PN junctions and diodes
Why does no current flow externally in a PN junction in thermal equilibrium?
- ABecause carriers do not move at all in the depletion layer
- BBecause the diffusion current and the drift current due to the built-in field cancel each other
- CBecause all electrons and holes have recombined
Show answer and explanation
Answer: B. Because the diffusion current and the drift current due to the built-in field cancel each other
In a PN junction in equilibrium, the diffusion current caused by the difference in carrier density balances the drift current flowing in the opposite direction due to the built-in field. The net current is zero for both electrons and holes, so no current flows externally.
- A. Carriers do move, but diffusion and drift balance so that the net flow is zero.
- B. Correct. The diffusion and drift currents balance.
- C. Not all carriers recombine; majority carriers remain in the neutral regions on both sides.