AlN/GaN HEMTs with <i>f</i><sub>max</sub> Exceeding 300 GHz by Using Ge-Doped <i>n</i><sup>++</sup>GaN Ohmic Contacts
Liuyun Yang, Wei Huang, Ding Wang, Baoqing Zhang, Yibin Zhang, Junyu Zhang, Tangsheng Chen, Weikun Ge, Shaobing Wu, Bo Shen, Xinqiang Wang
Abstract
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 × 10 20 cm –3 ) with a crack-free surface and consequently leads to a low contact resistance of R c ∼ 0.09 Ω·mm for the HEMT. Thereupon, an enhanced current gain cutoff frequency and maximum oscillation frequency of f T / f max ∼ 140/301 GHz for a 70 nm gate length is achieved, along with a large maximum drain current density of I max ∼ 1.54 A/mm and a high transconductance of G m ∼ 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..