What is P-type semiconductor?
when a small amount of trivalent impurity is added to a pure semiconductor, it is called a p-type semiconductor. The addition of a trivalent impurity provides a large number of holes in the semiconductor.
Trivalent impurities: Gallium, Indium, Boron ,Aluminum
The impurities which produces p-type semiconductors are known as acceptor impurities because the holes created can accept the electron.
Conduction in p-type semiconductor
The current conduction in a p-type semiconductor is predominating by holes i.e. positive charges and is called p-type or hole-type conductivity. When a potential difference is applied across the p-type semiconductor the holes (produced by impurity) in the crystal will drift towards the negative terminal, called hole current. In p-type conductivity, the valance electron moves from one covalent bond to another unlike the n-type where current conduction is by free electron.
On adding trivalent impurities into silicon such as Boron ,it is forced to bond with neighboring silicon atom, only three electron will participate in bonding and 4th is not present That is why hole formation takes place in the valance bond (the missing electron in the bond is called hole).The hole acts as a positive charge. Therefore, for each boron atom added one hole will be available in the silicon crystal .Through each boron atom added provides one hole ,yet an extremely small amount of boron impurity provides enough atoms to supply millions of holes.
If n and p be the electron and hole concentration in the semiconductor and Na be the acceptor concentration.
Then, hole concentration is given by:
The conductivity in p-type semiconductor is mainly due to hole concentration which can be calculated by,
Energy Level Diagram of P type semiconductor
A lager number of holes in p type semiconductor are created by trivalent impurities such as boron, gallium, aluminum ,indium.
The binding energy is very small nearly equal to 0.05ev, so at room temperature, all the acceptor sites are ionized by thermal vibration in the lattice. When the boron accepts electrons from neighbors using the si-si bond, the hole is shifted away from the boron to the si-si bond in the valence bond. The boron atom accepts an electron from the si-si bond and resulting in a semiconductor called a p-type semiconductor.