What are intrinsic and extrinsic semiconductors?

Name two impurities that will make pure $\mathrm{Si}$ an extrinsic semiconductor.

Name the charge-carriers at room temperature in a conductor, an intrinsic semiconductor and an insulator.

Are the mobilities of electron and hole equal in a semiconductor?

Is the ratio of number of holes and number of conduction electrons in $n$-type extrinsic semiconductor more than, less than or equal to $1$?