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9.2 Semiconductor Diod

Semiconductor Materials

Semiconductors are materials which conduct electricity better than insulator, but no so well as ordinary conductors.

The following table shows the comparison between insulator, conductor and semiconductor:

 

Insulator

Semiconductor

Conductor

Example material

Glass,

ceramic,

polythene

Silicon,

germanium,

selenium

Copper,

aluminium,

iron

Charge carrier

No free electrons

Free electrons

and holes

Free electrons

Resistance

High

Between insulator and conductor

Low

Conductivity

Decrease

when the temperature

Increase when the temperature increase.

Also increase when light shines on it or with presence of impurities

Decrease when the temperature

Charge carriers in semiconductors

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In a pure crystal of a semiconductor( intrinsic semiconductor) such as silicon, each atom of silicon has four electrons in outermost orbit that are involved in covalent bonding.

The vibrations of atoms causes some electrons to break free the bonds.

When an electron is removed from a covalent bond, it leaves behind “a vacancy” and is called “a hole” in the bonding . Free electrons( negatively charged) and holes (positively charged) are known as charge carriers .

Conduction in a semiconductor is by means of a movement of free electrons and holes in opposite direction.

Semiconductors cannot conduct electricity as well as metals because they have smaller numbers of free electrons and holes. The conductivity of the semiconductors can be increased by a process is called “doping”

Doping of Semiconductors

Doping is a process of adding a small amount of impurities into the pure crystal of semiconductor (intrinsic semiconductor).

Atoms of the impurities added should have almost the same size as the atoms of the intrinsic semiconductor.

Type of Semiconductors Material

Different kinds of impurities are added produce different types of semiconductor ; the p-type and the n-type.

(i) p-type semiconductor

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A p-type semiconductor is produced when trivalent atoms are added to intrinsic semiconductor atoms.

Only three of the four bonds formed by the trivalent atoms are complete. The vacancy is a hole with positive charge.

The holes are now the majority charge carriers in the p-type semiconductor since there are more holes than free electrons.

Examples of trivalent atoms are Indium,Boron and Gallium and called acceptor atoms.

(ii) n-type semiconductor

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A n-type semiconductor is produced when pentavalent atoms are added to intrinsic semiconductor atoms.

Each pentavalent atom donate a free electron ,because there will be one extra electron.

The electrons are now the majority charge carriers in the n-type semiconductor since there are more free electrons than holes.

Examples of pentavalent atoms are Arsenic,Phosphorus and Antimony and called donor atoms.

Comparison between p-type semiconductor and n-type semiconductor


 

p-type semiconductor

n-type semiconductor

Pure Semiconduktor

Silicon,

Germanium

Silicon,

Germanium

Doping substance

Indium,Boron,

Gallium

Phosporus,

Antimony,

Arsenic

Function of doping substance

Aceptor atom

Donor atom

Valency of doping substance

Pentavalent

Trivalent

Majority charge carrier

Hole

Electron

Minority charge carrier

Electron

Hole

Semiconductor Diode

A diode is a component (device) that allows electric current to flow in one direction only.

A diode acts like a one-way valve to electric current.

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The structure and the symbol of a semiconductor diode

A semiconductor diode can be made by joining pieces of n-type and p-type semiconductor.

The semiconductor diode is also called p-n junction diode.

The following figure shows structure and the symbol of a semiconductor diode :

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How does the p-n junction diode work?

When p-type semiconductor material in contact

with n-type semiconductor material , a layer called the depletion layer is formed in the middle.

At this junction , electrons from n-type material drifts across the junction to fill in the holes in p-type.

The holes from p-type material drift in the opposite direction to unite with free electrons in the n-type material. As a result a depletion layer is a very narrow region which has lost all its available free electrons and holes and thus behaves almost like pure silicon,i.e with high resistivity.

Any further movement of charges across the boundry in the depletion layer will be repelled by the charges in the layer.

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The layer of the negative charge in the p-type region will prevent the majority charge carriers from the n-type region(the electrons) from crossing the boundary. Similarly , the positive charge layer in the n-type region will prevent the majority charge carriers from the p-type region(the holes) from crossing the boundry in the opposite direction. Thus, a potential difference ,known as the junction voltage.In its normal state a p-n junction delivers no current since the charges are in equilibrium.

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The effect of this junction voltage is to prevent charge carriers from drifting across the junction.

The junction voltages for germanium and silicon are approximately 0.1 V and 0.6 V respectively.

In order for electric current to flow through the diode, the voltage applied across the diode must exceed the junction voltage.

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When the in forward-biased arrangement, the cell voltage greater than the junction voltage. The depletion layer is narrow , and the resistance of diode decreases. Hence a large current flows through the diode.

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When the in reverse-biased arrangement, the cell voltage lower than the junction voltage. The depletion layer is wide , and the resistance of diode increases. Hence only a very small current (leakage current) flows through the diode.

Graph of current, I against voltage V

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Experiment to show a semiconductor diode flows current in one direction only.

image

 

The diode is connected to the cell in the forward-biased arrangement as shown in Figure(a). The bulb light up.

The experiment is repeated with the reverse-biased arrangement as shown in Figure (b) The bulb does not light up.

The experiment shows that a diode allow the current in one direction only when the diode in the forward-biased arrangement.

Diode as a Rectifier

A diode can act as a rectifier because it can convert alternating current(a.c.) into direct current(d.c).

The process of converting a.c. to d.c. is called rectification.

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There are two types of rectification process :
(1) Half -wave rectification

(2) Full – wave rectification

Half- wave rectification

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For half of the cycle, A is more positive than B ,the diode conducts.

For the other half cycle, A is more negative than B , no current can flow.

Full-wave rectification

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For half of the cycle, A is more positive than B ,the diode conducts and the current flows through the resistance.

For the other half cycle, A is more negative than B ,

the current flows through the resistance in the same direction as before.

Capacitor

A capacitor is device which can

(1) store electric charge

(2) smooth out waveform in the rectified output

(3) separate the a.c and d.c (as a filter)

Smoothing output wave by a capacitor

By connecting a capacitor parallel to the resistance , the half-wave and the full-wave rectified waveform could be partially smoothed out.

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For half of the cycle, the capacitor is charged up. Energy is stored in the capacitor.

For the other half cycle, the capacitor releases its charge (discharges)

So the capacitor can produced a steady output or output is stabilised.

One Response

  1. Did you create your own blog or did a program do it? Could you please respond? 65

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