The role of selective pattern etching to improve the Ohmic contact resistance and device performance of AlGaN/GaN HEMTs
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In this work, we report the processing and DC performance of fabricated AlGaN/GaN HEMT devices using 3 different patterned Ohmic contact structures. The types of Ohmic contact patterns used are horizontal, vertical and chess. A device with a conventional Ohmic contact was also fabricated for comparison. Two different etch depths were investigated, a ~ 9 nm and ~ 30 nm for a shallow and deep Ohmic recess etching, respectively. The lowest contact resistance of 0.32 Ω.mm was observed for a deep horizontal patterned structure. The fabricated device with this structure also demonstrated the highest maximum saturation drain current of 1285 mA/mm and maximum transconductance of 296 mS/mm compared to other devices. The horizontal patterned structure utilizes the uneven AlGaN layer thickness underneath the Ohmic metal contacts. The formation of sidewall areas on AlGaN surface during the patterned etching process provides better contact of Ohmic metal resulting in more tunnelling current between the Ohmic metal and AlGaN barrier thus reducing the contact resistance. This approach also provides the lowest contact resistance due to removal of AlGaN barrier layer (patterned etching) and it is in parallel with the lateral current of the 2DEG resulting in better tunnelling current compared to the vertical and chess patterned structures. The contact resistance can be further improved by optimization the etching depth prior to Ohmic metal deposition. The results indicate the potential of the Ohmic patterned etching structure to further improving the performance of GaN devices.