n and p Type Semiconductors
An intrinsic semiconductor is one to which a small amount of impurity has been added. Roughly one in a million atoms is replaced by an atom of another element in a process called doping. Doping can change the electrical properties of the material. Adding one in a hundred million foreign atoms to germanium will increase the conductivity by a factor of 12 at 300 Degrees Kelvin. Very precise doping may be achieved by neutron irradiation.
The most common semiconductor is silicon. Silicon is made up of tetravalent (capable of forming four bonds) atoms joined in a lattice. Two types of semiconductor can be made by doping with different impurities.
An atom from group V has five electrons in the outermost shell, so will act as an electron donor in a silicon structure where each atom has four outermost electrons so can form four bonds. Conversely, an atom from group III has three electrons in the outermost shell, so will generate a space – hole – for an electron in a silicon structure where each atom has four outermost electrons so can form four bonds.
Suppose silicon is doped with aluminium (group 3) to produce a p – type semiconductor. The lowest empty energy levels of aluminium lie just above the full valence band of silicon. Electrons can easily jump from the full valence band to the empty conduction band of aluminium, called acceptor levels. When an electron jumps up to one of these levels it leaves behind a hole in the valence band. It is the movement of holes within the valence band that causes most of the current flow in p – type semiconductors.
Suppose silicon is doped with phosphorus (group 5) to produce an n – type semiconductor. The energy levels of phosphorus lie just below the empty conduction band of silicon. Electrons can easily jump from the energy levels of phosphorus – called donor levels - to the empty conduction band of aluminium. In n – type semiconductors, conduction takes place mainly due to these electrons.
In fact both type of conduction occur in both types of semiconductors. In each type of semiconductors, the major charge carriers – holes or electrons – are called majority carriers and the minor charge carriers are called the minority carriers.