Wednesday, November 19, 2014

Intrinsic and Extrinsic Semi Conducting Meterials

Basing on the ability of a material that whether it can pass the current through it are not,the materials can be divided into three categories.

The materials that can pass the current through them because they’re having the free availability of the charges are called conductors. Almost all the metals and human bodies of the some of the examples for the conductors .

The materials who cannot pass the current through them because they are not having the availability of the free charges are called insulators.Plastic, rubber, cotton and paper are some of the examples of the insulators.

The materials who behave  like insulators at absolute zero temperature but behaves like conductors once if the temperature is raised a little bit is called a semiconducting material. Silicon and germanium are some of the examples of the semiconducting materials.

Here we are going to have further detailed discussion regarding the semiconducting materials.Many are for modern-day electronic gadgets are having a lot of the integrated parts of the semiconducting materials. It is impossible to imagine electronic gadgets without having a semiconducting material inside it. Diodes, transistors, integrated circuits(IC’s) are very much made up of semiconducting materials.

Properties of the semiconducting materials:

1. The resistivity is more than that of the conductors but less than that of the insulators.
2. They have negative temperature coefficient which means with the increase in temperature corresponding resistance is going to decrease.
3. At absolute zero there is no availability of the free charges and hence it behaves like insulators.
4. Its conductivity can be increased by adding a little bit of impunity to it and this phenomena is called doping.

Energy band theory:

To explain the conducting nature of a given material we can depend on the energy band theory.We know that it electrons are distributed in the different orbits like S, P, D,F. The electrons in the outer orbits are called as valency electrons and they are always having a chance of going to some other body if they got enough energy. The valency electrons in the outermost orbits are bit close to each other and their form together like a band. This band of electrons is called valency electrons and it is either completely filled are partially filled in any kind of material.

Once if this electrons becomes free they’re ready to conduct and all that kind of electrons together could be called as conduction electrons.The conduction band is either partially filled are completely empty depending on the nature of the material.

The band gap between the valency band in the conduction band is called forbidden band gap. The size of this gap depends on the nature of the material.

In the case of the conductor valency band and conduction band overlap with each other electrons from valency band can easily move to the conduction band and hence there is a flow of current quite easily.

In the case of insulators there is a big gap between these two bands and hence it becomes very difficult for the valency electrons to travel from that band to the conduction band and hence it always behaves like insulators.

In the case of the semiconducting material the band gap is somewhere in between the conductors and insulators. By default the charges have not enough energy to cross this band and hence it behaves like insulators. Once if the temperature is raised a little bit, the charges get enough energy to cross the and hence the start behaving like a conductors.

Types of semiconducting materials:

Semiconducting materials are broadly of two types. Those who are naturally available are called pure semiconducting materials or intrinsic semiconducting materials. If a electron leaves a place in a energy band , there is an empty place left in the band that empty spaces called hole. In the case of a intrinsic semiconducting material ,number of electrons is equal to the number of holes. The flow of current in this kind of semiconducting materials is equal due to both electrons and holes.

At 0° Kelvin in the crystalline structure of the semiconducting material who is having four valency electrons  and each electron is having a covelent bond with its neighboring electron. There’s no electron is free and hence it is behaving like insulators. Once if a little bit of temperature is given, by obserbing that heat energy some of the covalent bonds can break and hence it starts behaving like a conductor because of the availability of the free charges.

Anyway practically use of this kind of intrinsic semiconductor is very much limited because they are by default behaving like insulators. We are looking for a semiconducting material or can pass the current directly without raising the temperature and for that sake we are going to use a phenomena called doping. Doping is the phenomenon of adding a little bit of impunity to the pure semiconducting material therefore there will be availability of the free charges.

When we are doping we have two options.We can add tiavalent  impunity and hence there will be one excess hole and hence it start behaving like a P type semiconducting material who is having access holes.

We can also had a pentavalent and impurity to the semiconducting material and hence there will be availability of excess electrons in the material. This type of the material is called N type. These are called impure semiconducting materials or  extrinsic semiconducting materials.

In practical life we use extrinsic semi conductor materials in making electronic devices because of its ability to conduct current at ordinary temperatures.

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