(Invited) The Effect of Thermally Induced Piezoelectricity on GaN HEMT Device Characteristics

Wide bandgap semiconductor-based High Electron Mobility Transistor (HEMT) power amplifier devices are coming into widespread use in a variety of applications, including hybrid electric vehicles.  These devices dissipate considerable power, which, when combined with shrinking package size, leads to high densities of heat flux.  To remove this heat, novel cooling approaches are used which can create significant thermal gradients.   This study will discuss these packaging and thermal management approaches focusing on the piezoelectric defect generation in AlGaN/GaN high electron mobility transistors’ (HEMTs) resulting from thermomechanical stresses produced by these thermal gradients and the effect on electrical reliability. Two different multiphysics modeling software packages were used to determine the thermal gradients in a model of a HEMT device, convert them to thermomechanical stresses and then voltage gradients, and then assess defect creation resulting from the voltage gradient.  First a thermal gradient of between 30°C to 50°C was imposed across the device.  For this condition, a principal stress of 0.73GN/m² was seen in the device along with a 9V drop across the AlGaN/GaN barrier. Next, devices were probe tested at a power density of 6.9W/mm in the GaN and a baseline temperature of 120°C in the substrate. Evident drain and gate current and on-resistance degradation were observed after 30 minutes.