TMAH Pretreatment to Minimize Ohmic Contact Resistance in InAlN/GaN-on-Si RF HEMTs

We report a novel tetramethylammonium hydroxide (TMAH) pretreatment to minimize the contact resistance $\left(R_{C}\right)$ using Ti-based Ohmic contacts to InAIN/GaN-on-Si RF HEMTs. The TMAH treatment is shown to etch the dislocation sites on the surface, which act as a medium for Ti diffusion into GaN during annealing. This enhances the TiN contact inclusion formation inside the GaN channel layer and results in an $R_{C}$ of $0.23 \pm 0.01 \Omega-\mathrm{mm}$ , which is threefold lower compared to a sample without TMAH treatment. The impact of $R_{C}$ on RF performance is studied using sub-quarter micron HEMTs. For the sample with TMAH pretreatment, a high $f_{T}$ of 95 GHz and a $f_{T}-L_{G}$ product of $19 \mathrm{GHz}-\mu \mathrm{m}$ are achieved in a device with 200 nm gate and 900 nm source-to-drain separation.