Practice portal › Semiconductor Electronics › p-n Junction Formation
Asked in GSEB Board July 2021 · Diffusion and drift currents
Idea: diffusion is driven by a difference in concentration alone, not by any force. Particles wander at random, and more wander out of a crowded region than into it.
The p-side is made hole-rich by its trivalent dopant while the n-side is hole-poor, so there is a steep hole concentration gradient across the junction.
That gradient by itself sends holes from p to n, and electrons the other way, the moment the junction is formed -- with no battery connected and no field to help.
The holes that leave expose negative acceptor ions behind them and the electrons that leave expose positive donor ions, so a potential difference builds up across the depletion layer, and that barrier then opposes any further diffusion.
So holes diffuse from p to n because the hole concentration in the p-region is higher than in the n-region.
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