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# Semiconductors

Solid state physics has classified the material that exist in nature into three types- Insulators, semiconductors and conductors. The material with fully occupied valence bands and empty conduction bands are insulators, the material with almost fully filled valence band and almost empty conduction band are semiconductors whereas the material with empty conduction band and fully filled conduction band are conductors.The semiconductor behaves as insulator at $0K$ and as temperature increase it behaves as conductor as temperature increases.

## Definition

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A substance that is usually solid element or a compound which can conduct electricity under certain conditions that makes it a good medium for the passing of current is known as a semiconductor. Basically, semiconductors are the materials which are conductive between an insulator that are non-conductors and metals that are conductors. They can either be a pure element or even compounds. They are an important part of our lives because of the role played by them in the electronic devices.

## Properties of Semiconductors

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Some of the properties of semiconductors which have been seen so far and are also being explored till now are listed below.
1) Electrical Conductivity

The process of doping makes it easier to have control on the electrical conductivity. When the dopant is changed, the semiconductor can conduct both holes and electrons without being changed. When the doping is done on a higher level, degeneracy happens which can make the material behave like a metal. If an amorphous film of the semiconductor is formed, then it will behave like an insulator only. On the basis of these properties, many electronic devices came into the picture like transistors, diodes etc.

2) Optical Properties

There may be a direct or indirect band gap existing in semiconductors. The semiconductors with direct band gap are able to emit a photon when excitation is done by the wavelength required. On the basis of this property, the development of applications like LEDs, lasers etc came in action. The semiconductors with indirect band gap can be made of the type direct band gap by simply alloying.

3) Photo Conductivity

An electron-hole pair can be produced by illuminating a $p – n$ junction. These electron-hole pairs can then be collected in at the used electrodes in the circuit which will now pass current. Because of this property, the development of photodetectors and solar cells was possible.

## Types of Semiconductors

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We can classify semiconductors in two categories:
• Intrinsic
• Extrinsic

INTRINSIC SEMICONDUCTOR

The material of this type of semiconductor is of chemically very pure form. Also, this type of material has poor conductivity. These materials have an equal number of positive and negative carriers. Positive carriers are holes and negative carriers are electrons.

The energy gap that is forbidden is very minute in such semiconductors and hence, even the room temperature energy is sufficient enough for the electrons to jump across the band of conduction. Also, the Fermi level of such materials lies in between the conduction band and the valence band.
EXTRINSIC SEMICONDUCTOR

An extrinsic semiconductor is simply an improved type of intrinsic semiconductor. This is done by addition of a small amount of impurities being added by the process called doping. This process only alters the electrical properties of the semiconductor thereby increasing the conductivity of the material. Basically, the increase of impurities in a semiconductor results in improvement and controlling of their conductivity. The materials that are chosen for the process of doping are in such a manner that in their valence band, there are either $5$ or $3$ electrons only. on the basis of a number of electrons the dopants are called pentavalent if there are $5$ electrons and trivalent if there are $3$ electrons. The dopant type also gives rise to two types of extrinsic semiconductors namely $N$ – type and $P$ – type.

## Semiconductors Transistors

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A transistor is also a type of semiconductor device. This device is used to switch or amplify the electronic signals and power. A transistor consists of the type of semiconductor material that has at least three terminals for making a connection to any external circuit. A current or voltage that is applied to one pair of the terminal of the transistor changes the current via another pair of the terminals of the transistor. The transistor can amplify the signal because the controlled power of output at times can be higher than the controlled power of input.

When a transistor works as an amplifier, a tiny current is taken by it from one end and a very high electric current is produced at the other end. That is, the current is boosted. This is much useful in instruments like hearing aids which have a tiny microphone in it that are able to pick up sounds and converts it into electric currents in fluctuating form.

When a transistor works as a switch, a tiny electric current that flows through one part of the transistor, can be used to make a higher current flow via another part of it.  Simply we can say that, the small switches over a larger current. This is the type of functionality used in computer chips. A small memory chip is consisting of millions or billions of such transistors, where each of such transistors can be individually switched on or off. Because of the possibility of being in two states on or off, every transistor in such case store two different numbers that are zero and one.
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