What are n-type and p-type semiconductors? How is each made? Draw their energy band diagrams and explain why each is electrically neutral.

1. 1 · Define doping. Adding a tiny, controlled amount of impurity to a pure semiconductor makes an extrinsic semiconductor.
2. 2 · Explain n-type. Pentavalent donor (As, P): the fifth electron is freed; electrons are the majority carriers.
3. 3 · Explain p-type. Trivalent acceptor (B, In): one bond is missing, making a hole; holes are the majority carriers.
4. 4 · Draw the band diagrams. [Diagram: n-type with donor level Ed just below the conduction band and many electrons; p-type with acceptor level Ea just above the valence band and many holes]
5. 5 · Explain neutrality. Each free carrier is balanced by a fixed dopant ion, so total charge is zero.
6. 6 · Close in one line. So, the type of dopant decides the majority carrier, but the crystal stays neutral.

Scoring tip: Name one dopant for each type and write 'donor' and 'acceptor' beside the dashed levels in your diagrams.