Performance Limitations of GaN HEMTs with Quaternary InAlGaN and ScAlGaN Barrier Layers

Quaternary alloys such as InAlGaN and ScAlGaN are promising barrier materials for GaN-based advanced high electron mobility transistors (HEMTs) due to the higher achievable two-dimensional electron gas (2DEG) densities compared to conventional AIGaN barriers. In this work, we analyze the 2DEG density, mobility, on-resistance and transconductance of InAIGaN and ScAIGaN barrier HEMTs using a comprehensive in-house modeling framework, and validate our results with available experimental data for a lattice-matched InAIGaN/AIN/GaN HEMT. We explore the dependence of 2DEG density and mobility on barrier In/Sc-content and Al-content, and obtain an on-resistance below $90 \Omega / \square$ for the ScAIGaN barrier HEMT at Sc-contents below 0.18 and Al-contents above 0.78. We also find that a lattice-matched ScAIGaN barrier device achieves a 2DEG density higher by a factor of 3 at zero gate bias compared to the InAIGaN reference device. This means that a thinner ScAlGaN barrier can be utilized to obtain the same threshold voltage, resulting in a transconductance higher by a factor of 1.78 at an overdrive voltage of 4 V. The study gives insight into the physical mechanisms governing the analyzed figures of merit and provides guidelines for the design of future quaternary barrier HEMTs.