Processing and characterization of GaN/SiC heterojunctions and SiC bipolar transistors
Abstract: Silicon Carbide (SiC) and Gallium Nitride (GaN) are bothwide bandgap semiconductors that have been suggested for highpower, high voltage, and high temperature applications. Themost investigated SiC devices so far are the Schottky diode,PiN diode and the field effect transistor. However, untilrecently very few results have been presented for the BipolarJunction Transistor (BJT), which is the focus of this thesis.Since GaN can be grown epitaxially on SiC, the fabrication ofHeterojunction Bipolar Transistors is an interesting path tofurther enhance the operation of the bipolar transistor. Devicesimulations have also been extensively used in the analysis ofthe fabricated structures. Since the processing of wide bandgapdevices is still a relatively immature technology, a bipolarprocess was also developed as part of this thesis.The bipolar transistors were fabricated from n-type wafers,with up to four epitaxial layers grown on the substrate. Theprocess development was mainly focused on dry etching of SiC,passivation of etch damage, and metallization. For the dryetching of SiC an Inductively Coupled Plasma (ICP) etch wasused, and etch rates up to 0.3 µm/min was achieved with SF6as reactive gas. However, the damage increaseswith etch rate, thus the rate was usually lowered to 0.1µm/min for device fabrication. Even at these low etchrates the surface was damaged, which was investigated togetherwith the passivation of the damage. Metallization schemes wereinvestigated for both n-type and p-type SiC.The fabricated BJTs showed a current gain of 10 times and anopen base breakdown voltage up to 600 V. A SiC BJT withimplanted emitter was also fabricated, using phosphorus asimplanted n-type dopant. However, only unity current gain wasobtained on these transistors, and further development of theimplantation process is necessary.The bipolar process was also used to fabricate GaN/SiC HBTs,with the GaN as the top emitter layer. The dry etching of GaNwas performed using a conventional Reactive Ion Etch (RIE). Themetallization to n-type GaN and AlGaN was also developed, andthe Al-Ti alloy worked excellent as ohmic contact, with contactresistivities in the 10-5? cm2range. Since these HBTs showed poor transistoroperation, a detailed electrical characterization of GaN/SiCheterojunctions was initiated. The heterojunctions werecharacterized using current-voltage (IV) andcapacitance-voltage (CV) measurements. A model was presentedfor a recombination process assisted by tunneling, which couldexplain the poor operation of the HBTs. The type II (ECfor GaN below ECfor SiC) alignment of the GaN/SiC heterojunctionmay also enhance the recombination process. Differentcompositions of AlGaN were investigated as heterojunctions onSiC, and the Al0.3Ga0.7N, showed a change in the characteristics thatindicated a decreased recombination.Keywords:silicon carbide, gallium nitride, devicesimulation, bipolar junction transistor, heterojunction,heterojunction bipolar transistor, current-voltage measurement,capacitance-voltage measurement, self-heating.
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