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Semiconductors

A-Level semiconductor extension: energy bands, intrinsic and doped materials, p–n junction behaviour, applications, simulations and dedicated practice.

  • A-Level Semiconductor Physics extension
  • 6 lessons
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Before you begin

Semiconductor behaviour makes sense when you connect three models: available electron energies, the balance of mobile carriers, and the junction field. This is an extension beyond the H2 syllabus; complete the core electricity topics first if exam time is limited.

Be comfortable with: quantum physics, current of electricity and basic diode behaviour from rectification.

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.

  1. Formation of Energy BandsBuild the qualitative band model and compare metals, semiconductors and insulators.Beyond the syllabus
  2. Intrinsic Semiconductors and Charge CarriersTrack electron–hole pair creation and explain the temperature dependence of conductivity.Beyond the syllabus
  3. Doping SemiconductorsDistinguish donor and acceptor doping, majority and minority carriers, and bulk neutrality.Beyond the syllabus
  4. The p–n JunctionExplain diffusion, the depletion region, the built-in field and forward or reverse bias.Beyond the syllabus
  5. Why Are Semiconductors Important?Connect controllable conductivity to diodes, sensors, light emitters and transistor switching.Beyond the syllabus
  6. Semiconductors Checkpoint QuestionsAnswer short written questions that expose band, doping and junction misconceptions.Beyond the syllabus

Practise and check

Topic reference

The organising model

Energy-band comparison for metals, semiconductors and insulatorsThree labelled energy diagrams compare overlapping bands, a small forbidden gap and a large forbidden gap.Metalconduction statesvalence statesbands overlapmany mobile electronsSemiconductorconduction statesvalence statessmall gapcarriers can be excitedInsulatorconduction statesvalence stateslarge gapvery few carriersEnergy increases upwards; the drawing is qualitative and not to scale.
Scroll diagram horizontally to read all labels.
Electrical behaviour depends on the occupied states and the energy needed to reach available conducting states: overlap for a metal, a modest gap for a semiconductor and a much larger gap for an insulator.

The band picture explains why carrier availability can change strongly with temperature, light or doping. The circuit picture then explains what those carriers do under an applied field.

Interactive device lab

Connect intrinsic carriers, doping and junction bias to diagrams and device behaviour.

Concept Explorer: Semiconductor Devices Lab

Move from doping and depletion regions to diode bias and rectification without losing the carrier logic.

BetaReviewed enrichmentElectricityBest for: A Level semiconductor device revision
  • Doping Logic
  • Carrier Reasoning
  • Bias & Depletion Region
  • Rectification
Open the full interactive simulation on its own page

Use the standalone simulation page for the live controls, SVG scene, run modes, and scoring flow.

Open Interactive Simulation

The lesson stays lightweight and links out to the dedicated simulation page.

Revision checkpoints

Energy bands and carriers
  • A metal has accessible conducting states without a large energy input.
  • A semiconductor has a modest band gap, so heat or light can generate electron–hole pairs.
  • A hole is a mobile vacancy model, not a separate positive particle added to the crystal.
  • Conductivity depends on both carrier number density and mobility.
Doping and junctions
  • Donors provide electrons as majority carriers; acceptors provide holes as majority carriers.
  • Doping changes carrier concentration while fixed dopant ions preserve bulk neutrality.
  • Junction diffusion leaves a depletion region and built-in field.
  • Forward bias reduces the barrier; reverse bias increases it. A real diode still has a non-linear current–voltage relation.