AlGaN/SiC heterojunction bipolar transistor Ya. I. Alivov a , Q. Fan a , X. Ni a , S. Chevtchenko a , I. B. Bhat b , and H. Morkoç a a Virginia Commonwealth University, Department of Electrical Engineering Richmond, VA, 23284; b Department of Electrical, Computer and Systems Engineering, Rennsselaer Polytechnic Institute, Troy, New York 12180 ABSTRACT AlGaN/6H-SiC heterojunction bipolar transistors (HBTs) were fabricated, and the device performance as well as the electrical properties of the n-AlGaN/p-SiC heterojunction were studied by temperature dependent current-voltage characterization. Current gain β=I C /I B calculated from I-V characteristics varied from sample to sample in the range of 75-100. A barrier height of 1.1 eV is derived from the Arrhenius plot and its origin is discussed. Keywords: Silicon carbide, aluminum gallium nitride, Arrhenius plot, heterojunction bipolar transistor 1. INTRODUCTION It is well-known that a heterojunction bipolar transistor (HBT) with a heterojunction between the base and emitter has many advantages over the conventional homojunction bipolar junction transistor (BJT) since the wider bandgap emitter material limits reverse injection from the base to emitter. This allows one to increase base doping and or the reduce the base thickness, leading to a decrease in base transit time which in turn results in an increase of speed, because the injection efficiency of a heterojunction is nearly independent of the doping concentrations on both sides. The emitter and base can be, respectively, lightly and heavily doped without deterioration of the current gain. For high-temperature and high-power applications wide-bandgap HBTs are used. Because of the large conduction-band offset, large energy-gap differences, high thermal conductivity, high breakdown voltages, and relatively good lattice matching III-nitrides and 4H, 6H-SiC materials are well-suited for the realization of high-power, high temperature HBTs, particularly considering all GaN based varieties are poor due to the relatively low materials quality and low hole concentration of p-type GaN which forms the base. GaN/SiC HBTs have been fabricated and their properties were reported by several groups [1,2,3]. There has been a number reports indicating that the GaN/SiC system has a type II band alignment, while the AlGaN/SiC is a type I system (for example, [4]), the conduction and valence band offset values being dependent on the Al composition. From this point alone AlGaN seems to be a better choice compared to a GaN emitter for realizing high performance HBT. In this report we present results on the fabrication and performance of such a transistor. 2. EXPERIMENT A 0.3 µm thick n-type Al 0.15 Ga 0.85 N layer, the wide bandgap emitter, was grown on 6H type p-SiC/n-SiC by metal- organic chemical vapor deposition (MOCVD). The 0.1 µm thick p-SiC layer of the stack, which served as the base, was grown homoepitaxially by chemical-vapor deposition on 0.3 µm thickness n-SiC layer, which served as the collector. n- SiC layer in turn was grown on a highly conductive n-type 6H-SiC substrate. Carrier concentration of p-type and n-type SiC layers, and that of the substrate were 3x10 18 cm -3 , 1x10 18 cm -3 , and 3x10 19 cm -3 , respectively. AlGaN layer was grown by MOCVD, with trimethylgallium (TMGa), trimethylaluminum (TMAl) and ammonia as the Ga, Al and N sources, respectively. The growth was carried out at a relatively low chamber pressure of 30 torr at 1060 o C. The flow rate for Ga, Al, NH 3 were kept at 55 µmol/min, 8 µmol/min, 1000 sccm, respectively. According to the x-ray diffraction (XRD) results, which are shown in Fig.1, from one test growth on a GaN template using the same growth condition, the composition of Al for the AlGaN layer was around 15%. Reasonably high crystal quality of the grown AlGaN layers were observed as seen from FWHM value of x-ray rocking curve for (0002) and (10 12) peaks 3.7 arcmin and 12 arcmin, respectively, - values comparable to the reported figures [5,6]. The HBT mesa structures were fabricated using conventional photolithography on a 10x10 mm 2 size substrate. A schematic of the fabricated AlGaN/SiC HBT is shown in Fig.2. The HBT mesa structures were isolated from each other by tranches. Ohmic contacts to n-AlGaN emitter, p-SiC Gallium Nitride Materials and Devices III, edited by Hadis Morkoç, Cole W. Litton, Jen-Inn Chyi, Yasushi Nanishi, Euijoon Yoon, Proc. of SPIE Vol. 6894, 68941W, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.763143 Proc. of SPIE Vol. 6894 68941W-1 2008 SPIE Digital Library -- Subscriber Archive Copy