P type semiconductor

1.09 P type semiconductor

The pure silicon is doped with a group 3 element such as boron, aluminium or indium. These materials have atoms with three valence electrons (trivalent atoms). The three electrons will form covalent bonds with neighbouring silicon atoms. However there are not enough electrons to form the fourth covalent bond. This leaves a hole in the covalent bond structure and therefore a hole in the valence band of the energy level diagram. Every impurity atom will produce a hole in the valence band. These holes will drift to produce an electrical current if a voltage is applied to the material and the P type semiconductor is a much better conductor than the intrinsic pure silicon material.

p_type
  • The silicon atoms form a square lattice
  • The green atoms represent the donor atoms
  • three of the four covalent bonds are formed with neighbouring silicon atoms
  • The fourth bond cannot be formed as there are not enough electrons, this leaves a hole in the valence band
  • Each donor atom produces a hole in the valence band

Note it is important to point out that the material is called P type semiconductor because the majority of charge carriers which contribute to an electrical current are positively charged holes produced by the doping process. There will be some contribution to the current flow from negatively charged electrons due to electron hole pair generation but these electrons are the minority charge carriers in this material. The P type material itself is not positively charged because the negative charge of the electrons of the donor atoms are balance by the positive charge in the nucleus.

Energy band diagram
The diagram below shows an energy band diagram for P type semiconductor. The valence band contains holes due to the incomplete covalent bond around each donor atom. The conduction band is empty as there are no free electrons.

p_energy_bands

Note this diagram does not show the electron hole pairs that would be present due to thermal energy. The electron hole pairs are minority charge carriers in P type semiconductors, the majority being the holes produced by the doping process.

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