Novel Oxides for Passivating AlGaN/GaN HEMT and Providing Low Surface State Densities at Oxide/GaN Interface
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Novel Oxides for Passivating AlGaN/GaN HEMT and Providing Low Surface State Densities at Oxide/GaN Interface F. Ren1, B. Luo1, J. Kim1, R. Mehandru1, B. P. Gila2, A. H. Onstine2, C. R. Abernathy2, S. J. Pearton2, R. Fitch3, J. Gillespie3, T. Jenkins3, J. Sewell3, D. Via3, A. Crespo3 and Y. Irokawa4 1. Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA 2. Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA 3. Air Force Research Laboratory, Sensors Directorate Wright-Patterson AFB, OH 45433, USA 4. Toyota Central Research and Development Laboratories, Inc., Nagakute, Aichi, 480-1192, Japan Both MgO and Sc2O3 are shown to provide low interface state densities (in the 10 eV-1 cm -2 range) on n- and p-GaN, making them useful for surface passivation layers to mitigate current collapse in GaN/AlGaN high electron mobility transistors(HEMTs) and also gate dielectrics for metal-oxide semiconductor(MOS) devices . Clear evidence of inversion has been demonstrated in gate-controlled MOS p-GaN diodes using both types of oxide. Charge pumping measurements on diodes undergoing a high temperature implant activation anneal show a total surface state density of ~3 × 1012 cm-2. On HEMT structures, both oxides provide effective passivation of surface states and these devices show improved output power. The MgO/GaN diodes and Sc2O3 passivated HEMT are also found to be quite radiation-resistant, making them attractive for satellite and terrestrial communication systems requiring a high tolerance to high energy(40MeV) protons. 11
INTRODUCTION AlGaN/GaN high electron mobility transistors (HEMTs) appear well-suited to high speed and high temperature applications including high frequency wireless base stations and broad-band links, commercial and military radar and satellite communications[1-34]. These devices appear capable of producing very high power densities (>12W/mm), along with high breakdown voltage and low noise figures. The use of metal-oxidesemiconductor (MOS) or metal-insulator-semiconductor (MIS) gates for HEMTs produces a number of advantages over the more conventional Schottky metal gates, including lower leakage current and greater voltage swing[2, 22, 26, 28-30].The materials reported for gate oxide/insulators include SiO2[3, 22-27, 35], Gd(Ga2O3)[22, 23, 27, 33], AlN[34], SiNX[14,31,32], MgO[24, 36] and Sc2O3[36]. These materials can also be employed as surface passivation layers on HEMTs. A commonly observed problem in these devices is the so-called “current collapse” in which the application of a high drainsource voltage leads to a decrease of the drain current and increase in the knee voltage[4, 6,12-17, 20]. This phenomenon can also be observed by a current dispersion between dc and pulsed test conditions or a degraded rf output power. The mechanisms include the presence of surface states on the cap layer or trapping centers in the resistive buffer
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