  Low-dimensional photoconductors have extraordinarily high photoresponse and
gain, which can be modulated by gate voltages as shown in literature. However,
the physics of gate modulation remains elusive. In this work, we investigated
the physics of gate modulation in silicon nanowire photoconductors with the
analytical photoresponse equations. It was found that the impact of gate
voltage varies vastly for nanowires with different size. For the wide nanowires
that cannot be pinched off by high gate voltage, we found that the
photoresponses are enhanced by at least one order of magnitude due to the
gate-induced electric passivation. For narrow nanowires that starts with a
pinched-off channel, the gate voltage has no electric passivation effect but
increases the potential barrier between source and drain, resulting in a
decrease in dark and photo current. For the nanowires with an intermediate
size, the channel is continuous but can be pinched off by a high gate voltage.
The photoresponsivity and photodetectivity is maximized during the transition
from the continuous channel to the pinched-off one. This work provides
important insights on how to design high-performance photoconductors.
