Question

3. Many parameters of crystals are dependent on the band gap. a) From the optical absorption spectrum of a certain semiconductor, one finds that the longest wavelength of radiation absorbed is 1.40 m. What is the band gap for this semiconductor? Explain how the band gap gives rise to this behavior. b) What band gap would be needed to make blue (450 nm) LEDs? Explain why. c) Most of the sun's radiation has wavelengths shorter than 1000 nm. For a solar cell to absorb all of this, what band gap ought the material have? Explain your reasoning. d) The band gap in Germanium is 0.72 eV. When used as a photon detector, roughly how many electrons can be made to jump from the valence to the conduction band by passage of a 760 keV photon that loses all its energy in the crystal?

Answer 1

Solution of above question is given below-

longert ware length - 1.4m Energy asssouated with = + photon of above wardenyth = 6.626 X 10 39 x 3x200 E 1:42 X 1025 J 1.42 x 10 Y - eN 16x 10-15 = 8.87 x 107 en [Band gap or semiconducts = 8.87% 10 er) In semiconductor, photon gines its energy to electoon election may have collision with other elections of atoms en valance band before going into Conduction band. Some enersy is lost ein this process. So Minimum enersy of photon will send election from valance band to conduction band. which gives rise to max warelayth of radiation in optical absosption spectum. (6) 2=450 nm 1-24x10 bevom Energy of photon (E)= he = 1 - 7 - 950x109 m. E = 2.75 ev Band gap for blue LED = 2.75N (0) À = 1ooo nm & - be - 1.29 X 10 ev-m 1000x109 m E = 1.84 ev gap for solar cell = 1.24 en 1 Band