Low resistance ohmic contacts are difficult to form on p-type GaN and AlGaN due to the relatively high ionization energy of Mg in GaN and AlGaN. A carbon-doped GaAs grown on p-GaN prior to ohmic metallization has been shown to improve contact resistance to p-GaN. npn AlGaN/GaN heterojunction bipolar transistors and GaN bipolar junction transistors have been demonstrated by employing a regrowth C-doped GaAs to the p-GaN base regions. GaN/AlGaN epilayers were grown with a molecular beam epitaxy system and C-doped GaAs(10(20) cm(-3)) was regrown on the devices (similar to 500 A) by metal organic chemical vapor deposition using SiO2 as mask. Very high current densities were achieved for common base operation in both device types and devices were operable at 250 degreesC.