A silicon photodevice to operate in a photon flux integrated mode

Abstract

An integrated structure which uses the nearly ideal switch characteristics of an insulated gate field effect transistor to sample a pn photodiode will be described. This structure permits operation of the pn photodiode in a photon flux integration mode. This mode of operation is based on the fact that if a pn junction is charged to a reverse voltage less than its breakdown voltage and then open circuited, the rate of decay of charge stored on the space charge capacitance with zero incident illumination is independent of junction area and can have a time constant in the order of tenths of a second. Under illumination this rate of decay of charge depends linearly on the intensity of the incident illumination. That is, the charge removed at any instant of time is proportional to the time integral of illumination. The insulated gate field effect transistor provides a high conductance when in the ON condition to charge the space charge capacitance and a very low conductance during the integration time. This structure lends itself very nicely to monolithic integration of linear arrays where individual photodetectors are sampled periodically on a time-shared basis. Such arrays may be sampled in any order providing the possibility of spatially preprocess information. This device provides not only better sensitivity but also improved S/N ratio over other electronic imaging devices. Several advantages offered by this device are: (1) linear dependence of signal on light intensity over several orders of magnitude; (2) electronically controllable sensitivity; (3) ease of integration into arrays for image sensing.