Phototransistors

Phototransistors are used in many applications. Photodiodes are widely used as optical detectors. Avalanche photodiode (APD) and positive-Intrinsic-Negative (PIN) semiconductor photodiodes are examples of photodiodes which are extensively used in LIDAR and fiber optic communication. Phototransistor is an optical detector which is capable of providing an internal gain of the photocurrent. In a phototransistor this gain is accomplished by ‘transistor action’. Same property is exhibited by avalanche photo diodes but action is due to avalanche multiplication. In a phototransistor, base remains unconnected. Photosensitive base-collector junction acts as a light gathering element. The absorbed light favours an increase in the base current. The transistor action amplifies this base current which leads to the increase of the primary photocurrent. Liquid phase epitaxy method is used for the construction of phototransistors. When a phototransistor is exposed to light, radiation passes unattenuated through the wide bandgap emitter. Here it gets absorbed by the base layer, base collector depletion region and the collector. A large secondary photocurrent generated between the collector and emitter increases the forward bias on the device. Doping concentration in base is made large and low in emitter and collector. This heterostructure permits a low value of capacitance between emitter-base and the collector-base junction with a low value of base resistance. This heterostructure also creates a potential barrier at the emitter base junction which allows elimination of the hole injection from the base when the junction is forward biased.