Photoconductivity: Fundamental Concepts

Photoconductivity is defined, and the importance of electrical contacts are highlighted by examining the origin of the dark current flowing through a photoconductor. The Shockley–Ramo theorem is explained and the photoconductive gain and the necessary prerequisites for its manifestation are discussed. Major recombination kinetics are addressed, including the Shockley–Read–Hall statistics, Simmons–Taylor formulation, and Langevin recombination, and their main features are highlighted. Photoconductivity experiments have been extensively used by numerous researchers to characterize various semiconductor materials. Principles of steady-state and modulated photoconductivity (frequency-resolved photoconductivity) are introduced along with their main features in extracting material characteristics such as the density of states in the bandgap. The effects of traps on transient photoconductivity and modulated photoconductivity are discussed in simple terms and extended to include the Taylor–Simmons formulation. The phase and the amplitude of the modulated photocurrent are explained in relation to the density of states in the bandgap. Major noise sources in a photoconductor are described and the importance of generation–recombination and flicker noise are highlighted. Particular attention is given to noise in a-Si:H films and its dependence on doping.