  Graphene field effect transistors commonly comprise graphene flakes lying on
SiO2 surfaces. The gate-voltage dependent conductance shows hysteresis
depending on the gate sweeping rate/range. It is shown here that the
transistors exhibit two different kinds of hysteresis in their electrical
characteristics. Charge transfer causes a positive shift in the gate voltage of
the minimum conductance, while capacitive gating can cause the negative shift
of conductance with respect to gate voltage. The positive hysteretic phenomena
decay with an increase of the number of layers in graphene flakes. Self-heating
in helium atmosphere significantly removes adsorbates and reduces positive
hysteresis. We also observed negative hysteresis in graphene devices at low
temperature. It is also found that an ice layer on/under graphene has much
stronger dipole moment than a water layer does. Mobile ions in the electrolyte
gate and a polarity switch in the ferroelectric gate could also cause negative
hysteresis in graphene transistors. These findings improved our understanding
of the electrical response of graphene to its surroundings. The unique
sensitivity to environment and related phenomena in graphene deserve further
studies on nonvolatile memory, electrostatic detection and chemically driven
applications.
