why does electrical conductance rise for a semi conductor with increase in temperature?

Answer 1

When you heat up the crystal lattice of the semiconductor, the valence electrons gain energy from the heat. That extra energy translates directly into them needing less energy to move into the conduction band (which means lower resistance, higher conductance).

At 25C if it takes 0.6 electronvolts of energy to move an electron from the valance band to the conduction band, and you heat the thing up to 100C, you would add about 6 millielectonvolts of energy to that electron (KT/q), so now the energy reqired to move it to the conduction band is only 0.594 eV -- less resistance, more conductance.

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