Question 4 (i) Explain how you would use optical absorption measurements to differentiate between a direct and indirect gap semiconductor? (ii) Where would you expect to measure the optical absorption edge in germanium at 4 K? The indirect band gap is 0.74 eV at this temperature. Hint: the phonon energies for germanium at the L point are Longitudinal acoustic (LA): 0.027 eV Transverse acoustic (TA): 0.008 eV Longitudinal optic (LO): 0.030 eV Transverse optic (TO): 0.035 eV

Photoconductive Cell

Photoconductive cells are light-sensitive resistors in which resistance is inversely proportionated to light intensity when illuminated. These devices consist of a thin single-crystal or polycrystalline film of composite semiconductor materials. Cadmium sulfide (CdS) is used to make the most commercially available photoconductive particles which are sensitive to light in the visible spectrum. Other less commonly used substances respond to infrared light, but not to the visible spectrum. Such substances are lead sulfide (PbS), lead selenide (PbSe), and lead telluride (PbTe), and so on.