Intrinsic Semiconductors

Intrinsic Semiconductors

Intrinsic Semiconductors

Semiconductor devices, such as diodes and transistors, are made from a single crystal of semiconductor material (Germanium or Silicon). To make a semiconductor device, the very first step is to obtain a sample of semiconductor in its purest form.

Such a semiconductor is called an Intrinsic Semiconductor.

A semiconductor is not truly intrinsic unless its impurity content is less than one part impurity in 100 million part of semiconductor.

 To understand the phenomenon of conduction of current in a semiconductor, it is necessary to study its crystal structure.

Crystal structure of Semiconductor

When atoms bond together to form molecules of matter, each atom attempts to acquire eight electrons in its outermost shell. If the outermost shell of an atom has eight electrons, it is said t be filled. It then becomes a stable structure.

An intrinsic semiconductor has only four electrons in its outermost shell. To fill the valence shell, each atom requires four more electrons. This is done by sharing one electron from each of the four neighboring atoms. The atom align themselves in a uniform three-dimensional pattern so that each atom is surrounded by four atoms. Such patterns is called a crystal.

Following figure shows the 2D structure of an atom crystal.

Crystal Structure of Semiconductor

The core represent the nucleus and all the orbiting electrons except the valence electrons. Since there are as many protons in the nucleus as there are electrons orbiting it, the core will have contain 14 protons but only 10 electrons.

The valence electrons are shown around each core. Each of the four valence electrons take part in forming covalent bonds with the four neighboring atoms.

 

A covalent bond consists of two electrons, one from each adjacent atom. Both the electrons are shared by the two atoms. At absolute zero, all the valence electrons are tightly bound to the parent atoms. No free electrons are available for electrical conduction. The semiconductor therefore behaves as a perfect insulator at absolute zero.