Free Electron and Hole
- If a bonded electron absorbs heat energy from the surrounding, it may be promoted to higher energy level.
- These electrons are free to move when they are at a higher energy level.
- If an electron is promoted to a higher level, a vacancy is left in the valence shell, and it is called a hole.
- A hole has the tendency to pull electrons. Therefore a hole is assumed carries a positive charge.
- Both of the free electrons and the holes can help to conduct electric current.
- 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
- 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.
- 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.