Ohmic Contacts for Wide Bandgap Devices
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Bipolar devices based on GaN and SiC have gained tremendous popularity as an alternative to Si based devices, primarily due to the ability to sustain high temperature and high voltage operations that can be attributed to their high breakdown field and saturation velocity. Any bipolar device based on wide bandgap technology requires high performing ohmic contacts that have low specific contact resistivity and exhibit linear I-V behavior, as opposed to Schottky characteristics. A range of high work function metals like Ni, Au, Pd, Pt, Al can be used to realize these contacts. Further, annealing under specific conditions is required to ensure extremely high doping in the near surface layer. In this research, the Ni/Au stack for p-type contacts, and the Ti/Al/Ti/Au layer for n-type contacts have been specifically investigated over a range of annealing time periods and temperatures. Control wafers with GaN:Mg (p-type) or GaN:Si and GaN/AlGaN (n-type) formed the basis of this study. The Transmission Line Model (TLM) technique was used to conduct measurements, and obtain the specific contact resistance. Specific contact resistances as low as 9.44*10-3 Ω-cm2 for n-type, and 5.73*10-3 Ω-cm2 for p-type were achieved. These shall form the basis for ultimate fabrication of an InGaN/GaN HBT with a high current gain, breakdown voltage (VBR) and current density (J). The quality of resulting contact is seen to depend on the initial doping/bulk resistance, determination of a unique time-temperature window, and careful process control. A further investigation is conducted into non-linear behavior exhibited by p-type contacts for low separation.