  We have observed the Hall effect in the field-induced accumulation layer on
the surface of small-molecule organic semiconductor. The Hall mobility mu_H
increases with decreasing temperature in both the intrinsic (high-temperature)
and trap-dominated (low-temperature) conduction regimes. In the intrinsic
regime, the density of mobile field-induced charge carriers extracted from the
Hall measurements, n_H, coincides with the density n calculated using the
gate-channel capacitance, and becomes smaller than n in the trap-dominated
regime. The Hall data are consistent with the diffusive band-like motion of
field-induced charge carriers between the trapping events.
