Production Scale Growth of AlGaN/GaN Field Effect Transistors
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PRODUCTION SCALE GROWTH OF AlGaN/GaN FIELD EFFECT TRANSISTORS David Gotthold∗, Shawn Gibb, Boris Peres, Ian Ferguson1, Chris Palmer and Eric Armour EMCORE Corporation, Somerset, NJ 08873 1 Now at School of ECE, Georgia Institute of Technology, Atlanta, GA 30332 ABSTRACT This paper addresses issues with the manufacturability of AlGaN/GaN FET structures. A robust Metalorganic Chemical Vapor Deposition growth process has been developed that will now allow reliability measurements to be obtained on the resulting devices. During a small scale production run mobilities in excess of 1600 cm2/V.s, sheet charge (Ns) between 0.8x1013 and 1.2x1013 cm-2, and Rs2” diameter) and rapid device level testing. Reliable and cost-effective production processes must be established while still achieving high performance indicated by early devices before AlGaN/GaN FETs can be effectively commercialized.
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Corresponding email: [email protected]
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2. EXPERIMENTAL DETAILS All the device structures reported in this work were grown in an EMCORE D-180 Discovery series MOCVD reactor, using trimethylgallium (TMGa), trimethylaluminum (TMAl) and ammonia (NH3) as precursors. The reactor was capable of growing on 6x2”diameter wafers and process was been developed for transfer to a large area system capable of growing on 5-8x34”diameter wafers. Crystal quality was monitored during growth via in-situ optical reflectivity that allowed for the real time observation and control of the growing layer. Devices were grown on, single side polished, commercially available, 2” c-plane sapphire substrates employing an AlN or GaN nucleation layer grown at low temperature, nominally 500°C. The growths on SiC were completed on 2” 4H semi-insulating substrates using an AlN nucleation layer grown at 1030°C. Identical structures were grown on the sapphire and SiC substrates apart from some differences in the nucleation layers. A typical device structure consists of a SI GaN layer (13µm thick) with a 20-40 nm thick AlGaN cap layer depending on the aluminum concentration, normally 20-30%. The devices and individual layers were characterized using a variety of techniques, including: Nomarski microscopy, contactless sheet resistivity/conductivity measurements, high-resolution x-ray diffraction (HR-XRD), capacitance-voltage (CV), and Hall measurements prior to device fabrication. 3. DISCUSSION 3.1 Semi-Insulating GaN Buffer Layer An important element for the manufacturability of the AlGaN/GaN FETs is the quality and reproducibility of the SI GaN buffer layer. This layer must be very resistive to facilitate “pinch off” of the FET during operation yet contain a low density of defects that may affect the performance and reliability of the device. One problem in producing SI GaN is the difficulty of measuring the resistivity of the as grown material. This limits the ability to optimize or monitor changes due to variations in the growth process. IV measurements on high quality SI-GaN do not indicate any detectable leakage current at voltages up to 350V. Moreov
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