  Graphene-insulator-graphene vertical tunneling structures are discussed from
a theoretical perspective. Momentum conservation in such devices leads to
highly nonlinear current-voltage characteristics, which with gates on the
tunnel junction form potentially useful transistor structures. Two prior
theoretical treatments of such devices are discussed; the treatments are shown
to be formally equivalent, although some differences in their implementations
are identified. The limit of zero momentum conservation in the theory is
explicitly considered, with a formula involving the density-of-states of the
graphene electrodes recovered in this limit. Various predictions of the theory
are compared to experiment.
