Effect of Buffer Charge Redistribution on RF Losses and Harmonic Distortion in GaN-on-Si Substrates

Understanding and mitigation of substrate RF losses and signal distortion are critical to enable high-performance GaN-on-Si front-end-modules. While the origin of RF losses and consequently a decreased effective substrate resistivity $({\rho }_{eff})$ in GaN-on-Si substrates is now understood to be diffusion of Al and Ga atoms into the silicon substrate during III-N growth, the effect of upper III-N buffer layers on the ${\rho }_{eff}$ degradation under stressed conditions remains unclear. In this paper, we show that up to 50% variation in ${\rho }_{eff}$ at 2 GHz can take place over more than 1,000 s when the substrate is stressed at 50 V. Additionally, Coplanar Wave Guide (CPW) large-signal measurements under the same experimental conditions show a variation of $2^{\mathrm{ nd}}$ harmonic power of up to 5dB. A thermally activated stress and relaxation behavior shows the signature of traps which are present in the C-doped layers. With the help of a simplified TCAD model of the GaN-on-Si stack, we link this behavior to slow charge redistribution in the C-doped buffer continuously modifying the flat-band voltage ( $\text{V}_{\text {FB}}$ ) of the Metal-Insulator-Semiconductor (MIS) structure. Free carrier transport across the buffer is shown to have the greatest contribution on the large time constants, highlighting the importance of vertical transport paths in GaN-on-Si stacks.