Semiconductor

                                            Semiconductor

Semiconductor is defined as the solid whose electrical conductivity lies between conductors and insulators. It may be noted that semiconductors show conductivity due to presence of impurity and defect. Semiconductors are of two type: Intrinsic semiconductor and Extrinsic semiconductor.

It may be noted that in intrinsic type of semiconductor, equal number of holes and electrons are present. This is due to the reason that, these are normally insulator at room temperature but become semiconductor when temperature will be increased. For example, pure silicon and germanium are insulator at room temperature because they have network of lattice of four covalent bond and no free electrons are present. when temperature is increased then some of the covalent bond may break due to thermal vibration which produces free electrons. These free electrons make them semiconductors. It may be noted that when the crystal is connected to electric current these electrons move leaving behind positive hole at the site of missing electron. Now the crystal can conduct electricity because when electric current is applied then electrons moves in one direction and electrons move toward other direction. So, due to this, equal no of holes and electrons are present.

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Doping plays a very important role to increase the conductivity of Si and Ge by doping it with some other element. Si and Ge belongs to group 14 of the periodic table and have four valence electrons. Hence in their crystal Si and Ge atom can form four covalent bond with its neighbors. Now consider some of the atom of group 15 having five valence electrons such as Arsenic and phosphorous are added to the Ge and Si crystal. Hence fours of the five electrons of group 15 are shared by group 14 and form covalent bond while fifth electron of group 15 remain free. Hence this delocalize electron is responsible for increase in the conductivity of Si and Ge. So, in this case conductivity increases due to negative charge electron. that’s why this type of semiconductor is called N-type semiconductors.