Question

Crystal types are sometimes classified based on the type of bonding, for example ionic crystals. Name two other types of crystal. (b) A crystal is formed from N atoms. Give a brief description of the origin of energy bands in solids. (c) Sketch the band structure of an undoped semiconductor, label the conduction and valence bands, and the relevant energies. Mark the position of the Fermi energy. Make a second sketch and assume the semiconductor has been doped n - type, add in and label the appropriate levels (you don't have to add the Fermi energy on this sketch). (d) At room temperature germanium has an almost completely filled valence band separated by a 0.67 eV gap from an almost completely empty conduction band and so is a poor electrical conductor. Determine the maximum wavelength of electromagnetic radiation that will cause a marked increase in the conductivity. In what part of the electromagnetic spectrum does the photon lie? (e) In a pure germanium sample determine the probability, f(E), that a state at the bottom of the conduction band is occupied for the temperatures 300 K and 320 K. What is the fractional change in the probability of occupancy between the two temperatures? Forming a junction between two pieces of the same semiconductor material doped n-type and p-type, respectively, forms a p-n junction. A simple model predicts that the current through the junction, I, depends in the applied voltage, V, in the form I = I_s(e^ev/kT - 1) (f) Briefly explain how it may be possible to dope germanium w-type and p-type, name the majority carriers that result. (g) Determine the current through a p-n junction with a saturation current of 0.500 mu A operating at 300 K. with an applied voltage of 0.3 V, repeat the calculation for a voltage of-1 V. Briefly comment on the values an on the types of carries contributing to the current in both cases.

Answer 1

Giemaniuy ) au imbuity like Antimony , Axsehic eadt Cne rties ike Boren aluniniu galli to to dlifive into Gre Cystal tanless ekeles armajarty caies casters.

(C CB Ec Ec C.8. Devet V B Ev Undobed Sewti Conduct Send conductor C. B Cc EAccustey level V B ト·沈 Se네.corduchor -34 max

a) ellic ts Holecular Crysta b) 4 N states 2ees св VB 4N-states 4N - elechons -) Lattice Sbactn 0h lottuct as shou pacth atouts ,etteng ed-cheer and ㅘlitng f ibaudsoecus a Badsaumaty eyeh lled o partially filed