electron leaves a vacancy to complete bonding. This ‘deficiency’ of electron is termed as a ‘hole’ Note The total current I is always the sum of the electron current and the hole current. That is, I h An intrinsic semiconductor behaves like an insulator at K. The increase in temperature increases the number of charge carriers (electrons and holes).
The schematic diagram of the intrinsic semiconductor in band diagram is shown in Figure . (b). The intrinsic carrier concentration is the number of electrons in the conduction band or the number of holes in the valence band in an intrinsic semiconductor . .
. Extrinsic semiconductors The carrier concentration in an intrinsic semiconductor is not sufficient enough to develop efficient electronic devices. Another way of increasing the carrier concentration in an intrinsic semiconductor is by adding impurity atoms. The process of adding impurities to the intrinsic semiconductor is called doping.
It increases the concentration of charge carriers (electrons and holes) in the semiconductor and in turn, its electrical conductivity. The impurity atoms are called dopants and its order is approximately ppm (parts per million). On the basis of the type of impurity added, extrinsic semiconductors are classified into: Free electron (a) Si + Si + Si + Si + Si + Si + Si + Si + P + Donor energy levels Conduction band Valence band Figure . n -type extrinsic semiconductor: (a) Free electron which is loosely attached to the lattice (b) Representation of donor energy level - - - - - - - - Unit electronics and Communication each impurity atom provides one extra electron to the conduction band in addition to the thermally generated electrons.
These thermally generated electrons leave holes in valence band. Hence, the majority carriers of current in an n -type semiconductor are electrons and the minority carriers are holes. Such a semiconductor doped with a pentavalent impurity is called an n -type semiconductor. ii) p -type semiconductor In p -type semiconductor, trivalent impurity atoms (from III group of periodic table) such as boron, aluminium, gallium and indium are added