N‐polar n+ GaN cap development for low ohmic contact resistance to inverted HEMTs

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AbstractThis study investigates the development and application of Si‐doped N‐polar n+ GaN capping layers used for the reduction of ohmic contact and access resistances in inverted HEMTs. By applying an optimized n+ GaN capping layer to a standard undoped inverted HEMT, we find that non‐alloyed Ti/Al/Ni/Au contacts demonstrate a very low ohmic contact resistance of 80 Ω‐µm. To eliminate the n+ GaN shunting pathway in HEMT devices, we have used e‐beam lithography and dry etching to define windows that establish an effective source‐drain spacing. 0.3 µm gate‐length inverted HEMTs with n+ GaN caps and 1 µm effective source‐drain spacing were found to have a high maximum current density of 1.94 A/mm and an extrinsic transconductance of 335 mS/mm. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)