Distinguish between intrinsic and extrinsic semiconductors.

Explain briefly what is meant by extrinsic semiconductors, including the terms donor and acceptor impurities as well as $p$-type and $n$-type semiconductors.

Using the concept of electron and hole currents, derive expression for the conductivity of a semiconductor.

What do you understand by band theory of energy in solids? Define valence band, conduction band and forbidden gap.

What is doping in semiconductors? Explain $p$ and $n$-type of semiconductors by selecting an element from third and fifth group and also explain acceptor and donor nature of these elements mixed with a pure semiconductor.