220GHz fT and 400GHz fmax in 40-nm GaN DH-HEMTs with re-grown ohmic

We report record RF performance in 40nm-gate GaN-HEMT technology. Through vertical scaling in an AlN/GaN/AlGaN double heterojunction (DH) HEMT structure and reduction of access resistance using MBE re-growth of n⁺-GaN ohmic contacts, fully-passivated 40-nm devices exhibited excellent DC characteristics, such as an R_on of 0.81Ω·mm, an I_dmax of 1.61A/mm, a BV_off of 42V, and a peak extrinsic g_m of 723mS/mm, resulting in a peak f_T of 220GHz and a peak f_max of 400GHz. The measured f_T and f_max are the highest ever reported in a GaN-HEMT technology. Small signal model and delay time analysis showed that the parasitic charging time was only 10% of total delay time and the gate transit time scaled with the gate length (L_g) down to 40nm, demonstrating high scalability of the new technology.