Operation of p-n Junction Photodetectors in a Photon Flux Integrating Mode

A technique for operating a p-n junction photodiode in a photon flux integration mode is described. In this mode the p-n junction is charged to a reverse voltage (less than its breakdown voltage) and then open-circuited. The voltage across the junction, with zero incident illumination, will decay at a rate that is independent of junction area. Time constants in the order of seconds may be achieved with silicon planar structures at room temperature. Under illumination, the rate of decay of charge depends linearly on the intensity of the incident illumination, so that the total charge removed is proportional to the time integral of illumination. Operation of p-n junction photodiodes is analyzed for this mode and boundary conditions are established. A practical structure utilizing this mode of operation is discussed. This structure makes use of the nearly ideal switch characteristics of an insulated gate field-effect transistor to periodically sample a photodiode. Advantages offered by this device structure include 1) linear dependence of signal charge on light intensity over several orders of magnitude; 2) electronically controllable sensitivity; 3) ease of integration into arrays for image sensing.