Electrical and structural dependence of operating temperature of AlGaN/GaN HEMTs

Understanding the distribution of the considerable heat generated in the active region of high power AlGaN/GaN high electron mobility transistors (HEMTs) at the sub-micron length scales relevant to the failures being observed in these devices is crucial for understanding device performance and reliability. In addition, electrical bias conditions and structural characteristics such as field plates alter the electric field distribution and thermal path within the device leading to changes in the heat generation profile across the channel. This in turn influences the value and location of the device peak temperature and the channel to ambient (or case or base-plate) thermal resistance. The channel temperature distribution of AlGaN/GaN HEMT structures with and without source connected field plates were examined via micro-Raman spectroscopy and coupled electro-thermal simulation. For both type of structures, high V _ds conditions lead to significantly higher channel temperature compared to that for low V_ds conditions for the same power dissipation level. This is important because the industry standard Arrhenius relation assumes the total power is sufficient to describe the device channel temperature and that the bias condition is irrelevant [1]. We explore the level of agreement between modeling and experiment, and also the extent to which variability in input parameters for the modeling affects model results. We show that operating bias condition has a significant role in device reliability by altering value and location of the peak temperature, which then alters the type and rate of thermally induced degradation taking place at critical locations such as the drain side corner of the gate. Specifically, care must be taken when extrapolating results of an accelerated life test to usage conditions at dissimilar bias conditions to consider if the results will be applicable.