The two energy bands in solids are valence band and the conduction band. Depending on the gap between the two bands, solids are classified as conductors, insulators and semiconductors. Semiconductors are those whose conductivity can be increased by doping and also by increasing the temperature. Doping results in P - type and N - type semiconductor. The P and N can be fused to form a PN junction diode which is used to rectify A.C. By sandwiching a P type between two N type and vice versa, transistor is produced which is used to amplify and sustain oscillation in an oscillator.
