Finite Element Analysis of Fabrication- and Operation-Induced Mechanical Stress in AlGaN/GaN Transistors

Mechanical stress is an important factor influencing the performance and reliability of GaN-based devices. In highly piezoelectric materials like AlGaN/GaN, mechanical stress directly influences the piezoelectric polarization, and hence the charge density of the 2-D electron gas. Fabrication processes for devices as well as device operating conditions can change the mechanical stress in AlGaN/GaN transistors. In this paper, the mechanical stress resulting from fabrication and operation of AlGaN/GaN transistors has been modeled using a finite element analysis approach. Specifically, the stress induced in AlGaN/GaN by chemical vapor deposition of silicon nitride films is discussed. Furthermore, the temperature-induced mechanical stress during on-state device operation is computed and the effect of source field plates and substrate materials has been investigated. Finally, the above-mentioned stress components are compared with those due to lattice mismatch, epi growth, and inverse piezoelectric effect. The potential impact of mechanical stress on device reliability is also discussed.