Low-level response of phototransistors operating in the charge storage mode

A formula is derived relating the peak output voltage of the phototransistor operating in the charge storage mode to the average illumination current for low signal levels. This formula reveals the non-linearity in response apparent at low signal levels caused by the presence of the base-emitter junction. Sensitivity formulas are derived which enable the calculation of uniformity in response from device to device in an image sensing array.