  We theoretically investigate negative differential resistance (NDR) of
massless and massive Dirac Fermions in double barrier resonant tunneling diodes
based on sufficiently short and wide graphene strips. The current-voltage
characteristics calculated in a rotated pseudospin space show that, the NDR
feature only presents with appropriate structural parameters for the massless
case and the peak-to-valley current ratio can be enhanced exponentially by a
tunable band gap. Remarkably, the lowest NDR operation window is nearly
structure-free and can be almost solely controlled by a back gate, which may
have potential applications in NDR devices with the operation window as a
crucial parameter.
