  We report GaN n++/p++ interband tunnel junctions with repeatable negative
differential resistance and low resistance. Reverse and forward tunneling
current densities were observed to increase as Si and Mg doping concentrations
were increased. Hysteresis-free, bidirectional negative differential resistance
was observed at room temperature from these junctions at a forward voltage of
~1.6-2 V. Thermionic PN junctions with tunnel contact to the p-layer exhibited
forward current density of 150 kA/cm^2 at 7.6 V, with a low series device
resistance of 1 x 10^-5 ohm.cm^2.
