Phototransistors

All phototransistors are based on a photodiode as the photon-to-current converter but include an emitter diffusion creating an npn transistor. The current produced by the photodiode is increased by the h_FE of the transistor structure yielding light currents in the milliamp to tens of milliamps. However, the speed-of-response of the photodiode is compromised by the transistor structure with the internal base resistance and the base-collector capacitance (r_b-b’ C_C) time constant being the primary driver.

Phototransistors have been a primary sensor for decades and continue to be the sensor-of-choice when speed is not of primary importance, but there is need for significant output light current.