- P-type Semiconductor Definition: A p-type semiconductor is defined as a semiconductor doped with trivalent impurity atoms, creating holes as the majority charge carriers.
- Role of Trivalent Impurity Atoms: Trivalent impurities, like boron, have three valence electrons that form bonds with the semiconductor atoms, leaving one incomplete bond or hole.
- Hole Movement: Holes move within the crystal when neighboring electrons fill these vacancies, creating a new hole in the previous electron’s place.
- Majority and Minority Carriers: In a p-type semiconductor, holes are the majority carriers, while electrons, generated by thermal excitation, are the minority carriers.
- Thermally Generated Electron-Hole Pairs: Thermal energy at room temperature can break covalent bonds, creating additional electron-hole pairs in the 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 the electrical behaviour of the crystal is drastically changed.
The trivalent impurity atoms replace some semiconductor atoms in the crystal. Each trivalent atom, with its three valence electrons, forms covalent bonds with three neighboring semiconductor atoms, leaving it with seven electrons in its outer orbit.
However, the trivalent impurity atom lacks one electron in its outer orbit, resulting in three complete covalent bonds and one incomplete bond. This vacancy is called 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.
Soon, an electron from a neighboring covalent bond fills the hole, forming a new covalent bond. This movement creates a new hole where the electron was. Thus, it appears as though the hole moves from one position to another.
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.
Apart from the holes created by trivalent impurities, p-type semiconductors also have thermally generated electron-hole pairs. These pairs form when thermal energy at room temperature breaks covalent bonds, adding free electrons to the p-type semiconductor.
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.
