  Double barrier GaN/AlN resonant tunneling heterostructures have been grown by
molecular beam epitaxy on the (0001) plane of commercially available bulk GaN
substrates. Resonant tunneling diodes were fabricated; room temperature
current-voltage measurements reveal the presence of a negative differential
conductance region under forward bias with peak current densities of ~6.4
$kA/cm^2$ and a peak to valley current ratio of ~1.3. Reverse bias operation
presents a characteristic turn-on threshold voltage intimately linked to the
polarization fields present in the heterostructure. An analytic electrostatic
model is developed to capture the unique features of
polar-heterostructure-based resonant tunneling diodes; both the resonant and
threshold voltages are derived as a function of the design parameters and
polarization fields. Subsequent measurements confirm the repeatability of the
negative conductance and demonstrate that III-nitride tunneling
heterostructures are capable of robust resonant transport at room temperature.
