- 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.
A p-type semiconductor is a semiconductor crystal doped with a small fraction of trivalent atoms so holes become the majority carriers. Each tetravalent host atom bonds to four neighbours and fills its outer shell to eight electrons. Adding those trivalent atoms to an intrinsic semiconductor changes how the crystal conducts.
Each trivalent atom takes the place of a host atom. It has three valence electrons, so it completes bonds with only three neighbouring atoms and is left with seven electrons in that shell.
That missing electron leaves three complete bonds and one incomplete bond. The vacancy is 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.
The same fill happens at the new site, so the hole moves again. That is hole motion in the crystal. A p-type semiconductor therefore has many holes moving through the lattice.
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.
Total holes equal acceptor holes plus thermal holes. Free electrons come only from those thermal pairs, so they are far fewer. Holes are therefore the majority carriers and electrons the minority carriers.
Common acceptors for a p-type semiconductor are boron, gallium and indium.
