Q.Write two differences between intrinsic and extrinsic semiconductors.

A pure semiconductor -- pure silicon or pure germanium, with no impurity added -- is called INTRINSIC. Each silicon or germanium atom has a valence of four and forms four covalent bonds with its neighbours in a tetrahedral arrangement; at absolute zero, every bond is complete and no free carriers exist. At room temperature, thermal agitation breaks a small number of these bonds, promoting a valence electron into the conduction band and leaving behind a vacancy -- a HOLE -- in the valence band, which behaves as an effective positive charge. Because every broken bond produces one electron and one hole together, an intrinsic semiconductor always has equal carrier densities, $n_e=n_h=n_i$ (the intrinsic carrier density), and conduction happens through BOTH carrier types drifting in opposite directions when a field is applied -- electrons toward the positive terminal, holes (via a relay of neighbouring electron jumps) toward the negative terminal. Because so few of a crystal's roughly $5\times10^{22}$ atoms per $cm^3$ actually contribute a carrier this way, intrinsic conductivity is comparatively low and not easily controlled. …