Semiconductor3 Comments

“Semiconductors are solid materials, either non-metallic elements or compounds, which allow electrons to pass through them so that they conduct electricity in much the same way as a metal”.
Semiconductors possess the following characteristics:

1. The resistivity is usually high.
2. The temperature co-efficient of resistance is always negative.
3. The contact between a semiconductor and a metal forms a layer which has a higher resistance in one direction than the other.
4. When some suitable metallic impurity (e.g., Arsenic, Gallium etc.) is added to a semi-conductor, its conducting properties change appreciably.
5. They exhibit a rise in conductivity with the increasing temperature, with the decreasing temperatures their conductivity falls off, and at low temperatures semiconductors become dielectrics.
6. They are usually metallic in appearance but (unlike metals) are generally hard and brittle.

Both the resistivity and the contact effect are as a rule very sensitive to small changes in physical conditions, and the great importance of semiconductors for a wide range of uses apart from rectification depend on the sensitiveness
 
Types of Semiconductor: Semiconductor can be classified into two types:
1. Intrinsic Semiconductors or Pure of semiconductors
2. Extrinsic Semiconductor or Impure of Semiconductors

Instrinsic semiconductors: Two normal (pure) silicon and Germanium are intrinsic semiconductor.They posses  all essential conducting characteristics of a semiconductor. The number of electrons present in the outermost orbit of intrinsic semiconductor is four,hence they are termed as tetra valent . So,intrinsic semiconductor are tetra valent in nature

Extrinsic Semiconductors: The process of adding impurities to an intrinsic semiconductor is known as Doping. With respect to the type of  Impurities  added, extrinsic semiconductor are classified into two types.

1. N-type semiconductors
2. P-type semiconductors

DOPING PROCESS

The pure semiconductor mentioned earlier is basically neutral. It contains no free electrons in its conduction bands. Even with the application of thermal energy, only a few covalent bonds are broken, yielding a relatively small current flow. A much more efficient method of increasing current flow in semiconductors is by adding very small amounts of selected additives to them, generally no more than a few parts per million. These additives are called impurities and the process of adding them to crystals is referred to as DOPING. The purpose of semiconductor doping is to increase the number of free charges that can be moved by an external applied voltage. When an impurity increases the number of free electrons, the doped semiconductor is NEGATIVE or N TYPE, and the impurity that is added is known as an N-type impurity. However, an impurity that reduces the number of free electrons, causing more holes, creates a POSITIVE or P-TYPE semiconductor, and the impurity that was added to it is known as a P-type impurity. Semiconductors which are doped in this manner – either with N- or P-type impurities – are referred to as EXTRINSIC semiconductors.

N-Type Semiconductor :

The N-type impurity loses its extra valence electron easily when added to a semiconductor material, and in so doing, increases the conductivity of the material by contributing a free electron. This type of impurity has 5 valence electrons and is called a PENTAVALENT impurity. Arsenic, antimony, bismuth, and phosphorous are pentavalent impurities. Because these materials give or donate one electron to the doped material, they are also called DONOR impurities.

An N-type semiconductor (N for Negative) is obtained by carrying out a process of doping, that is, by adding an impurity of valence-five elements to a valence-four semiconductor in order to increase the number of free (in this case negative) charge carriers.

P-Type Semiconductor

The second type of impurity, when added to a semiconductor material, tends to compensate for its deficiency of 1 valence electron by acquiring an electron from its neighbor. Impurities of this type have only 3 valence electrons and are called TRIVALENT impurities. Aluminum, indium, gallium, and boron are trivalent impurities. Because these materials accept 1 electron from the doped material, they are also called ACCEPTOR impurities.

A P-type semiconductor (P for Positive) is obtained by carrying out a process of doping, that is adding a certain type of atoms to the semiconductor in order to increase the number of free (in this case positive) charge carriers.

• Basic Electronics