Submicrometer Copper T-Gate AlGaN/GaN HFETs: The Gate Metal Stack Effect

We demonstrate the realization of ~0.2-mum T-shaped copper gate AlGaN/GaN heterojunction field-effect transistors (HFETs) and compare the performance achieved with Cu, Cu/Au, Ni/Cu, and Ni/Au gate metal stack configurations in otherwise nominally identical T-gate transistors that are fabricated on the same chip. Surprisingly, simply replacing the Ni/Au gate stack by a Cu metallization increases the f_TtimesL_G product by up to 40%, yielding a current-gain cutoff frequency as high as 82 GHz without further modifications to the process. Cu-based metallizations reduce the gate leakage current by one to two orders of magnitude and also result in broader transconductance characteristics. The results call for a detailed investigation of gate metallurgy effects in the AlGaN/GaN-based HFETs