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9.2.1 Semiconductors

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  1. Semiconductor is a class of crystalline solid with conductivity between a conductor and an insulator.
  2. Example of semiconductors are:
    1. Silicon
    2. Germanium
    3. Boron
    4. Tellurium
    5. Selenium

The Silicon Crystal


  1. The typical example of semiconductor is silicon.
  2. Silicon has 4 valence electrons. Each of these 4 electrons are shared with another 4 silicon atoms to form 4 pairs of covalent bond, as shown in the diagram above.
  3. The bonded valence electrons are not free to move. Therefore silicon is not a good conductor at room temperature.
  4. At room temperature, a silicon crystal acts approximately like an insulator because only a few free electrons and holes are presence.

Free Electron and Hole


  1. If a bonded electron absorbs heat energy from the surrounding, it may be promoted to higher energy level.
  2. These electrons are free to move when they are at a higher energy level.
  3. If an electron is promoted to higher level, a vacancy is left in the valence shell, and it is called a hole.
  4. A hole has the tendency to pull electrons. Therefore a hole is assumed carries positive charge.
  5. Both of the free electrons and the holes can help to conduct electric current.
  6. Therefore, with the presence of the free electrons and holes, the conductivity of a semiconductor is higher than an insulator.

Resistance Change Due to Temperature Change

  1. As the temperature increases, more and more electrons are getting promoted to become free electrons and at the same time creating more and more holes. Therefore the conductivity of a semiconductor increases as the temperature increases.
  2. The graph below shows the resistivity change of a conductor and semiconductor against the temperature. The resistance of a semiconductor decreases as the temperature increases.

Flows of Free Electrons and Holes

  1. We have learned that, there are 2 types of charge carrier in a semiconductor, the free electrons and the holes.
  2. The free electrons carry negative charge whereas the holes carry positive charge.
  3. If a potential difference is applied to a semiconductor, the electrons and holes will start to flow.
  4. The electrons will flow to the negative terminal whereas the holes will flow to the positive terminal.
  5. Video below explain how the free electrons and holes flow in an electric field.