Effects of SiN passivation by catalytic chemical vapor deposition on electrical properties of AlGaN/GaN heterostructure field-effect transistors

We investigated the effects of SiN passivation by catalytic chemical vapor deposition (Cat-CVD) on the electrical properties of AlGaN/GaN heterostructure field-effect transistors. The two-dimensional electron density (N_s) greatly increased after the Cat-CVD SiN deposition, and the tendency of the increase was enhanced with decreasing AlGaN barrier thickness. As a result of the large increase in N_s, the sheet resistance (R_sh) significantly decreased after the deposition, and it had low values of 320–460 Ω/◻ for extremely thin AlGaN barriers of 4–10 nm. The increase in N_s showed little dependence on SiN thickness, indicating that the stress applied to the AlGaN barrier by SiN cannot be the origin of the increase. Cat-CVD SiN also improved the in-plane uniformity of mobility for extremely thin-barrier structures, which in turn improved the uniformity of R_sh. Moreover, we found that Cat-CVD was more effective than plasma-enhanced chemical vapor deposition in increasing N_s. A comparison of theoretical calculations and experimental results indicated that these behaviors can be explained by a decrease in the AlGaN surface barrier height due to the SiN deposition.