Electron Hole Pairs

Electron‒Hole Pairs

• When a valence electron absorbs the energy, it becomes free electorn.

• Thus a valence electron leaves the valence band and enters the conduction band.

• When a valence electron drifts from the valence band, a vacancy is created in the valence band. Such a vacancy is called a hole. As negative charged electron leaves behind a hole, the hole is treated as positively charged.

• Whenever an electron becomes free, a hole gets genereted in valence band.

• Fig. 1.2.1 shows a formation of electro‒hole pair.

• Such a generation of electro‒hole pairs due temperature is called thermal thermal generation in semiconductors.

Concept of Hole Current

• When a battery is connected to a semiconductor, free electrons move towards positive of battery and constitute a current called an electron current. The free electrons move in conduction band.

• Consider three atoms as shown in Fig. 1.2.2.

• There is a hole in atom 1. When battery is applied, electron from atom 2 shifts into hole of atom 1, creating hole in atom 2.

• Then electron from atom 3 shifts into hole of atom 2 creating hole in atom 3.

• Thus hole moves from atom 1 to atom 3 and like this it gets attracted towards negative of battery.

• As holes are positively charged, movement of hole constitutes a current called hole current. The holes always move in valence band.

• In semiconductors, the total current is combination of electron current and hole current.

• The direction of conventional current is always opposite to the direction of free electrons i.e. from positive to negative of battery, external to the battery.

Review Question

1. Explain the concept of hole current.