Mini Physics
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Why Are Semiconductors Important?

Why semiconductors matter: controllable conductivity enables diodes, transistors, ICs, and modern computing.

  • A-Level Semiconductor Physics extension
On this page

Learning objectives

  • Explain p-n junction formation and bias, and analyse diode, LED, Zener, rectifier-smoothing, sensor and transistor-switching circuits.
Syllabus note (9478 H2)

This page is enrichment / legacy. For current H2 Physics, focus on device behaviour (e.g. diode I–V and thermistor characteristics) in:

1. Definitions (Must Know)

Semiconductor (idea)

A semiconductor is a material whose electrical conductivity can be controlled (much more than a metal) using:

  • temperature,
  • light (photons),
  • impurities (doping),
  • electric fields.

Why “controlled conductivity” matters

If you can control whether a material conducts well or poorly, you can build:

  • switches (on/off) for digital logic,
  • amplifiers for signals,
  • sensors (light, temperature, etc.).

2. Key Ideas (What Earns Marks)

  • Semiconductors have a small band gap (compared with insulators), so carriers can be created by heat/light.
  • Doping changes carrier concentration dramatically:
    • n-type: electrons are majority carriers,
    • p-type: holes are majority carriers.
  • A p–n junction is the basis of the diode, and diodes/transistors are the building blocks of modern electronics.

3. Detailed Explanations

A. From diodes to computers

Modern computers work because semiconductors can act like:

  • a reliable switch (transistor on/off),
  • made very small and packed into billions on a single chip (integrated circuits).

B. Common real-world semiconductor devices

  • Diodes (one-way conduction; rectification)
  • Transistors (switching + amplification)
  • LEDs / laser diodes (light emission)
  • Photodiodes / solar cells (convert light → electrical signal/power)
  • Thermistors / LDRs (sensors)

4. Common Mistakes

  • Mixing up electrons and holes (holes behave like positive charge carriers in p-type material).
  • Saying “a semiconductor is just a poor conductor” without mentioning controllability (temperature/doping).

5. Exam Tips

  • When describing n-type vs p-type, always state the majority carrier.
  • For p–n junction questions, mention the depletion region and built-in electric field qualitatively.

6. Worked Examples

Modelled example 1

Temperature and intrinsic conductivity

Core

Problem

What generally happens to the conductivity of a pure semiconductor when its temperature increases?
Study the worked solution

  1. Track carrier creation

    Method

    More electrons gain enough energy to enter the conduction band.

    Reason

    Thermal energy helps electrons cross the semiconductor’s small band gap.

    Working

    T↑ ⇒ more electrons cross E_g

  2. Count both carrier types

    Method

    More electron–hole pairs are created.

    Reason

    Each promoted electron leaves a hole in the valence band.

    Working

    nₑ↑ and nₕ↑

  3. Infer conductivity

    Method

    The intrinsic semiconductor’s conductivity generally increases.

    Reason

    The larger mobile-carrier concentration outweighs the qualitative mobility change in this lesson’s model.

    Working

    σ↑

7. Mind Stretchers

Mind stretcher 1: ChallengeExtension

Why can semiconductors be both good switches and good sensors, while metals are usually poor switches/sensors?