Semiconductor

From ArticleWorld

A semiconductor refers to any material that has electrical conductivity (at room temperatures) that is more than that of an insulator but lesser than that of a conductor. The behaviour of pure semiconductors changes according to the temperature. At low temperatures, they behave like insulators. However, under higher temperatures or with incident light the conductivity of semiconductors can increase to much higher levels. The most widely used semiconductor material is silicon. Germanium, gallium nitride and gallium arsenide are other examples of semiconductor materials. Semiconductors are used for the manufacture of transistors, diodes, microprocessors, thermistors and solar cells.

Effect of temperature and light on semiconductors

In a pure semiconductor, increase in conductivity with temperature, light or impurities arises from the increase in the number of conduction electrons. The valence or outermost electrons of a semiconductor atom are paired and form a covalent bond with other atoms. These electrons, once excited by temperature/light, can be set free to conduct current, once they enter the so-called ‘conduction band’. Deficiencies in charge called ‘holes’ are left behind. These holes contribute to the passage of electricity and are said to be carriers of positive electricity. The energy which is needed to excite electrons is called energy gap.

How semiconductors work

There are two types of semiconductors: intrinsic, in which no impurities are added, and extrinsic, in which impurities (dopants) are added. As mentioned above, physical changes can bring about an increase or decrease in the conductivity of a semiconductor. Apart from that, the addition of impurities, in a process called doping, can also produce free carriers of electricity. The process of adding dopants gives rise to two types of semiconductors, namely, p-type semiconductors and n-type semiconductors. The type depends upon the difference in the number of valence electrons between the dopants and host. As a result, positive (p-type) carriers of electricity or negative (n-type) carriers of electricity are formed. Doping is the sole reason why semiconductors can be used in electronics. Conduction can be greatly increased, hence the use in ICs. Consider an example of a doped silicon crystal. Each silicon atom has four valence electrons, two of which are required to form a covalent bond. In n-type semiconductors, atoms such as phosphorus with five valence electrons can take the place of some silicon atoms and provide extra electrons for conduction. In p-type semiconductors, atoms with three free electrons such as aluminium can be added as dopants. These lead to a deficiency of electrons resulting in holes, which behave as positive carriers of electricity.

The band structure of a semiconductor The working of a semiconductor can be explained on the basis of the band structure. The uppermost band of occupied electron energy states, referred to as the valence band is full. This is true at absolute zero. At room temperature, electrons may jump the so-called ‘forbidden-energy gap’ to enter the conduction band. These electrons, which leave behind holes after breaking covalent bonds, become charge carriers. Insulators have large forbidden gaps while conductors or heavily-doped semiconductors may have overlapping valence and conduction bands.

Concentrations of the carriers The concentration of the carriers depends on the type of semiconducting material and its level of doping. After doping, the product of majority and minority carriers is equal to the square of the intrinsic carrier concentration.

Uses of semiconductors Semiconductors can be used as a series of p-n junctions to make transistors and several other devices such as solar cells, p-n junction lasers and rectifiers. Semiconductor chips with millions of transistors are manufactured on a large scale in various applications of electrical engineering.