A phototransistor is a promising candidate as an optical power monitor in Si
photonic circuits since the internal gain of photocurrent enables high
sensitivity. However, state-of-the-art waveguide-coupled phototransistors
suffer from a responsivity of lower than 103 A/W, which is insufficient for
detecting very low power light. Here, we present a waveguide-coupled
phototransistor consisting of an InGaAs ultrathin channel on a Si waveguide
working as a gate electrode to increase the responsivity. The Si waveguide gate
underneath the InGaAs ultrathin channel enables the effective control of
transistor current without optical absorption by the gate metal. As a result,
our phototransistor achieved the highest responsivity of approximately 106
A/W among the waveguide-coupled phototransistors, allowing us to detect light
of 621 fW propagating in the Si waveguide. The high responsivity and the
reasonable response time of approximately 100 μs make our phototransistor
promising as an effective optical power monitor in Si photonics circuits