  Two-dimensional transition metal dichalcogenide phototransistors have been
object of intensive research during the last years due to their potential for
photodetection. Photoconductivity in these devices is typically caused by a
combination of two physical mechanisms: photoconductive effect (PCE) and
photovoltaic effect (PVE; also known as photogating). However, the interplay
and working regimes of these two effects are yet not fully understood. Here we
investigate the photoresponse of a fully h-BN encapsulated monolayer MoS2
phototransistor at low temperature (5 K). For gate voltages below the threshold
voltage Vth of the MoS2 channel we observe a small but measurable signal due to
PCE, while for voltages above Vth the photoresponse largely increases due to a
fast-responding PVE. Using a simple model for the photocarrier dynamics we can
reproduce the illumination power dependence of the photoresponse for the two
gate voltage regimes and extract information on the microscopic origin of the
observed PVE.
