High-responsivity silicon photodetectors for optoelectronic integrated systems

This paper presents two types of high-gain photodetectors, a photo-FET and a bipolar NPN phototransistor, with modified structures that allow the optical coupling to the waveguides. Because the photo-FET structure is different from that of the conventional field-effect-phototransistors, we had to develop a model for this device. The model takes into account two effects of the incident illumination: the variation of channel conductivity and the variation of channel dimensions due to the photovoltaic effect across the channel-gate junction. It has been concluded that, for low optical power and high gate bias resistance, the optical radiation controls the drain current by changing the channel conductance rather then its conductivity. The effect of the photovoltage was verified on a test structure. The two types of photodetectors can achieve high responsivities and can replace both a photodiode and a amplifier.