(ii) Extrinsic Semiconductor

EXTRINSIC SEMICONDUCTOR

Impure semiconductors in which the charge carriers are produced due to impurity atoms are called extrinsic semiconductors. They are obtained by doping an intrinsic semiconductor with impurity atoms.

Based on the type of impurity added they are classified into

(i) n-type semiconductor

(ii) p-type semiconductor

(i) n-type semiconductor

n- - type semiconductor is obtained by doping an intrinsic semiconductor with pentavalent (5 electrons in valence band) impurity atoms like phosphorus, arsenic, antimony, etc.,

The 4 valence electrons of the impurity atoms bond with 4 valence electrons of the semiconductor atom and the remaining 1 electron of the impurity atom is left free as shown in Fig. 2.12.

Therefore number of free electrons increases. As the electrons are produced in excess, they are the majority charge carriers in n-type semiconductor and holes are the minority charge carriers.

Since electrons are donated in this type of semiconductor the energy level of these donated electrons is called donor energy level (E_d) as shown in Fig. 2.13.

E_d is very close to conduction band and hence even at room temperature the electrons, are easily excited to conduction band. The current flow in this type of semiconductor is due to electrons.

(ii) p-type semiconductor

p- type semiconductor is obtained by doping an intrinsic semiconductor with trivalent (3 electrons in valence band) impurity atoms like boron, Gallium, Indium, etc.,

The three valence electrons of the impurity atom pairs with three valence electrons of semiconductor atom and one position of the impurity atom remains vacant, this is called hole as shown in Fig. 2.14.

Therefore the number of holes are increased with the impurity atoms added to it. Since holes are produced in excess, they are the majority charge carriers in p-type semiconductor and electrons are the minority charge carriers.

Since the impurity can accept the electrons this energy level is called acceptor energy level (E_a) and is present just above the valence band as shown in Fig 2.15.

Here, the current conduction is mainly due to holes (holes are shifted from one covalent bond to another)

Effect of Impurity States

• Due to impurity atoms the energy band gap is very much reduced say upto 0.01 eV.

• Since there is no interaction between the impurity atoms, the energy levels of the impurity atoms will not appear as bands. Therefore the energy levels appear as isolated dots.

• In the case of n-type semiconductor the donor energy level is very close to the unfilled energy band (conduction band) so it can easily donate an electron to that unfilled state.

• In the case of p – type semiconductors the acceptor energy level is very close to the filled energy band (valence band) so it can easily accept the electrons from the filled state.

Note: The impurities added are of very small amounts, for example addition of Boron to Silicon in the proportion of 1 boron atom to 10⁵ silicon atoms increases the conductivity even at room temperature.