Although potentiostats are the foundation of modern electrochemical research, they have seen relatively little application in resource poor settings, such as undergraduate laboratory courses and the developing world. One reason for the low penetration of potentiostats is their cost, as even the least expensive commercially available laboratory potentiostats sell for more than one thousand dollars. An inexpensive electrochemical workstation could thus prove useful in educational labs, and increase access to electrochemistry-based analytical techniques for food, drug and environmental monitoring. With these motivations in mind, we describe here the CheapStat, an inexpensive (<$80), open-source (software and hardware), hand-held potentiostat that can be constructed by anyone who is proficient at assembling circuits. This device supports a number of potential waveforms necessary to perform cyclic, square wave, linear sweep and anodic stripping voltammetry. As we demonstrate, it is suitable for a wide range of applications ranging from food- and drug-quality testing to environmental monitoring, rapid DNA detection, and educational exercises. The device's schematics, parts lists, circuit board layout files, sample experiments, and detailed assembly instructions are available in the supporting information and are released under an open hardware license.

AlGaN/GaN current aperture vertical electron transistors with regrown aperture and source regions have been fabricated and tested. A 2 μm thick GaN:Si drain region followed by a 0.4 μm GaN:Mg current-blocking layer were grown by metalorganic chemical vapor deposition on a c-plane sapphire substrate. Channel apertures were etched, and a maskless regrowth was performed to grow unintentionally doped GaN inside the aperture as well as above the insulating layer, and to add an AlGaN cap layer. Cl2 reactive ion etching was used to pattern the device mesa, and source, drain, and gate metals were then deposited. Devices were achieved with a maximum source-drain current of 750 mA/mm, an extrinsic transconductance of 120 mS/mm, and a 2-terminal gate breakdown of 65 V while exhibiting almost no DC-RF dispersion for 80 μs pulsed I–V curves. The suppression of DC-RF dispersion was shown to result from the absence of the large electric fields at the surface on the drain-side edge of the gate that are present in high electron mobility transistors. Parasitic leakage currents, which were present in all devices, have been studied in detail. Three leakage paths have been identified, and methods to eliminate them are discussed.

AlN/GaN single and multilayer structures with various AlN and GaN layer thicknesses were grown by metalorganic chemical vapor deposition. Step flow growth of AlN was achieved using trimethylindium as a surfactant. Defect formation in the AlN layer could be largely prevented for AlN layers thinner than 2.9 nm. In the multiquantum-well samples, which consisted of five (AlN/GaN) periods, a two-dimensional electron gas (2DEG) was formed at the interface between the GaN base layer and the first AlN barrier layer. When the thickness of the AlN barrier layer in the multiquantum well was increased from 0.9 to 2.6 nm at a constant GaN well thickness of 5 nm, the sheet carrier density of the 2 DEG increased from 5×1012 to 2.1×1013 cm−2 and the electron mobility measured at 77 K decreased from 11780 to 3140 cm2/V s. The effect of the GaN well thickness was also investigated.

AlGaN ∕ GaN current aperture vertical electron transistor (CAVET) was fabricated and optimized for band gap selective photoelectrochemical wet etching. The large polarization induced voltage offset (around 2.5–4eV) observed in the first generation CAVET was reduced to 0.7V in this structure by employing a δ Si doping layer buried 60Å below the In0.03Ga0.97N (60nm thick) and bottom GaN interface to screen the polarization fields. Other sample structures were studied to achieve an aperture with both good undercut etching and a small voltage offset. It was clearly demonstrated that etch selectivity in the GaN∕InGaN∕GaN undercut structures was influenced by hole confinement and the chemical activity of the N-face GaN.

GaN grown on c-plane sapphire substrates has been reactive ion etched successfully in a SF 6 plasma with an etch rate of 29 nm/min. The etch rate does not change with substrate temperatures between 10 and 50°C. Optical transitions have not been destroyed after etching, instead, two additional lower energy transitions appear close to the band-edge luminescence. The two additional transitions are related to defect states that bind excitons. The defect-bound states exhibit different behavior compared to the free excitonic states in that their normalized intensities decrease more rapidly as temperature increases, the peaks exist only up to 80 K, and their line energies show no temperature dependence.