GaN and related materials (especially AlGaN) have recently attracted a lot of interest for applications in high power electronics capable of operation at elevated temperatures. Although the growth and processing technology for SiC, the other viable wide bandgap semiconductor material, is more mature, the AlGaInN system offers numerous advantages. These include wider bandgaps, good transport properties, the availability of heterostructures (particularly AlGaN/GaN), the experience base gained by the commercialization of GaN-based laser and light-emitting diodes and the existence of a high growth rate epitaxial method (hydride vapor phase epitaxy) for producing very thick layers or even quasi-substrates. These attributes have led to rapid progress in the realization of a broad range of GaN electronic devices, including heterostructure field effect transistors (HFETs), Schottky and p–i–n rectifiers, heterojunction bipolar transistors (HBTs), bipolar junction transistors (BJTs) and metal-oxide semiconductor field effect transistors (MOSFETs). This review focuses on the development of fabrication processes for these devices and the current state-of-the-art in device performance, for all of these structures. We also detail areas where more work is needed, such as reducing defect densities and purity of epitaxial layers, the need for substrates and improved oxides and insulators, improved p-type doping and contacts and an understanding of the basic growth mechanisms.

Fabrication and performance of GaN electronic devices

Theoretical possibilities of InxGa1-xN tandem PV structures

An overview is presented of progress in GaN electronic devices along with recent results from work at UCSB. From 1995 to 2001, the power performance of AlGaN/GaN high electron mobility transistors (HEMT) improved from 1.1 to 11 W mm-1, respectively. The disadvantage of the low thermal conductivity of the sapphire substrate was mitigated by flip-chip bonding onto AlN substrates, yielding large periphery devices with an output power of 7.6 W. A variety of HEMT amplifier circuits have been demonstrated. The first AlGaN/GaN heterojunction bipolar transistor (HBT) was demonstrated in 1998, with a current gain of about 3. By developing the technique of emitter regrowth, a current gain of 10 was achieved in both GaN BJTs and AlGaN/GaN HBTs. A common emitter current gain cutoff frequency of 2 GHz was measured. Critical issues involved in the growth of high quality AlGaN/(AlN)/GaN heterostructures and GaN:Mg by metal-organic chemical vapour deposition (MOCVD) and molecular beam epitaxy (MBE) and the device fabrication are discussed.

https://iopscience.iop.org/article/10.1088/0953-8984/13/32/317/pdf

Gallium nitride based transistors

The development of a self-aligned fabrication process for small emitter contact area ( 2×4 μ m2) GaN/AlGaN heterojunction bipolar transistors and GaN bipolar junction transistors is described. The process features dielectric-spacer sidewalls, low damage dry etching and selected-area regrowth of p-GaAs(C) on the base contact or n-GaN/AlGaN on the emitter contact. Series resistance effects are still found to influence the device performance.

Self-aligned process for emitter- and base-regrowth GaN HBTs and BJTs

The diffusion and reactions of hydrogen in GaN are described by applying differential equations for the concentration profiles of H species, charged dopants, and carriers with simultaneous solution of Poisson’s equation This approach dispenses with the simplifying assumptions of local equilibrium among states and local charge neutrality that were employed previously by us to treat high-temperature H behavior in uniform layers The result is a more general modeling capability which encompasses nonequilibrium conditions and space-charge effects such as are encountered in devices Density-functional theory, previously used by us to treat equilibrium H energies, is employed herein to examine activation barriers and wave-function overlaps affecting the rates of relevant H and carrier reactions, thereby guiding the selection of mechanisms to be included and influencing the evaluation of some rate parameters The model is applied to H-containing p-n junctions, with detailed consideration of the reversible, meta

Theoretical description of H behavior in GaN p-n junctions

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.

GaN PN junction issues and developments

http://esl.ecsdl.org/content/3/3/144.full.pdf

High Current, Common‐Base GaN ‐ AIGaN Heterojunction Bipolar Transistors

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.

High temperature characteristics of GaN-based heterojunction bipolar transistors and bipolar junction transistors

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.

Simulation of GaN/AlGaN heterojunction bipolar transistors: part II – pnp structures

http://esl.ecsdl.org/content/3/7/333.full.pdf

D.J King

Papers

GaN PN junction issues and developments

High Current, Common‐Base GaN ‐ AIGaN Heterojunction Bipolar Transistors

High temperature characteristics of GaN-based heterojunction bipolar transistors and bipolar junction transistors

Simulation of GaN/AlGaN heterojunction bipolar transistors: part II – pnp structures

Common‐Base Operation of GaN Bipolar Junction Transistors