浙大光电子学课件chapter 5-1.pptVIP

Fundamentals of Photonics Chapter 5-1Photons in Semiconductors Semiconductors A semiconductor is a solid material that has electrical conductivity in between a conductor and an insulator. Semiconductors can be used as optical detectors, sources (light-emitting diodes and lasers), amplifiers, waveguides, modulators, sensors, and nonlinear optical elements. A. Energy bands and charge carriers Energy bands in semiconductors The solution of the Schr?dinger equation for the electron energy in the periodic potential created by the atoms in a crystal lattice, results in a splitting of the atomic energy levels and the formation of energy bands. Energy bands in semiconductors Each band contains a large number of finely separated discrete energy levels that can be approximated as a continuum. The valence and conduction bands are separated by a “forbidden” energy gap of width Eg bandgap energy Energy bands in semiconductors Electrons and holes In the absence of thermal excitations, the valence band is completely filled and the conduction band is completely empty. Thus, the material cannot conduct electricity. As the temperature increases, some electrons will be thermally exited into the empty conduction band, result in the creation of a free electron in the conduction band and a free hole in the valence band. Electrons and holes Energy-momentum relations Effective mass Effective mass Direct- and indirect-gap semiconductors Semiconducting materials Si: widely used for making photon detectors but not useful for fabricating photon emitters due to its indirect bandgap. GaAs, InP GaN etc.: used for making photon detectors and sources. Ternary and quaternary semiconductors: AlxGa1-xAs, InxGa1-xAsyP1-y etc. tunable bandgap energy with variation of x and y. Semiconducting materials Doped semiconductors Dopants: alter the concentration of mobile charge carriers by many orders of magnitude. n-type: predominance of mobile electrons p-type: predominance of ho