Semiconductor Fundamentals · Topic 9 of 13
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PN junction
Where n-type and p-type meet — the building block that conducts in one direction.
What you’ll learn
- What forms at a p–n interface
- The depletion region and built-in potential
- Forward vs reverse bias
Why it matters
The p–n junction is the heart of diodes, solar cells, LEDs, and every transistor.
Explanation
When p-type and n-type material meet, electrons and holes near the interface recombine, leaving a carrier-free 'depletion region' with a built-in electric field. This field opposes further diffusion and sets up a built-in potential.
Applying forward bias (p positive) shrinks the depletion region and lets current flow; reverse bias widens it and blocks current. That asymmetry is rectification.
Visual explanation
Key terminology
- Depletion region
- The carrier-free zone at the junction.
- Built-in potential
- The voltage across the depletion region at equilibrium.
- Bias
- An externally applied junction voltage.
Example
Common mistakes
Watch out for:
- Assuming current flows equally both ways — the junction is a one-way valve.
- Forgetting the depletion region has no free carriers.
Real-world application
Solar cells generate power by separating light-created carriers across a p–n junction.
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PN junction
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Diode
Semiconductor Fundamentals
A single p–n junction packaged as a one-way current valve.
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