Conductor, Insulator, Semiconductor

Conductor:

In a conductor, the conduction and valence bands overlap as shown in Fig. 1. There is no forbidden energy band region. When an external electric field is applied, the electrons acquire additional energy and to higher energy level. These mobile electrons constitute a current. The overlapping of conduction and valence bands gives rise to high conductivity. The resistivity of conductors is of the order of 10^-7

Ω (ohm). Most metals are good conductors.

Fig:1: Conduction Band formation of Conductor

Insulator:

The energy band structure of insulators is shown in Fig. 2. A large forbidden band of several electron volts, exists between valence and conduction bands. The valence band is completely filled at absolute zero temperature. The valence electrons remain tightly bound to the nucleus. As the temperature of the insulator is increased, the added heat energy enables some valence electrons to jump the forbidden gap and occupy the unfilled level above. At room temperature the number of such electrons is negligible and conductivity is very small. The resistivity of an insulator is very high and is around 10¹⁰ to 10¹⁶ Ω (ohm).

Fig:2: Energy Band Diagram

Semiconductor:

In some substances the forbidden gap between conduction and valence bands is small (about 1 ev). These materials are known as semiconductors. Germanium and silicon are the two examples of semiconductor. At 0K temperature the forbidden gap in germanium is 0.785 ev and in silicon it is 1.21 ev. Even at room temperature some valence electrons have enough energy and can move into the conduction band. Therefore, these materials are slightly conducting. The forbidden energy gap decrease with increase in temperature. The resistivity of semiconductors is of the order of 1 ohm-m.