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 the electrical behaviour 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 bonds with three neighbouring 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 we refer to this vacancy as a hole.
Each hole gets created from one impurity atom. So far we have explained, about the creation of holes but did not focus how a hole associated with static impurity atom can move in the crystal. 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 neighbouring 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 relevant point of view, we can say that the hole moves from its previous position to a new position.
Same things will happen at a new position of the hole, and hence, the hole will further move to another new position. That 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 get caused due to the breakdown of covalent bonds 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, the number of free electrons in a p-type semiconductor is much smaller than the number of holes in it. That is why we consider holes as majority carriers, 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.
