**2. Energy bands**

Silicon is a semiconductor whose number of free electrons is less than conductor but more than that of an insulator. Two kinds of energy band which are conduction and valence. Series of energy levels having valence electrons forms valance band in solid. At 0°K, the energy levels of valence band are filled with electrons. This band contains maximum of energy when the electrons are in valence band. Conduction band is the higher energy level band, which is the minimum of energy. Conduction band is partially filled by the electrons, which are known as the free electrons as they can move anywhere in solid. These electrons are responsible for current flowing. There is a gap of energy. The difference between the conduction band and valence band is called energy gap. For semiconductors, the gap is neither large nor the bands get overlapped (**Figure 1**).
