Conductivity of semiconductors increases with increase in temperature. At absolute zero, electrons do not have any thermal energy and do not have the ability to conduct at all. Hence, at absolute zero, semiconductors behave like an ideal insulator.
Inside a solid crystal, each electron has a different energy level because of slightly different patterns of the surrounding charges. These electron energy levels form a continuous energy variation called as the Energy Bands. Energy bands of more tightly bound electrons have lower energy (more negative energy) as compared to that of loosely bound electrons.
Energy band gap of a semiconductor is defined as: EG=EC−EV where EC: Lowest energy level in conduction band EV: Highest energy level in valence band Note: 1. Energy band gap is the lowest energy required for a material to conduct. 2. At 273K, Eg,Si=1.14eV,Eg,Ge=0.67eV
Energy gaps in semiconductors
Semiconductors have a finite but small bandgap (<3eV). Because of this, at room temperature some electrons from valence band can acquire enough energy to to cross the energy gap and enter the conduction band. Hence, resistivity of semiconductors is not high as insulators.