The reverse breakdown voltage of p-GaN Schottky diodes was used to measure the electrical effects of high density Ar or H2 plasma exposure. The near surface of the p-GaN became more compensated through introduction of shallow donor states whose concentration depended on ion flux, ion energy, and ion mass. At high fluxes or energies, the donor concentration exceeded 1019 cm−3 and produced p-to-n surface conversion. The damage depth was established as ∼400 Å based on electrical and wet etch rate measurements. Rapid thermal annealing at 900 °C under a N2 ambient restored the initial electrical properties of the p-GaN.

Schottky contacts were formed on n- and p-type GaN after either a conventional surface cleaning step in solvents, HCl and HF or with an additional treatment in (NH4)2S to prevent reformation of the native oxide. Reductions in barrier height were observed with the latter treatment, but there was little change in diode ideality factor. A simple model suggests that an interfacial insulating oxide of thickness 1–2 nm was present after conventional cleaning. This oxide has a strong influence on the contact characteristics on both n- and p-type GaN and appears to be responsible for some of the wide spread in contact properties reported in the literature.

Al x Ga 1−x N  (x=0–0.25) Schottky rectifiers were fabricated in a lateral geometry employing p+-implanted guard rings and rectifying contact overlap onto an SiO2 passivation layer. The reverse breakdown voltage (VB) increased with the spacing between Schottky and ohmic metal contacts, reaching 9700 V for Al0.25Ga0.75N and 6350 V for GaN, respectively, for 100 μm gap spacing. Assuming lateral depletion, these values correspond to breakdown field strengths of ⩽9.67×105 V cm−1, which is roughly a factor of 20 lower than the theoretical maximum in bulk GaN. The figure of merit (VB)2/RON, where RON is the on-state resistance, was in the range 94–268 MW cm−2 for all the devices.

Edge-terminated Schottky rectifiers fabricated on quasibulk GaN substrates showed a strong dependence of reverse breakdown voltage VB on contact dimension and on rectifier geometry (lateral versus vertical). For small diameter (75 μm) Schottky contacts, VB measured in the vertical geometry was ∼700 V, with an on-state resistance (RON) of 3 mΩ cm2, producing a figure-of-merit VB2/RON of 162.8 MW cm−2. Measured in the lateral geometry, these same rectifiers had VB of ∼250 V, RON of 1.7 mΩ cm2 and figure-of-merit 36.5 MW cm−2. The forward turn-on voltage (VF) was ∼1.8 V (defined at a current density of 100 A cm−2), producing VB/VF ratios of 139–389. In very large diameter (∼5 mm) rectifiers, VB dropped to ∼6 V, but forward currents up to 500 mA were obtained in dc measurements.

Planar geometry, lateral Schottky rectifiers were fabricated on high resistivity AlxGa1−xN (x=0–0.25) epitaxial layers grown on sapphire substrates. The reverse breakdown voltages of unpassivated devices increased with Al composition, varying from 2.3 kV for GaN to 4.3 kV for Al0.25Ga0.75N. The reverse current–voltage (I–V) characteristics showed classical Shockley–Read–Hall recombination as the dominant mechanism, with I∝V0.5. The reverse current density in all diodes was in the range 5–10×10−6 A cm−2 at 2 kV. The use of p+ guard rings was effective in preventing premature edge breakdown and with optimum ring width increased VB from 2.3 to 3.1 kV in GaN diodes.