P Type Semiconductor
We all know that in semiconductor crystal each tetra valiant atom creates covalent bond with four neighboring atoms. In this way, each of the atoms in semiconductor crystal gets eight electrons in outermost orbit. Now if a small percentage of tri valiant impurity atoms are doped in the pure or intrinsic semiconductor crystal then electrical behavior of the crystal is drastically changed. Let us explain how the impurity atoms displace the same number of semiconductor atoms in the crystal and occupy their positions. Now three valence electrons of each trivalent impurity atom create covalent bond with three neighboring semiconductor atoms. In this way, each impurity atom gets 7 valence electrons at outermost orbit. But still there is lack of one electron in the outermost orbit of the impurity atom. In other words, there are three complete covalent bonds and one incomplete covalent bond with one electron. Hence, there is a vacancy for one electron and this vacancy is referred as hole.
Each hole is created from one impurity atom. So far we explained, about creation of holes but did not focus how a hole can move in the crystal as it is associated with static impurity atom. But in a semiconductor crystal holes can also move like electrons but the mechanism of movement is different. When one hole that is one incomplete covalent bond created, it will not remain incomplete lifelong.
Very soon electron of other neighboring covalent bonds breaks out and seats on that hole and makes a new covalent bond. The electron when breaks out from a covalent bond it creates a hole behind it. If we look at the matter in relative point of view, we can say that the hole is moved from its previous position to a new position. Same things will happen at a new position of the hole and hence, hole will further move to another new position. This is how the holes move in a semiconductor crystal. Finally, we can say that in a p-type semiconductor has plenty of holes move randomly inside the crystal.
In addition to holes generated due to trivalent impurity atoms in the p-type semiconductor crystal, there will also be thermally generated electron-hole pairs. Thermally generated electron-hole pairs mean those electron-hole pairs which are generated due to the breakdown of covalent bond due to thermal excitations at room temperature. These thermally generated electrons contribute free electrons in the p-type semiconductor crystal. Hence, the total number of holes in a p-type semiconductor is a sum of holes due to trivalent impurity atoms and holes generated due to thermal excitation whereas free electrons are only due to thermal excitation. Hence, number of free electrons in a p-type semiconductor is much smaller than number of holes in it.This is why holes are considered as majority carries and electrons are called minority carriers in a p-type semiconductor.
The trivalent impurity used for doping purpose of a p-type semiconductor are boron, gallium and indium.