Common-base GaN bipolar junction transistors (50-100 mm emitter diam) were operated at current densities up to 3.6 kA cm(-2) and temperatures up to 300 degrees C. The devices were fabricated with a low damage, Cl-2/Ar dry etch process, and the structure was grown by molecular beam epitaxy. The reverse breakdown voltage of the base-collector junction decreased with increasing temperature in these unpassivated devices. Simulations show that the bipolar junction transistors are expected to have similar current gain to GaN/AlGaN heterojunction transistors at collector densities above similar to 500 A cm(-2).