Before Knowing what intrinsic semiconductor is, let’s learn what a semiconductor is. It is not possible to think life without it. They are used in all modern electronic gadgets. They are generally made of silicon, which is very cheap and easily available.
What is Semiconductor?
Semiconductors are materials whose conductivity is between conductors (generally metals) and insulators (such as wood).
Material in which gap between valance band and conduction band is less than 3eV and electrons move to conduction band from valance band on supplying a maximum of 3eV are semi conductors.
There are two types of semiconductor. They are:-
- Intrinsic Semiconductor
- Extrinsic Semiconductor
In this article we shall deal with intrinsic semiconductor only.
Intrinsic Semiconductor
Semiconductors, which are free from impurities are termed as intrinsic semiconductors.
Thus, we can say that they are pure substances. They are chemically pure and electrically neutral. Therefore, the number of holes and electrons are equal. In intrinsic semiconductors, the number of excited electrons is equal to the number of holes; n = p.
Note: In previous sentence we used word excited electrons. Total electrons are not equal to number of holes. Rather, excited electrons are equal to intrinsic semiconductors.
They are also termed as undoped semiconductors or i-type semiconductors. We will further see what is doping.
Silicon and germanium are examples of i-type semiconductors. These elements belong to the 4th Group of the periodic table. Their atomic numbers are 14 and 32 respectively.
Working Mechanism of Intrinsic Semiconductor
We shall talk about Silicon (Si) and Germanium (Ge), as they are most widely used semiconductors.
We know that Si and Ge have four valence electrons. In its crystalline structure, every Si or Ge atom share its four valence electrons with each of its four nearest neighbour atoms. Also, it takes share of one electron from each such neighbour. This is covalent bond with valency 4.
We can assume that the two shared electrons move back and forth between the associated atoms holding them together strongly. This is similar analogy of shuttle of badminton.
The figure above shows a picture with no broken bonds. This happens only at low temperatures. As the temperature increases, more thermal energy becomes available to these electrons and some of these electrons break away. Thus, becoming free electrons contributing to conduction.
Note that all electrons do not become free.
The thermal energy effectively ionizes only a few atoms in the crystalline lattice and creates a vacancy in the bond as shown in figure below.
The neighbourhood, from which the free electron (with charge –q) has come out, leaves a vacancy with an effective charge (+q ). This vacancy with the effective positive electronic charge is called a hole.
The hole behaves as a free particle with positive charge equal to charge of electron.
In intrinsic semiconductors, the number of free electrons, ne is equal to the number of holes, nh.
That is, ne = nh = ni
where ni is intrinsic carrier concentration.
Semiconductors posses the unique property in which, apart from electrons, the holes also move. Usually we have seen only electrons move but here holes also move.
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