Semi Conductor: The material objects, on the basis of electrical properties, are classified in three groups:
- Conductor
The substances in which electric current flows easily are called conductors. Usually, metals are good conductors. For example gold, silver, copper, aluminum and other metals. Our body is also a good conductor of electricity. Earth is also a very big conductor, in conductors free electrons are present in very large numbers due to which electric current can easily flow through them.
- Insulator
These are the substances through which electric current does not flow. For example mica, plastic, rubber, glass etc. in these substances all the electrons are bound with their respective atoms and cannot move freely. It is because of this reason that the electric current does not flow through them.
- Semi-conductor
These are the substances, the ability of which to conduct current at room temperature lies midway between conductors and insulators. Germanium and silicon are two main semi-conductors. Let us see how these substances develop the property of semi-conductor.
Both germanium and silicon belong to fourth group of the periodic table. As such both of them have four valence electrons. If we examine the cross-section of a crystal of germanium or silicon from front side. We find that its atoms re arranged as in Fig. 19.4 (a). Fig. 19.4
(b) shows these atoms along with their four valence electrons. It can be seen that each central atom has eight electrons instead of four in its outermost orbit. It is so because it has four valence electrons of its own and borrows four from its neighboring atoms, one from each. As such it has eight electrons in its orbit. In this way, each atom has eight electrons in its outermost orbit.
We know that if the outermost orbit possesses eight electrons, it represents a stable configuration and the electrons do not change this stable state as if they are bound in the orbit. This binding force due to which the electrons are not allowed to leave the orbit is known as covalent bond as shown in fig. 19.4 (c)
In this way all the valence electrons present in crystals of silicon or germanium are bound in their orbits b covalent bonds and no free electrons are available for the flow of electric current. So a crystal of germanium of silicon in its pure from behaves as an insulator. This situation occurs near zero Kelvin temperature. At ordinary temperature the electrons have a kinetic motion due to thermal kinetic energy. The motion of some of the electrons is so vigorous that the covalent bond is unable to keep them in their orbits. Such electrons break their covalent bonds and get free, leaving behind the vacancy for an electron in the orbit as shown in Fig. 19.5.
this vacant place for an electron in the orbit is known as a hole. Whenever a covalent bond breaks, it creates an electron-hole pair. It is worth nothing that in addition to free electrons, electric current also flows due to holes. Whenever electron from a certain location moves to fill a hole, a hole is created at that particular location. When an electrons begin to move because of the presence of holes.
This motion becomes a means for the flow of current . thus, at ordinary temperature, a crystal of germanium or silicon becomes a semi-conductor due to presence of free electrons and holes present in ti is much smaller as compared to the number of free electrons in a conductor.