CBSE Class 12-science Answered
The pure semiconductor whose conductivity is governed by the electrons thermally excited from valence band to conduction band is called an intrinsic semiconductor.
P-type semiconductor:
If we dope intrinsic semiconductor of Si, with a controlled amount of trivalent atoms say indium (In) or boron (B) or aluminium (Al), trivalent (say indium) impurity atoms will occupy places of some Si-atoms and there will be one incomplete covalent bond with a neighboring Si-atom, due to the deficiency of an electron. This is completed by taking an electron from one of the Si-Si bonds, thus completing the In-Si bond. This makes it ionized (negatively charged), and creates a hole or an electron deficiency in the covalently bonded electron system in the crystal as shown in figure. The trivalent atoms are called acceptor atoms and this extrinsic semi- conductor is known as p - type semiconductor.
n-type semiconductor:
If we dope silicon, which has four valence electron, with a controlled amount of pentavalent atoms, say arsenic (As) or antimony (Sb) or phosphorus (P), which have five valence electrons, the atoms, of the impurity element will substitute the silicon atoms (see Fig). Four of the five valence electrons of As are shared in covalent bonding while the fifth electron is comparatively free to move. The pentavalent atoms are called the donor atoms because they donate electrons to the host crystal. Extrinsic semiconductor is called n- type. On giving up their fifth electron, the donor atoms become positively charged. However, the material remains electrically neutral as a whole.