Current collapse in AlGaN/GaN transistors studied using time-resolved Raman thermography

Current collapse in AlGaN/GaN high electron mobility transistors was investigated using time-resolved Raman thermography. The virtual-gate mechanism was visualized by changes in the device temperature distribution, illustrating an effective gate lengthening up to 0.6 um. Two devices with different levels of current collapse are compared, demonstrating that the effective gate length increases for greater current collapse. A comparison of two-dimensional drift diffusion simulations with experimental data was used to estimate a lower limit for the charge trapping density in the virtual-gate region. This was found to be of the order of 2×1013 cm−2 for a device exhibiting relatively little current collapse.