P-n Junction Formation
Immediately after a p-n junction is formed and left without any external battery, a thin region near the boundary becomes free of mobile carriers. Which statement correctly describes this depletion region at equilibrium?
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About This Question
- Subject
- physics
- Chapter
- semiconductor electronics
- Topic
- p-n junction formation
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
It contains immobile ions producing a potential barrier that stops further diffusion
When p-type and n-type regions are joined, the steep concentration difference makes electrons diffuse from the n-side to the p-side and holes diffuse the other way. As carriers cross and recombine near the boundary, they leave behind the fixed donor and acceptor ions of the lattice: positive donor ions on the n-side and negative acceptor ions on the p-side. These uncovered immobile charges create an internal electric field and an associated potential barrier (about 0.7 V in silicon) directed so as to oppose further diffusion. Equilibrium is reached when this barrier exactly balances the diffusion tendency, leaving a carrier-free depletion layer. The option describing free-flowing electrons is wrong because the region is precisely where mobile carriers have been swept out. The option saying the region disappears is wrong; it persists as long as the junction exists. The option of a large steady current is wrong because at equilibrium the diffusion and drift currents cancel, giving zero net current. As a check, the existence of a built-in barrier with no net current is exactly what makes an unbiased diode non-conducting.
This easy difficulty physics question is from the chapter semiconductor electronics, covering the topic of p-n junction formation. It appeared in the 2025 exam.
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