or holes respectively.

This process of adding impurity atoms to semiconductor atoms (the order of 1

impurity atom per 10 million (or more) atoms of the semiconductor) is called

Doping.

But simply connecting a silicon crystal to a battery supply is not enough to extract

an electric current from it. To do that we need to create a "positive" and a

"negative" pole within the silicon allowing electrons and therefore electric current

to flow out of the silicon. These poles are created by doping the silicon withcertain impurities.

There are two more types- Intrinsic and Extrinsic.

Intrinsic semiconductor means extremely pure semiconductor. On the

other hand Extrinsic Semiconductor means a doped semiconductor.


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In order for our silicon crystal to conduct electricity, we need to introduce animpurity atom such as Arsenic, Antimony or Phosphorusinto the crystalline

structure making it extrinsic (impurities are added). These atoms have five

outer electrons in their outermost orbital to share with neighbouring atoms

and are commonly called "Pentavalent" impurities.

This allows four out of the five orbital electrons to bond with its neighbouringsilicon atoms leaving one "free electron" to become mobile when an

electrical voltage is applied (electron flow). As each impurity atom "donates"

one electron, pentavalent atoms are generally known as "donors".

The resulting semiconductor basics material has an excess of current-carrying electrons, each with a negative charge, and is therefore referred to

as an "N-type" material with the electrons called "Majority Carriers" while

the resulting holes are called "Minority Carriers".

When stimulated by an external power source, the electrons freed from thesilicon atoms by this stimulation are quickly replaced by the free electrons

available from the doped Antimony atoms. But this action still leaves an extra

electron (the freed electron) floating around the doped crystal making it

negatively charged.


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If we go the other way, and introduce a "Trivalent" (3-electron) impurityinto the crystalline structure, such as Aluminium, Boron or Indium,which

have only three valence electrons available in their outermost orbital, the

fourth closed bond cannot be formed. Therefore, a complete connection is

not possible, giving the semiconductor material an abundance of positively

charged carriers known as "holes" in the structure of the crystal where

electrons are effectively missing.

As there is now a hole in the silicon crystal, a neighbouring electron isattracted to it and will try to move into the hole to fill it. However, the

electron filling the hole leaves another hole behind it as it moves. This in turn

attracts another electron which in turn creates another hole behind it, and so

forth giving the appearance that the holes are moving as a positive charge

through the crystal structure (conventional current flow. Trivalent impurities

are generally knownas "Acceptors" as they are continually "accepting" extraor free electrons.

"P-type" material with the positive holes being called "Majority Carriers"while the free electrons are called "Minority Carriers.


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What is a diode?Diodes are basic unidirectional semiconductor devicesthat will only allow current

to flow through them in one direction only, acting more like a one way electrical

valve, (Forward Biased Condition).

However, unlike a resistor, a diode does not behave linearly with respect to the

applied voltage as it has an exponential I-V relationship and therefore cannot be

described simply by using Ohm's law as we do for resistors.

What is a diode made of?

Diodes are made from a single piece of Semiconductor material which has apositive "P-region" at one end and a negative "N-region" at the other, and

which has a resistivity value somewhere between that of a conductor and an

insulator. But what is a "Semiconductor" material?, firstly let's look at what

makes something either a Conductor or an Insulator.

Letslearn about some commercially important diodes- they areZener Diode, Photo diode and light emitting diode.

Zener Diode LED


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A Zener diode is a diode which allows current to flow in the forward direction in

the same manner as an ideal diode, but will also permit it to flow in the reverse

direction when the voltage is above a certain value known as thebreakdown

voltage, "zener knee voltage" or "zener voltage" or "Avalanche point".

The Zener iode RegulatorZener Diodes can be used to produce a stabilised voltage output with low ripple

under varying load current conditions. By passing a small current through the

diode from a voltage source, via a suitable current limiting resistor (RS), the zener

diode will conduct sufficient current to maintain a voltage drop of Vout. We

remember from the previous tutorials that the DC output voltage from the half or

full-wave rectifiers contains ripple superimposed onto the DC voltage and that as

the load value changes so to does the average output voltage. By connecting a

simple zener stabiliser circuit as shown below across the output of the rectifier, a

more stable output voltage can be produced.

I-V Characters of Zener Diode Symbol of Zener Diode


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A photodiode is a diode which generates a current when exposed to light.In utter darkness, they act as an open circuit, allowing no current to pass

through. When the light they are exposed to becomes bright, they conduct

current across from their cathode to anode. Photodiodes, like zener diodes,

connect to a circuit in reverse bias. This means that the cathode of the diode

is connected to the positive voltage and the anode to the negative voltage.

When the light intensity increases, current flows from the cathode to anode.

Current increases with light intensity.

Photodiodes have very fast response times (in the nanoseconds).Uses

They are used to detect optical signalsUsed in object counters and opto-couplers and sensors

An anatomy of photodiode, its symbol and a photodiode.


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Light emitting diodes, commonly called LEDs, are real unsung heroes in theelectronics world. They do dozens of different jobs and are found in all kinds

of devices. Among other things, they form numbers on digital clocks,

transmit information from remote controls, light up watches and tell you

when your appliances are turned on.

Basically, LEDs are just tiny light bulbs that fit easily into an electrical circuit.But unlike ordinary incandescent bulbs, they don't have a filament that will

burn out, and they don't get especially hot. They are illuminated solely by the

movement of electrons in a semiconductor material, and they last just as

long as a standard transistor. The lifespan of an LED surpasses the short life

of an incandescent bulb by thousands of hours. Tiny LEDs are already

replacing the tubes that light up LCD HDTVs to make dramatically thinner

televisions.

Their main advantages are that they are cheap and require low operating

voltages.They have stronger life and light weight. Different spectral colouroutputs are possible

Structure of a LED. I-V Characteristics of LED.


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Now I would like to conclude my project. We have learnt

about various applications of various diodes. We have also

learnt about various semiconductors and its applications.

So to conclude, Semiconductors and Diodes play an

important part of our lives. So we should learn about

them.


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www.enwikipedia .com

www.electronicstutorials.com

www.yahooanswers.com

www.howstuffworks.com

Physics HSC board textbook
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