When it is discovered that the conduction capabilities of an intrinsic semiconductor are not enough, then a new type of semiconductor is manufactured by doping impurities into the pure semiconductor. And today that is known as an extrinsic semiconductor. Extrinsic semiconductors are of two types n-type and p-type semiconductors.
In this article N-TYPE SEMICONDUCTOR, we discussed the n-type semiconductor which means today we focus only on the p-type extrinsic semiconductor.
Since the current conduction through the crystal is due to holes, which act as positive charge, such semiconductor is called the p-type extrinsic semiconductor.
Where ‘p’ stands for positive.
What is a p-type semiconductor?
That material is called a p-type semiconductor in which the trivalent impurities are added into the pure semiconductor crystal to increase the conduction capabilities of the intrinsic semiconductor.
When the trivalent impurity atom such as boron, gallium, or aluminium is added to the pure intrinsic semiconductor (silicon or germanium) they form the covalent bond between them. An atom requires 8 atoms to complete its outermost valence shell.
The trivalent impurities are group 13 elements i.e they have 3 electrons in their valence shell. On the other hand, the pure semiconductor Si or Ge are group 14 elements i.e. they have 4 electrons in their valence shell.
When the trivalent impurity atom and the pure semiconductor atom try to form a covalent bond they require 8 electrons to complete their octet. But they fall short with one electron which creates a vacant space in the semiconductor crystal known as a hole.
These impurities create holes that can accept electrons, so these impurities are also called acceptor impurities.
A small quantity of impurity can produce millions of holes that can have an appreciable effect on the conductivity.
Energy band of a p-type semiconductor
The energy band of p-type semiconductor is shown below:
When the p-type impurities are added to the intrinsic semiconductor, they produce an allowable discrete energy level that is just above the valence band. The produced energy level is called the acceptor energy level. The gap between the valence band and the acceptor energy level is so small that it requires very little energy for an electron to leave the valence band.
Since a very small amount of energy is required for an electron to leave the valence band and occupy the acceptor energy level, holes are created in the valence band by these electrons. The holes created constitute the larger number of carriers in the p-type extrinsic semiconductor material.
Conduction in a p-type semiconductor
P-type semiconductor contains trivalent impurity which constitutes a large number of holes in the semiconductor.
When the electric field is applied across the semiconductor, the current conduction is primarily due to holes. As the holes are positively charged, they are directed towards the negative terminal and constitute the hole current.
Unlike the n-type semiconductor where the current conduction is by the electrons, in the p-type semiconductor, the valence electrons move from one covalent bond to another covalent bond.
Conduction in the p-type semiconductor is mainly by the holes, so the majority charge carriers are holes and the minority charge carriers are electrons in the p-type extrinsic semiconductor.