Semiconductors, Doping and the p-n Junction
A small gap lets thermal excitation put carriers into the conduction band, and impurities decide which carrier dominates. Joining two differently doped regions builds a field at the boundary that lets current pass one way.
What a learner can do afterwards
- Relates carrier concentration to gap size and temperature
- Explains what donor and acceptor doping do to the Fermi level
- Describes the built-in field of a junction and why it rectifies
1 · Read
A semiconductor has a small gap, so warmth alone lifts some carriers into the conduction band. A smaller gap or a higher temperature means more carriers; that link between gap, temperature, and carrier count runs the whole story.
Impurities decide which carrier dominates. Donor doping adds electrons and lifts the Fermi level toward the conduction band, while acceptor doping leaves holes, empty seats in the nearly full band that carry current as positive charges, and pulls the Fermi level down.
Join p-type material to n-type and carriers wander across, leaving uncovered charges behind. That charge builds a field at the boundary pointing against further crossing: the built-in field of the junction.
The built-in field is why the junction rectifies. One push direction flattens the field and current flows; the opposite direction steepens it and only a trickle passes. Current goes one way.
Warmth fills the band, doping picks the carrier, and the junction field opens the road in one direction only.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.