Charge is observed at the regrowth interface for heterostructure field-effect transistors (HFETs) grown on semi-insulating (SI) bulk GaN substrates, even with Fe doping in the regrown buffer layer for reduction of the interface charge. Ultraviolet photoenhanced chemical (PEC) etching is used to treat the surface of SI bulk GaN substrates. Employing optimized etching conditions, a very smooth surface is achieved for the bulk GaN substrate after the etching. The charge at the regrowth interface is eliminated for HFETs grown on etched SI GaN substrates. Secondary ion mass spectrometry measurements show that the Si impurity concentration at the regrowth interface for HFETs grown on etched SI GaN substrates is much lower than that for HFETs grown on unetched SI GaN substrates, which suggests that the charge-containing layer on the SI substrate is removed by PEC etching and that the effects of the reduced charge layer near the regrowth interface can be eliminated by Fe doping for HFETs grown on etched SI substrates.

We report high-performance homojunction GaN avalanche photodiodes (APDs) grown on a low-defect GaN substrate and fabricated with an ion-implantation isolation method. High-quality p-i-n GaN layers were grown using metalorganic chemical vapor deposition (MOCVD), and an effective device isolation method using optimized nitrogen ion implantation was developed to provide significant leakage current suppression on the etched mesa sidewalls. Fabricated GaN APDs showed an ultralow dark current density <; 10 ^-9 A/cm ² up to 50% of the avalanching breakdown voltage region and achieved a photocurrent gain of >106 at a reverse bias of 71.5 V under deep-ultraviolet (DUV) illumination at λ = 280 nm. A further temperature-dependent study of the dark current indicated that a trap-assisted tunneling process predominates the leakage current component that also contributes to the carrier multiplication process near the avalanching breakdown.

We report high-current-gain n-p-n GaN/InGaN double-heterojunction bipolar transistors (DHBTs) using a direct-growth fabrication processing approach. The impact of the indium composition in the base layer was studied, and a burn-in effect using a constant-base-current stressing method was observed. We found that DHBTs with higher indium composition in the InGaN base layer may help reduce the base resistance and lower the surface recombination current but may result in higher bulk recombination current. A device burn-in effect was also studied. The postprocessing current stressing step helps increase free-hole concentration in the base, reduce the bulk recombination current, and enhance the current gain. As a result, a direct-growth GaN/In_0.03Ga_0.97N DHBT with a peak current gain of 105 and a collector current density > 6.5 kA/cm² was demonstrated on a sapphire substrate.

Lateral hole injection into AlGaN-based ultraviolet (UV) vertical-cavity light-emitting lasers (VCSELs) has been studied via numerical simulation. For blue and violet vertical cavity light emitters, indium tin oxide (ITO) is most commonly used as a transparent current spreading layer to increase the overlap between the optical mode and the radial current profile. However, ITO has very high optical losses in the UV spectrum, so alternative schemes for lateral current spreading have been investigated for use in UV-VCSELs. A modulation doped short-period superlattice (MD-SPSL) has been proposed as a transparent lateral current spreading layer in UV-VCSELs. The narrow bandgap unintentionally doped (uid) material maintains a high mobility due to reduced impurity scattering and has a high free hole concentration due to modulation doping, thus forming highly conductive channels which aid lateral hole transport. This has been shown to partially mitigate current crowding around the current aperture. To account for imperfect modulation doping due to the magnesium memory effects and other factors, the effect of varying the hole mobility in the uid-narrow bandgap layer of the MD-SPSL from 13-300 cm ² /(V · s) on the threshold current and slope efficiency has also been studied. Employing an MD-SPSL results in a significant reduction in the threshold current and slope efficiency compared to ITO, and the extent of the improvement depends on the hole mobility in the uid-AlGaN layer.

We report a study on the surface-leakage current in GaN/InGaN double heterojunction bipolar transistors (DHBTs) that are grown on a sapphire substrate. Surface-leakage-current densities on an unpassivated DHBT are 9.6 times 10^-5 - 5.8 times 10^-4 A/cm for JC = 0.5-50 A/cm². A fabricated n-p-n GaN/InGaN DHBT shows the common-emitter dc current gain of 42, the collector-current density of 5.2 kA/cm², and the common-emitter breakdown voltage (BV_CEO) of 75 V.