Barrier Material Improvement in AlGaN/GaN Microwave Transistors Under Gamma Irradiation Treatment
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Barrier material improvement in AlGaN/GaN microwave transistors under gamma irradiation treatment S.A.Vitusevich1, M.V.Petrychuk2, N. Klein1, S.V.Danylyuk1, A.E.Belyaev3, R.V.Konakova3, A.Yu.Avksentyev3, A.M.Kurakin3, P.M.Lytvyn3, B.A.Danilchenko4, V.Tilak5, J.Smart5, A. Vertiatchikh5 and L.F. Eastman5 1 Forschungszentrum Jülich, ISG, Jülich 52425, Germany 2 Taras Shevchenko Kiev National University, Kiev 01033, Ukraine 3 Institute of Semiconductor Physics, NASU, Kiev 03028, Ukraine 4 Institute of Physics, NASU, Kiev 03028, Ukraine 5 School of Electrical Engineering, Cornell University, Ithaca, New York 14853, U.S.A. ABSTRACT Effect of small dose gamma-irradiation on electrical characteristics of AlGaN/GaN high electron mobility transistors has been investigated. Decreasing of the leakage current and its noise has been registered after dose of 1x106 Rad. As-grown heterostructures used in further for the device fabrication have been examined after the same radiation treatment. The small dose radiation results are explained within a model that takes into account relaxation of elastic strains and structural-impurity ordering occurring in the barrier layer under irradiation. INTRODUCTION Microwave transistors manufactured on the basis of AlGaN/GaN heterostructures demand for improvement of material characteristics to attain high power and high speed operation. Today, GaN-based High Electron Mobility Transistors (HEMTs) possess still comparative high level of leakage current due to large dislocation density and material layers imperfections [1-3]. To decrease the leakage current an intermediate dielectric or ferroelectric layers have been grown before metalization to the barrier region [4, 5]. However, these layers result in lowering of the operating frequency due to an increase of the device capacitance. In this contribution we report an investigation of structural, transport and noise properties of HEMTs irradiated by gamma-quanta. At a certain dose of gamma irradiation an improvement of barrier properties of the transistors was revealed. The origin of observed effects is discussed. EXPERIMENTAL DETAILS The investigated structures are high performance AlGaN/GaN HEMT devices grown by organic metallic vapour phase epitaxy on sapphire substrates. Each structure consists of the following layers: a nucleation AlN layer, an undoped GaN buffer layer and an AlGaN (33% Al) undoped barrier layer. The devices with several gate lengths (150-350 nm) and widths (100-400 µm) were investigated. Detailed description of the structures preparation as well as characterization of materials and devices studied have been reported elsewhere [6, 7]. The transistor dc characteristics were investigated with a HP4145B Semiconductor Parameter Analyzer operated in the transistor mode. The noise spectra were measured in the
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