A survey of ohmic contact materials and properties to GaAs, InP, GaN will be presented along with critical issues pertaining to each semiconductor material. Au-based alloys (e.g. GeAuNi for n-type GaAs) are the most commonly used contacts for GaAs and InP materials for both n-and p-type contacts due to the excellent contact resistivity, reliability, and usefulness over a wide range of doping levels. Research into new contacting schemes for these materials has focused on addressing limitations of the conventional Au-alloys in thermal stability, propensity for spiking, poor edge definition, and new approaches for a non-alloyed contact. The alternative contacts to GaAs and InP include alloys with higher temperature stability, contacts based on solid phase regrowth, and contacts that react with the substrate to form lower bandgap semiconductors alloys at the interface. A

Unpassivated AlGaN/GaN high-electron-mobility transistors show significant gate lag effects due to the presence of surface states in the region between the gate and drain contact. Low-temperature (100 °C) layers of MgO or Sc2O3 deposited by plasma-assisted molecular-beam epitaxy are shown to effectively mitigate the collapse in drain current through passivation of the surface traps. These dielectrics may have advantages over the more conventional SiNX passivation in terms of long-term device stability.

Selective area ion implantation doping has been used to fabricate GaN junction field effect transistors (JFETs). p-type and n-type doping was achieved with Ca and Si implantation, respectively, followed by a 1150 °C rapid thermal anneal. A refractory W gate contact was employed that allows the p-gate region to be self-aligned to the gate contact. A gate turn-on voltage of 1.84 V at 1 mA/mm of gate current was achieved. For a ∼1.7 μm×50 μm JFET with a −6 V threshold voltage, a maximum transconductance of 7 mS/mm at VGS=− 2V and saturation current of 33 mA/mm at VGS=0 V were measured. These results were limited by excess access resistance and can be expected to be improved with optimized n+ implants in the source and drain regions.

A model for the stress relaxation of amorphous carbon films containing high concentrations of fourfold coordinated carbon is presented. The onset of stress relaxation in these materials occurs following thermal annealing at temperatures as low as 100~ and near full stress relaxation occurs after annealing at 600~ The stress relaxation is modeled by a series of first order chemical reactions which lead to a conversion of some fourfold coordinated carbon atoms into threefold coordinated carbon atoms. The distribution of activation energies for this process is derived from the experimental measurements of stress relaxation and is found to range from 1 eV to over 3 eV. Permanent increases in the electrical conductivity of the carbon films are also found following thermal annealing. The electrical conductivity is found to be exponentially proportional to the number of additional threefold atoms which are created upon annealing, with the increase in threefold atom concentration being deduced from the stress relaxation model. This indicates that the increase in electrical conductivity and the stress relaxation originate from the same fourfold to threefold conversion process and that electrical transport through these films is dominated by a hopping conduction process.

Ga 2 O 3 (Gd 2 O 3 ) was deposited on GaN for use as a gate dielectric in order to fabricate a depletion metal–oxide–semiconductor field-effect transistor (MOSFET). Analysis of the effect of temperature on the device shows that gate leakage is significantly reduced at elevated temperature relative to a conventional metal–semiconductor field-effect transistor fabricated on the same GaN layer. MOSFET device operation in fact improved upon heating to 400 °C. Modeling of the effect of temperature on contact resistance suggests that the improvement is due to a reduction in the parasitic resistances present in the device.

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"Ultra" wide-bandgap semiconductors are an emerging class of materials with bandgaps greater than that of gallium nitride (E G > 3.4 eV) that may ultimately benefit a wide range of applications, including switching power conversion, pulsed power, RF electronics, UV optoelectronics, and quantum information. This paper describes the progress made to date at Sandia National Laboratories to develop one of these materials, aluminum gallium nitride, targeted toward high-power devices. The advantageous material properties of AlGaN are reviewed, questions concerning epitaxial growth and defect physics are covered, and the processing and performance of vertical-and lateral-geometry devices are described. The paper concludes with an assessment of the outlook for AlGaN, including outstanding research opportunities and a brief discussion of other potential applications.

An AlN barrier high electron mobility transistor (HEMT) based on the AlN/Al0.85Ga0.15N heterostructure was grown, fabricated, and electrically characterized, thereby extending the range of Al composition and bandgap for AlGaN channel HEMTs. An etch and regrowth procedure was implemented for source and drain contact formation. A breakdown voltage of 810 V was achieved without a gate insulator or field plate. Excellent gate leakage characteristics enabled a high Ion/Ioff current ratio greater than 107 and an excellent subthreshold slope of 75 mV/decade. A large Schottky barrier height of 1.74 eV contributed to these results. The room temperature voltage-dependent 3-terminal off-state drain current was adequately modeled with Frenkel-Poole emission.