AlN/GaN HEMTs with f_max Exceeding 300 GHz by Using Ge-Doped n⁺⁺ GaN Ohmic Contacts

Here, we report a high-performance AlN/GaN high electron mobility transistor (HEMT) by using a heavily Ge-doped regrown GaN source and drain. We demonstrated that the compressive stress caused by Ge doping is proven for the first time via the lattice distortion along the [0001] axis in GaN by X-ray diffraction reciprocal space mapping (XRD-RSM) analysis. The Ge-doped GaN then affords a high electron concentration (n 2.5 x 10(20) cm(-3)) with a crack-free surface and consequently leads to a low contact resistance of R-c similar to 0.09 Omega.mm for the HEMT. Thereupon, an enhanced current gain cutoff frequency and maximum oscillation frequency of f(T)/f(max) similar to 140/301 GHz for a 70 nm gate length is achieved, along with a large maximum drain current density of I-max similar to 1.54 A/mm and a high transconductance of G(m) similar to 0.74 S/mm. These results are evidently preferable to those of the contrastive HEMT with rapid-thermal-annealed Ohmic contacts. This study provides a viable approach to better device gain and frequency performance for ultrascale GaN-based devices.