We have fabricated photoconductive and photovoltaic ultraviolet sensors from GaN single layers and pn-junctions. These sensors exhibit a sharp long wavelength cut-off in responsivity at the bandgap (365 nm). The active layers (GaN) were deposited using low pressure MOCVD. The p-type doping was accomplished using Mg as the dopant. Photoconductive, and schottky barrier detectors were then fabricated using photolithography, reactive ion etching and contact metallizations. These processing techniques were developed specific to the A1xGa1-xN material system. We will discuss growth, fabrication and characterization details for these various device types. The measured values of device parameters will be contrasted with those estimated from active layer material characterization.

AlxGa1-xN with it's tunable and direct bandgap is an important semiconductor system for visible and ultraviolet devices. The bandgap tunability from 200 nm at x = 1 to 365 nm at x= 0 combined with the tunable refractive index makes possible the fabrication of waveguide structures. Recently high quality single crustal films of AlN, GaN and AlGaN have been deposited over sapphire subs rates by several research groups using metalorganic chemical vapor deposition [1234]. N- and p- type doping [4,5] and high quality heterojunctions have also been reported[6]. These material quality improvements and control has resulted in several optoelectronic devices such as high responsivity UV detectors, Bragg reflectors, waveguides, transistors and visible LED's. We will briefly describe these recent advances and some current research directions.

In this letter we report the fabrication and characterization of a metal semiconductor field effect transistor (MESFET) based on single crystal GaN. The GaN layer was deposited over sapphire substrate using low pressure metalorganic chemical vapor deposition. MESFET devices were fabricated on isolated mesas using TiAu for the source and drain ohmic contacts and silver for the gate Schottky. For devices with a gate length of 4 μm (channel opening, i.e., source to drain separation of 10 μm), a transconductance of 23 mS/mm was obtained at −1 V gate bias. Complete pinch-off was observed for a gate potential of −12 V.

In this letter we report the fabrication and dc characterization of a high electron mobility transistor (HEMT) based on a n-GaN-Al0.14Ga0.86N heterojunction. The conduction in our low pressure metalorganic chemical vapor deposited heterostructure is dominated by two-dimensional electron gas at the heterostructure interface. HEMT devices were fabricated on ion-implant isolated mesas using Ti/Au for the source drain ohmic and TiW for the gate Schottky. For a device with a 4 μm gate length (10 μm channel opening, i.e., source-drain separation), a transconductance of 28 mS/mm at 300 K and 46 mS/mm at 77 K was obtained at +0.5 V gate bias. Complete pinchoff was observed for a −6 V gate bias.

In this letter we report the fabrication and characterization of Schottky barrier photodetectors on p-type GaN films. These films were grown over basal plane sapphire substrates using low pressure metalorganic chemical vapor deposition and magnesium as the p-type dopant. The current-voltage and capacitance-voltage characteristics were measured for Ti/Au Schottky barriers for a film with a p doping of 7×1017 cm−3. We measured a 1.5 V forward turn on and a 3 V reverse breakdown. The zero bias responsivity of a detector with 1 mm2 area was measured to be 0.13 A/W. For these photovoltaic detectors, the photoresponse was nearly constant from 200 to 365 nm and fell sharply by several orders of magnitude for wavelengths above 365 nm.

In this paper we report the fabrication and characterization of a metal insulator semiconductor field effect transistor (MISFET) based on single crystal GaN-AlxGa1-xN heterostructures. The device structure layers were deposited over sapphire substrate using low pressure metalorganic chemical vapor deposition. We discuss the fabrication and characterization of MISFET devices using single crystal GaN as the insulator layer. These were fabricated on isolated mesas using TiAu for the source and drain ohmic contacts and silver for the gate Schottky. For devices with a gate length of 4 fum (channel opening i.e. source to drain separation of 10 fum), a transconductance of 20 mS/mm was obtained at -1 volt gate bias. Complete pinchoff was observed for a gate potential of -15 volts. Device performance is compared to that for a GaN MESFET of identical dimensions. Due to its direct bandgap tunable from 3.4 to 6.2 eV AlxGa1-xN is an important semiconductor for devices in the ultraviolet and visible parts of the spectrum. The large bandgap, potential of heterojunction formation and the insulating nature of AlN make AlxGa1-xN an ideal material system for high performance metal semiconductor field effect transistor (MESFET) devices requiring high operation temperatures. Availability of the insulator (single crystal AlN or GaN) also makes possible the fabrication of metal insulator field effect transistor (MISFET) devices which has been an elusive goal for the GaAs based III-V materials technology. High quality single crystal GaN films have been deposited on sapphire and several other semiconductor substrates using metalorganic chemical vapor deposition (MOCVD)1,2, molecular beam epitaxy (MBE)3 and vapor phase epitaxy (VPE)4. Recently we have reported on quantum confinement5, room temperature stimulated emission6, and 2-dimensional electron gas in GaN-AlxGa1-xN heterostructures7,8. We have also deposited single crystal insulating GaN and A1N over sapphire using atomic layer epitaxy9,10 and fabricated high responsivity photoconductors11. We now report on the fabrication and characterization of FET devices using single crystal GaN-AlxGa1-xN heterostructures. This forms the basis of high temperature transistor devices based on the AlxGa1-xN material system.

A MOCVD based atomic layer epitaxy process is being developed as a potential solution to the problems of film thickness and interface abruptness control which are encountered when fabricating superconductor-insulator-superconductor (SIS) devices using YBa2Cu3O7-x. In initial studies, the atomic layer MOCVD process has yielded superconducting YBa2Cu3O7-x films with substrate temperatures of 605°C during film growth, and no post deposition anneal. The low temperature process yields a smooth film surface and will reduce interface degradation due to diffusion.