B. S. Shelton

Back illuminated AlGaN solar-blind photodetectors

We report the growth, fabrication, and characterization of AlxGa1−xN (0⩽x⩽0.60) heteroepitaxial back-illuminated solar-blind p-i-n photodiodes on (0001) sapphire substrates. The group III-nitride heteroepitaxial layers are grown by low-pressure metalorganic chemical vapor deposition on double polished sapphire substrates using various growth conditions. The back-illuminated devices exhibit very low dark current densities. Furthermore, they exhibit external quantum efficiencies up to 35% at the peak of the photoresponse (λ∼280 nm). Improvements were made to the growth technique in order to achieve crack-free Al0.4Ga0.6N active regions on a thick Al0.6Ga0.4N window layer and to obtain activated p-type Al0.4Ga0.6N layers.

Selective area growth and characterization of AlGaN/GaN heterojunction bipolar transistors by metalorganic chemical vapor deposition

The selective area growth (SAG) and properties of AlGaN/GaN heterojunction bipolar transistors (HBTs) grown by low-pressure metalorganic chemical vapor deposition (MOCVD) are described and analyzed. Transistors based on group III-nitride material are attractive for high-power and high-temperature applications. Much work has been focused on improving p-type material, as well as heterojunction interfaces. However, there have been very few reports on HBTs operating at room temperature, At this time, current gains for nitride-based HBTs have been limited to /spl sim/10. Selective area regrowth was applied to the growth of AlGaN/GaN HBTs to analyze its potential advantages as compared to more traditional growth techniques in order to realize improved electrical performance of the devices.

High-voltage mesa-structure GaN Schottky rectifiers processed by dry and wet etching

We have fabricated and investigated high-voltage GaN vertical Schottky-barrier rectifiers grown by metalorganic chemical vapor deposition. A mesageometry Schottky-barrier rectifier having a 5-μm-thick i region, and processed using reactive-ion etching, exhibited a reverse breakdown voltage of −450 V (at 10 mA/cm2) and an on-resistance of 23 mΩ cm2. For comparison, we have also applied wet chemical etching for the fabrication of mesageometry Schottky-barrier rectifiers. The 2-μm-thick i-region GaN mesa-Schottky rectifiers showed a breakdown voltage of −310 and −280 V for wet-etched and dry-etched devices, respectively, and an on-resistance of 8.2 and 6.4 mΩ cm2, respectively. These results indicate that the performance of the wet-etched rectifiers is comparable to or better than that of comparable dry-etched devices.

Temperature dependent common emitter current gain and collector-emitter offset voltage study in AlGaN/GaN heterojunction bipolar transistors

We have demonstrated state-of-the-art performance of AlGaN/GaN heterojunction bipolar transistors (HBTs) with a common emitter (CE) current gain of 31 at 175 K and 11.3 at 295 K. The increase in collector current and CE current gain at lower temperature can be attributed to the reduced base-emitter interface recombination current. We also observed an increase of collector-emitter offset voltage with decrease of temperature. The increase of V/sub CEOFF/ at lower temperature is related to an increase of V/sub BE/ as the base bulk current is increased, or to the reduction of the ideality factor n/sub BE/.

Simulation of the electrical characteristics of high-voltage mesa and planar GaN Schottky and p-i-n rectifiers

The use of GaN for the fabrication of Schottky and p-i-n rectifiers presents an opportunity to take advantage of the high-voltage and high-power handling characteristics of the III-nitride materials. We report the results of two-dimensional (2-D) device simulations to provide estimates of the performance limitations and a comparison for vertical- and horizontal-geometry GaN mesa and planar Schottky-barrier rectifiers and p-i-n devices. The simulated performance of devices with practical drift region thicknesses indicate that it is possible to realize Schottky-barrier and p-i-n rectifiers with turn-on voltages of V/sub 0N/ 1000 and 3000 MW/cm/sup 2/, respectively.

The growth of AlGaN/GaN heterojunction bipolar transistors by metalorganic chemical vapor deposition

The growth and properties of AlGaN/GaN heterojunction bipolar transistors grown by low-pressure metalorganic chemical vapor deposition (MOCVD) are described. The emitter-up structures are grown on (0001) sapphire substrates. Common-emitter characteristics and Gummel plots were measured on 120 x 120 μm 2 devices and useable DC current gains of β ∼ 8-14 were achieved at room temperature. The resistance of the base was a limiting factor in high-current operation of these devices.

Optical properties of undoped and modulation-doped AlxGa1−xN/GaN heterostructures grown by metalorganic chemical vapor deposition

Abstract The radiative recombination of carriers in a two-dimensional electron gas (2DEG) in undoped and modulation-doped Al x Ga 1− x N/GaN heterostructures grown by metalorganic chemical vapor deposition is investigated at low temperature using photoluminescence measurements. Temperature-dependent Hall effect and Shubnikov–de Haas measurements, verify the formation of a high-quality 2DEG. Radiative recombination is observed between the 2DEG in quantum states at the hetero-interface and the holes in the flat-band region or bound to residual acceptors both in undoped and modulation-doped Al x Ga 1− x N/GaN heterostructures. The luminescence peaks related to the 2DEG disappear when the top AlGaN layer is removed by reactive ion etching. In addition, the effect of the growth interruption time, laser excitation intensity, and doping conditions upon the photoluminescence is also described.