Indirect Bandgap: Optical Detectors and Phonons

This lecture discusses the principles of optical detectors, focusing on semiconductors with an indirect bandgap, such as silicon. The instructor explains the significance of energy and momentum conservation when transitioning electrons between the valence and conduction bands. The lecture includes exercises that explore the relationship between photon wavelengths and acoustic phonons, emphasizing the differences in their respective energies and momenta. The instructor highlights that while photons provide energy, phonons are necessary to conserve momentum, making the absorption process in indirect semiconductors less efficient than in direct ones. The band structure of silicon is analyzed, illustrating the alignment of valence band maxima and conduction band minima. The lecture concludes with a discussion on the effective mass of electrons and holes, as well as the group velocity of electrons, providing a comprehensive understanding of the dynamics within indirect bandgap materials and their implications for optical detection applications.