Explain the following, giving reasons.
(a) A doped semiconductor is electrically neutral.
(b) In a p-n junction under equilibrium, there is no net current.
(c) In a diode, the reverse current is practically independent of the applied voltage.
(a) A doped semiconductor is electrically neutral.
(b) In a p-n junction under equilibrium, there is no net current.
(c) In a diode, the reverse current is practically independent of the applied voltage.
Solution
(a) A dopant atom brings as many protons as electrons, so adding impurity atoms does not create net charge; the extra carriers are balanced by the fixed ionised impurity charges, keeping the doped crystal electrically neutral.
(b) At equilibrium the diffusion current of majority carriers across the junction is exactly balanced by the drift current of minority carriers driven by the barrier field, so the net current is zero.
(c) The reverse current is carried by the few thermally generated minority carriers; even a small reverse voltage sweeps essentially all of them across, so the current saturates and hardly changes with further voltage.
(b) At equilibrium the diffusion current of majority carriers across the junction is exactly balanced by the drift current of minority carriers driven by the barrier field, so the net current is zero.
(c) The reverse current is carried by the few thermally generated minority carriers; even a small reverse voltage sweeps essentially all of them across, so the current saturates and hardly changes with further voltage.
CBSE Class 12 Board · 2024 · Subjective