a) i) Use classical electron theory {Drude theory) to derive an expression for the electron mobility in the conduction band of a semiconductor when electrons of effective mass m* are exposed to an electric eld E, and the time between scattering events is 1. Explain in detail the steps in your derivation and state under which conditions your approximation is valid. ii) Two-dimensional semiconductors are known to have lower electron mobility than bulk silicon. Use the expression for electron mobility to explain this fact