  Trions are excitonic species with a positive or negative charge, and thus,
unlike neutral excitons, the flow of trions can generate a net detectable
charge current. Trions under favourable doping conditions can be created in a
coherent manner using resonant excitation. In this work, we exploit these
properties to demonstrate a gate controlled trion switch in a few-layer
graphene/monolayer WS2/monolayer graphene vertical heterojunction. By using a
high resolution spectral scan through a temperature controlled variation of the
bandgap of the WS2 sandwich layer, we obtain a gate voltage dependent vertical
photocurrent strongly relying on the spectral position of the excitation, and
the photocurrent maximizes when the excitation energy is resonant with the
trion peak position. Further, the resonant photocurrent thus generated can be
effectively controlled by a back gate voltage applied through the incomplete
screening of the bottom monolayer graphene, and the photocurrent strongly
correlates with the gate dependent trion intensity, while the non-resonant
photocurrent exhibits only a weak gate dependence -unambiguously proving a
trion driven photocurrent generation under resonance. We estimate a sub-100 fs
switching time of the device. The findings are useful towards demonstration of
ultra-fast excitonic devices in layered materials.
