Silicon Photonics: Fundamentals and Devices

8.6 Phototransistors in Standard Silicon Technology

By using the p+/n-well p-substrate, the p+/n-well polysilicon gate, or the n+/p-well p+ vertical or combined vertical-lateral phototransistors that are only bipolar, metal oxide semiconductors (MOS) or a combination of bipolar and MOS transistors can be made. Three examples are illustrated later for a vertical bipolar phototransistor, a combined vertical-lateral bipolar phototransistor, and a MOS phototransistor. Generally, bipolar transistors offer better photoresponse characteristics because they are bulk-transport rather than surface-controlled devices as in the case of a field effect transistor.

In a popular bipolar phototransistor structure, the base is left floating and the collector–base junction has a large window through which the light is incident. In this configuration and if it is a p–n–p transistor (collector (p), base (n), and emitter (p)), then the photogenerated holes in the reverse-biased collector–base will be swept to the collector terminal, giving rise to the photocurrent. The electrons from the photogenerated electron–hole pairs, on the other hand, will be swept into the base, causing the emitter–base potential to rise. This increase in emitter–base potential, in turn, causes more holes to be injected from the emitter into the base, where they diffuse to the collector, thus causing an increase in the collector current. Note also that since the base terminal is floating, current conservation requires that the ...