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Semiconductors of N-Type and P-Type

If we add a small amount of arsenic to silicon, several valence electrons of each arsenic atom will form covalent bonds with the neighbouring atoms of silicon. But only four electrons of each atom can take part in this process, because the fifth electron has no pair. As it is attached to the arsenic nucleus very loosely, it easily leaves the atom of arsenic and becomes a free electron. Thus each atom of arsenic will add one negative charge carrier to the silicon crystal and the resulting semiconductor will be called an N-type semiconductor. In this type of semiconductor electrons are majority charge carriers. Compared to the electrons the holes are in the minority in this semiconductor and they are called minority charge carriers. Sometimes an electron and a hole meet and recombination takes place.

If instead of arsenic, a group III element such as gallium is introduced into a silicon crystal, then each gallium atom will attempt to form a covalent bond with each of its four neighbouring silicon atoms. But gallium has only 3 valence electrons and only 3 bonds can be formed. Thus each atom of gallium introduces one hole into the crystal lattice. In this case holes are majority charge carriers. This type of impurity is called an acceptor impurity and the resulting semiconductor is known as a P-type semiconductor.

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