Semiconductors
Go beyond the A-Level syllabus with energy bands, pure and doped semiconductors, the p–n junction and the devices built from them.
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Before you begin
Semiconductors are the materials inside every diode, thermistor, LED and computer chip. This extension topic goes beyond the H2 syllabus to explain where their behaviour comes from. The lessons start with the energy bands of a solid, then show how electrons and holes carry current in a pure semiconductor, how doping controls them, and how a p–n junction makes a diode.
Be comfortable with: current of electricity, because the topic explains the resistivity of a thermistor and the I–V graph of a diode in terms of charge carriers.
Learning goals
- Explain energy bands, intrinsic conduction and how temperature and light change semiconductor resistance.
- Explain n-type and p-type doping, donor and acceptor levels and majority carriers.
- Explain p-n junction formation and bias, and analyse diode, LED, Zener, rectifier-smoothing, sensor and transistor-switching circuits.
Lessons
Work through them in order.
- Formation of energy bandsUse energy bands and the band gap to explain why metals conduct, insulators do not and semiconductors sit in between.Beyond the syllabus
- Intrinsic semiconductors and charge carriersExplain conduction by electrons and holes in a pure semiconductor, and why its resistivity falls as it warms.Beyond the syllabus
- Doping semiconductorsExplain how donor and acceptor atoms make n-type and p-type semiconductors, and name their majority carriers.Beyond the syllabus
- The p–n junctionExplain how a depletion region forms at a p–n junction, and why it conducts under forward bias only.Beyond the syllabus
- Why are semiconductors important?Link controllable conductivity to sensors, diodes and transistors, and an LED's colour to its band gap.Beyond the syllabus
Practise and check
Topic reference
Quick reference
| Material or device | Charge carriers | Key behaviour |
|---|---|---|
| Metal | many free electrons; bands overlap | resistivity rises with temperature as collisions increase |
| Insulator | almost none; large band gap | does not conduct at room temperature |
| Pure semiconductor | equal numbers of electrons and holes; small band gap | resistivity falls steeply with temperature as more pairs form |
| n-type | electrons (majority), holes (minority) | made with Group V donor atoms |
| p-type | holes (majority), electrons (minority) | made with Group III acceptor atoms |
| p–n junction | almost none in the depletion region | conducts under forward bias (p to +), hardly at all under reverse bias |
| LED | electrons falling into holes | photon energy hc/λ ≈ Eg |
Core knowledge to remember
- Band gap, Eg: the energy difference between the top of the valence band and the bottom of the conduction band.
- Hole: an empty state in the valence band, which behaves as a positive charge carrier.
- Intrinsic semiconductor: a pure semiconductor, whose carriers are thermally generated electron–hole pairs.
- Doping: adding a small amount of impurity to a semiconductor to increase its carrier number density.
- Depletion region: the layer at a p–n junction with almost no mobile carriers, containing fixed dopant ions.
- Forward bias: the p-side connected to the positive terminal; reverse bias: the p-side connected to the negative terminal.
Common mistakes
- Explaining the falling resistivity of a heated semiconductor by fewer collisions. The number of carriers rises.
- Calling a hole a proton.
- Thinking n-type material is negatively charged. Doped crystals are neutral.
- Taking the direction the electrons move as the direction of the current.