Photoconductive Sensors

Photoconductive devices normally have two terminals. Illumination of a photoconductive device changes its resistance. Conventional techniques are used to measure the resistance of the photoconductor. Frequently, small changes in conductivity need to be observed in the study of material or device characteristics. Also, in the measurement of light intensities of faint objects, one encounters small photoconductive signals. Variations in photon flux density incident on a photoconductor interact with the material to change the conductivity. These changes produce a signal voltage that is proportional to the input photon flux density. Minimum detectable signal power, that is, noise-equivalent power (NEP) is a convenient means to express detector sensitivity. NEP is expressed in units of watts or WHz. The photoresponse of an extrinsic detector occurs when a photon interacts with an impurity added to a host semiconductor material. The quantum-well infrared photodetector is generally comparable to extrinsic photoconductive detectors, in that both have lower than desirable quantum efficiency.