Physics · Modern physics. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
In a solid the electron energy levels spread into bands. What matters is the energy gap between the filled valence band and the conduction band. A conductor has no gap, an insulator has a wide gap and a semiconductor has a narrow one, so heat or light can lift electrons across it. A warmer semiconductor therefore conducts better.
Doping adds carriers: Group V atoms give free electrons (n-type) and Group III atoms give holes (p-type). The crystal stays electrically neutral in both cases.
A p–n junction conducts in one direction only, which makes the diode a rectifier. A transistor has two junctions; a small base current controls a much larger collector current, so it can amplify or switch. Switching circuits form logic gates.
Saying n-type material is negatively charged. Each donor atom brings one extra proton with its extra electron, so the material is neutral; only the majority carrier differs.
Answer: IB = 50 μA, IE = 4.05 mA, α ≈ 0.99.
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