  We investigate the origin of the hysteresis observed in the transfer
characteristics of back-gated field-effect transistors with an exfoliated MoS2
channel. We find that the hysteresis is strongly enhanced by increasing either
gate voltage, pressure, temperature or light intensity. Our measurements reveal
a step-like behavior of the hysteresis around room temperature, which we
explain as water-facilitated charge trapping at the MoS2/SiO2 interface. We
conclude that intrinsic defects in MoS2, such as S vacancies, which result in
effective positive charge trapping, play an important role, besides H2O and O2
adsorbates on the unpassivated device surface. We show that the bistability
associated to the hysteresis can be exploited in memory devices.
