Storage mode operation of a phototransistor and its adaptation to integrated arrays for image detection

It will be shown that a phototransistor possesses all the necessary requirements to operate in a photon flux integration mode; i.e. 1) a storage element, 2) a nearly ideal switch, and 3) a current source which depends on incident illumination level. The technique for realizing this mode of operation with a phototransistor and the resulting performance will be described. It will be shown that the signal charge will be enhanced by the gain of the transistor over that available from a comparable photodiode operated in the same mode. The simplicity of the structure permits fabrication of monolithic integrated arrays of closely spaced detectors without the need for electrical isolation, either with a dielectric or a junction. As a result a fundamental restriction on spacing density has been eliminated. Also, the restrictions imposed on spectral range and sensitivity by an isolation have been eliminated. Both linear and two-dimensional arrays may be fabricated using this structure. Sampling techniques will be described for a two-dimensional array which eliminates both crosstalk and the need of multiple load resistors for signal recovery.