Performance of Photoconductors

All semi-conductors and insulators are photoconductors. The characteristic parameter of a photoconductor is the life time of a free carrier. The photo-electron current is equal to the product of absorbed photon current and the ratio of life time to transit time of a free carrier. This relation holds as well for the commonly known types of barriers as for uniform photoconductors. The photo-electron current may range from a small fraction of the photon current to many powers of ten greater than the photon current depending on the ratio of life time to transit time. There is good evidence for life times in different photoconductors extending from 10-12 seconds to values approaching a second. The life times of free electrons and free holes are in general independent of each other and markedly different. Only at sufficiently high excitation rates for which the free carrier densities exceed the densities of bound states do the electron and hole life times necessarily become equal. The usual termination of the life time of a free carrier is by recombination with a deep lying bound state in the forbidden zone. These recombination processes are structure sensitive, complex, and generally not amenable to exact solution. A useful insight can be gained by dividing the bound states into deep-lying states, called ground states, that govern the rates of recombination and thereby the free carrier densities; and shallow lying bound states, called traps, that are responsible for observed response times exceeding the carrier life times.