A p-type or n-type semiconductor can be converted into a p-n junction by doping it with suitable impurity. The motion of majority charge carriers causes diffusion current across the junction while the barrier electric field causes motion of minority carriers for drift current. In case of unbiased diode, the diffusion and drift currents are equal. This equilibrium is disturbed by the biasing batteries. Diodes, therefore, allow currents in one direction. This property of diode is used in making rectifiers.
(i) Silicon is doped with which of the following to obtain p-type semiconductor ?
(A) Phosphorus
(B) Arsenic
(C) Boron
(D) Antimony
(ii) A semiconductor has an electron concentration of 5x1022 m-3. The concentration of holes is (given ni=1.5x1016 m-3)
(A) 5x1022 m-3
(B) 1.5x106 m-3
(C) 9x108 m-3
(D) 4.5x109 m-3
(iii) During forward biasing of a p-n junction diode, the
(A) current is mainly due to drifting of majority carriers.
(B) current is mainly due to drifting of minority carriers.
(C) diffusion and drift currents are equal.
(D) current is of the order of 1 A.
(iv) The threshold voltage for silicon diode is about
(A) 0.2 v
(B) 0.5 v
(c) 0.7 v
(D) 1.5 v
OR
When we dope Ge with a pentavalent element, four of its electrons bond with four germanium neighbours but fifth electron remains weakly bound. The ionisation energy for this electron is about
(A) 0.01 eV
(B) 0.05 eV
(C) 0.1 eV
(D) 0.15 eV
