  Recent developments in the technology of van der Waals heterostructures made
from two-dimensional atomic crystals have already led to the observation of new
physical phenomena, such as the metal-insulator transition and Coulomb drag,
and to the realisation of functional devices, such as tunnel diodes, tunnel
transistors and photovoltaic sensors. An unprecedented degree of control of the
electronic properties is available not only by means of the selection of
materials in the stack but also through the additional fine-tuning achievable
by adjusting the built-in strain and relative orientation of the component
layers. Here we demonstrate how careful alignment of the crystallographic
orientation of two graphene electrodes, separated by a layer of hexagonal boron
nitride (hBN) in a transistor device, can achieve resonant tunnelling with
conservation of electron energy, momentum and, potentially, chirality. We show
how the resonance peak and negative differential conductance in the device
characteristics induces a tuneable radio-frequency oscillatory current which
has potential for future high frequency technology.
