Excess Carriers in Semiconductors

Abstract

Most semiconductor devices operate by creation of charge carriers which are in excess of the charge carriers available at thermal equilibrium. Conduction in semiconductors are dominated by these excess carriers. Therefore, in this chapter optical absorption and its mechanism, absorption coefficient and factors affecting it, are explained. Luminescence, phosphorescence, electro-luminescence, mechanism of excitation and recombination are discussed. Carrier lifetime and its derivation, indirect recombination, steady state carrier generation and quasi-Fermi levels are described. Photoconductivity and photoconductive devices, photoconductive cell, photo-multiplier tube, and factors affecting the selection of semiconductors for photo-conducting purpose are explained. Concepts of diffusion of carriers, analysis of drift and diffusion carriers, Einstein relation, continuity equation for diffusion and recombination are presented. Equations have been derived for diffusion and diffusion length. Solution has been provided for charge transport and impurity distribution by error function method and Gaussian distribution method. Long diode and short diode are also discussed. Quantum efficiency, trapping centre, dark resistance of photoconductor, and derivation of junction-voltage etc. are briefed through solved solutions. Minute insight into the various topics are given through solved numerical and theoretical examples. Review questions, numerical problems and objective type questions are also given with their answers.