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.

A GaN/AIGaN heterojunction bipolar transistor structure with Mg doping in the base and Si doping in the emitter and collector regions was grown by Metal Organic Chemical Vapor Deposition on c-axis A1203. Secondary Ion Mass Spectrometry measurements showed no increase in the 0 concentration ( 2 -3~1 0 '~ ~m -~) in the AlGaN emitter and fairly low levels of C (-4-5~10'~ cm-3) throughout the structure. Due to the non-ohmic behavior of the base contact at room temperature, the current gain of large area (-90 pm diameter) devices was <3. Increasing the device operating temperature led to higher ionization fractions of the Mg acceptors in the base, and current gains of -10 were obtained at 300 OC. 1There is a strong interest in GaN-based electronics for applications involving high temperature or high power operation, based on the excellent transport properties of the III-nitride materials system.(*-7' Impressive advances in the performance of AlGaN/GaN high electron mobility transistors continue to be reported, due to in part to the formation of piezoelectrically-induced carriers in a 2-dimensional electron gas at the There is also interest in the development of GaN/AIGaN In this letter we report on the growth by MOCVD of a graded emitter HBT structure, DISCLAIMERPortions of this document may be illegible in electronic image products. Images are produced from the best available original document.Spectrometry (SIMS) since these could potentially have a strong influence on device performance, and finally on the dc characteristics of HBTs fabricated on this material.The layer structure is shown schematically in Figure 1, and was grown at -1050 "C following deposition of the GaN buffer at -550 "C on the c-plane A1203 substrate. The growth system has been described in detail previously,'2o' but in brief is a rotating (1200 rpm) disk MOCVD reactor. Ammonia (NH3), trimethylgallium (TMGa) and trimethylaluminum (TMAl) were used as precursors, while silane (SiH4) and biscyclopentadienyl-magnesium (CpzMg) were employed for n-and p-type doping, respectively. High purity H2 was used as the carrier gas. After growth the sample was annealed in the reactor at 850 "C for 20 min under 140 Torr of flowing N2 to activate the Mg acceptors.There are two important aspects to dopant and background impurity control in HBT structures. The first is that the p-type dopant should be confined to the base region, and not spill-over into the adjacent n-type emitter, where it could cause displacement of the junction and hence the loss of the advantage of the heterostructure. Figure 2 shows SIMS profiles of the A1 marker, signifying the position of AlGaN emitter layer, and also the Mg doping profile in the adjacent base layer. It is clear that the reactor memory effect for Cp2Mg has produced incorporation of Mg in the emitter, although the real situation is not quite as severe as it seems in the data because of "carry-over" of the matrix A1 signal during the depth profiling. The fact that working HBTs can still be made on this material is due...

GaN and Al0.25Ga0.75N lateral Schottky rectifiers were fabricated either with (GaN) or without (AlGaN) edge termination. The reverse breakdown voltage VB (3.1 kV for GaN; 4.3 kV for AlGaN) displayed a negative temperature coefficient of −6.0±0.4 V K−1 for both types of rectifiers. The reverse current originated from contact periphery leakage at moderate bias, while the forward turn-on voltage at a current density of 100 A cm−2 was ∼5 V for GaN and ∼7.5 V for AlGaN. The on-state resistances, RON, were 50 mΩ cm2 for GaN and 75 mΩ cm2 for AlGaN, producing figures-of-merit (VRB)2/RON of 192 and 246 MW cm−2, respectively. The activation energy of the reverse leakage was 0.13 eV at moderate bias.

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