Phototransistor operation in the charge storage mode

Phototransistor performance is considered for the charge storage mode, in which the phototransistor is briefly pulsed and then permitted to remain with both junctions reverse-biased while in the presence of illumination. The charge delivered with the next pulse is proportional to the integral of light received since the last pulse. A lumped model of the transistor, amended here to account for photogeneration, is presented which employs the relative excess densities of minority carriers as the terminal variables. By using the relative excess densities as variables, both the drift and diffusion components of current transported through a nonuniformly doped base region may be represented by a single element. In addition, all of the elements in the model are measured in electrical units, which allows simple use of the model without bringing in the detailed physics unless desired. Simple experimental measurements are outlined by which parameter values are determined, and the results for a particular device are presented. Using the model, a complete analysis of phototransistor operation in the charge storage mode is presented. The equations derived in the analysis are solved on a computer using the measured values of transistor parameters. The peak output voltage is predicted as a function of the light-generated current, along with the time response of the base-emitter and output voltages. The precise correspondence of the computed and measured results verifies the accuracy of both the model and the analysis.