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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 (1020 cm−3) was regrown on the devices (∼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°C.

The dc characteristics of pnp GaN/AlGaN heterojunction bipolar transistors are simulated using a quasi-3D-model. The effects of base doping and thickness, contact geometry and device operating temperature on dc current gain have been examined. Maximum gains of ∼50 are expected for high quality materials and an optimized layer design. For similar layer thicknesses, npn devices have higher gains than the pnp structures, but the latter does not suffer from a high base sheet resistance.

GaN Bipolar Junction Transistors and GaN/AlGaN Heterojunction Bipolar Transistors were characterized at temperatures up to 250–300°C and power densities >10 kW cm −2 . The breakdown voltage in both types of devices decreased at higher temperatures, with less degradation in the BJTs. At low current levels, the offset voltages were 2–3 V but increased at higher currents.

Critical nitride-based p-n junction issues relating to wide bandgap bipolar device performance include minority carrier lifetime, defect related current characteristics and ohmic contact properties. Recent developments in p-GaN deposition processes resulted in GaN p-i-n UV photodetectors with improved deep UV responsivity, visible light rejection and shunt resistance characteristics. From the device data, the electron diffusion length in p-GaN doped at 1·1018 cm−3 was estimated to be 790 A, and the minority carrier lifetime in the p-GaN was estimated to be 24 ps to 0.24 ns. Improved junction electrical characteristics were achieved using MBE deposition on GaN buffers grown by MOCVD. NiAu ohmic contacts were also made to p-GaN with specific contact resistances less than 10−4 Ω·cm2.

An extension to the traditional two-element array crosseye interferometric electronic jamming technique is investigated. Simulations are performed to obtain excitations for multiple-emitter linear arrays that produce distorted wavefronts over desired regions of space. This results in a greater number of degrees-of-freedom and a better ability to control the desired field pattern. The crosseye effect can be achieved over wider sectors, but only by increasing array power. The technique is extended to two-dimensional arrays and is applicable to multi-function radar antennas. Experimental measurements performed on a four-emitter array demonstrate the feasibility of the technique. Good agreement with predictions is shown.