  Transport in photoactive graphene heterostructures, originating from the
dynamics of photogenerated hot carriers, is governed by the processes of
thermionic emission, electron-lattice thermal imbalance and cooling. These
processes give rise to interesting photoresponse effects, in particular
negative differential resistance (NDR) arising in the hot-carrier regime. The
NDR effect stems from a strong dependence of electron-lattice cooling on the
carrier density, which results in the carrier temperature dropping
precipitously upon increasing bias. The ON-OFF switching between the NDR regime
and the conventional cold emission regime, as well as the gate-controlled
closed-circuit current that is present at zero bias voltage, can serve as
signatures of hot-carrier dominated transport.
