Low-light-level properties of the phototransistor charge-storage mode

The phototransistor charge-storage or integration mode at low light levels is described accurately using a simple model. The base-emitter junction governs charge readout at low levels, causing very nonlinear circuit behavior. Sluggish transient response, increased sensitivity of output to transistor current gain, and a nonlinear transfer characteristic degrade phototransistor low-light-level behavior more severely than junction dark current.