  We measure top-gated graphene field effect transistors (GFETs) with
nanosecond-range pulsed gate and drain voltages. Due to high-k dielectric or
graphene imperfections, the drain current decreases ~10% over time scales of
~10 us, consistent with charge trapping mechanisms. Pulsed operation leads to
hysteresis-free I-V characteristics, which are studied with pulses as short as
75 ns and 150 ns at the drain and gate, respectively. The pulsed operation
enables reliable extraction of GFET intrinsic transconductance and mobility
values independent of sweep direction, which are up to a factor of two higher
than those obtained from simple DC characterization. We also observe
drain-bias-induced charge trapping effects at lateral fields greater than 0.1
V/um. In addition, using modeling and capacitance-voltage measurements we
extract charge trap densities up to 10^12 1/cm^2 in the top gate dielectric
(here Al2O3). Our study illustrates important time- and field-dependent
imperfections of top-gated GFETs with high-k dielectrics, which must be
carefully considered for future developments of this technology
