semiconductors are solid materials, either non-metallic or compounds, which allow electrons to pass through them so that they conduct electricity in much the same way as metal.
Characteristics of semiconductors
Semiconductors possess the following characteristic
- The resistivity is usually high.
2. The temperature coefficient of resistance is always negative.
3. The contact between semiconductors and metal forms a layer that has higher in one direction than the other.
4. When some suitable metallic impurity for example. arsenic, gallium, etc. is added a to semiconductor its conducting properties change appreciably.
5. They exhibit a rise in conductivity the increasing temperature with decreasing temperatures their conductivity falls off and at low temperatures semiconductors become dielectrics.
6. They are usually metallic in appearance but unlike metal are generally hard and brittle. Both the resistivity and the contact effect are as a rule very sensitive to small changes in the physical condition and the great importance of semiconductors for a wide range of uses apart from rectification depends on the sensitiveness.
Examples of semiconducting materials
Difference between semiconductors and Conductors
- Their resistivity is usually high and the temperature coefficient of resistance is always negative.
- Both resistivity and contact effects are very sensitive to a small change in physical condition.
- These materials have filled energy bands and small forbidden zones.
- Moving carriers of electric current are originated due to the absorption of thermal radiant or electric energy from an external source.
- At low temperatures they become dielectrics.
- They show a rise in conductivity with an increase in temperature due to an increase in current carriers and vice-versa.
- The resistivity is very low and the temperature coefficient of resistance is not constant.
- Both resistivity and contact effect are very sensitive to small changes in physical condition.
- These are materials with unfilled or overlapping energy bands.
- Current carries here are free electrons which exist whether external energy is applied or not.
- At very low temperatures at absolute zero temperatures, they become superconductors.
- Their conductivity increases with decreasing temperature up to near absolute zero stage.
To understand how semiconductors work it is necessary to study briefly the structure of matter. All atoms are made of electrons, protons, and neutrons. Most solid materials are closed, from the standpoints of electrical conductivity, as conductors, semiconductors, and insulators. To be a conductor, the substance must contain some mobile electrons-one that can move freely between the associated with the valance electrons and bind adjacent atoms together. The inner electrons below the valence level, do not normally enter into the conduction process.
With the addition of suitable impurities to semiconductors, two types of semiconductors are,
1. N-type semiconductor.
The presence of even a minute quantity can produce an N-type semiconductor. If the impurity atoms have one valence electron more than the semiconductor atom that it has substituted, these extra electrons will be loosely bound to the atoms. For example, an atom of Germanium possesses valence electrons. When it is replaced in the crystal lattice of the substance by an impurity atom of antimony Sb which has five valence electrons, the fifth valence electron free electron produces extrinsic N-type conductivity even at room temperature. Such an impurity in a semiconductor is called a donor impurity. The conducting properties of germanium will depend upon the amount of antimony added.
2. P-type semiconductor.
A P-type extrinsic semiconductor can be produced if the impurity atom has one valence electron less than the semiconductor atom that it has replaced in the crystal lattice. This impurity atom cannot fill all interatomic bonds, and the free bond can accept an electron from the neighboring bond. This type of semiconductor conducting is by means of holes in the valence band. The process of conduction is called deficit conduction.
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