Question

Q6. [20 Marks] (a) Write short notes on electrical insulators, conductors, and semiconductors (3mrk) (b) Write notes on intrinsic and extrinsic semiconductors (3mrk) (c) Define the terms valence band, conduction band and forbidden gap (3mrk) (d) Describe in brief, the Band theory of formation of energy bands in solids (4mrk) (e) (i) Explain what is meant by Density of States (DOS) (1mrk) The density of states function for electrons in a metal is given by: Z(E)dE = 13.6x1027 E2 DE (a) Calculate the Fermi level at a temperature few degrees above absolute zero for copper which has 8.5 * 1028 electrons per cubic meter. (3mrk) (3mrk) (B) Calculate the velocity of electrons at the Fermi level in copper. 3

Answer 1

insulator The material that has no free elecfrom The band gap in insulator is so higes about (5 ev) that it can not carry electrical current by the action of applied voltage conductors : The material which has infinite numbe of free electrons. The valance band and Conduction band overlap in this case and it can carry current under application of applied electric field. semi-conductors :- semiconductors has medium arround it can carrry small amount of intrinsic semiconductor : The pure form of semiconductor with out dopping is called intrinsic semiconductor: extrinsic semiconductor : The pure semiconductor when dopped with diffent element As, is Called extrinsic semiconductor band gap lev. and Valance band :- The band which is occupied the valance election is called valance band. Conduction band when the electrons aquries high energy then they become free electron The band where and carry curet. free elections are occupied Conduction band US called conduction band Valance band partially filler for bidden gap!- the energ gap between valance band and conduction band is called forbidden gap completely filled

and than the An isolated atoom processes discrete energies of diffennt atom. when two atoms come close to each other, their wave function overbap. the energy of the combined system will be lower & and larger energy of separate atom. If n numbers of atoms Comes and interacts with each other they closely spaced energy wel band. levels and produce energy produce N