Modelling and Assessment of Threshold Voltage Instabilities in p-gate GaN HEMTs

• Applicant: Professor Dr.-Ing. Martin Pfost
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering ; Electrical Energy Systems, Power Management, Power Electronics, Electrical Machines and Drives
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 462842520

In this study, we intent to develop a verified compact model for p-gate GaN HEMTs that is accurate for simulating DC, AC, and large-signal transient simulations including threshold voltage instabilities which can be used for short-circuit and possibly health monitoring. The model will be usable for a wide range of voltage classes so that most existing devices can be considered. A common practice to demonstrate the threshold voltage instability is a quasi-static determination of the threshold voltage shift. However, due to the restrictions of this methodology, we developed a measurement procedure which is suitable to demonstrate that the change in threshold voltage is a time-dependent transient phenomenon. According to the acquisition of physically relevant quantities, we are convinced that the parameterization of the model can be done based on that measurements. Furthermore, the model will be based on a surface-potential approach (physically based) and by means of analytical equations implemented in VerilogA assuring the applicability for common circuit simulation tools.The mechanisms leading to a threshold voltage instability were identified to be caused by accumulation and trapping of carriers, which results in a transient change of charge in the 2-dimensional electron gas region under the gate structure. According to literature in conjunction with our measurements, the mechanisms to be considered comprise injection and trapping of electrons and holes. Therefore, thermionic emission, Pool Frankel emission, field assisted tunnelling, trap-release of existing states as well as thermal generation and recombination of carriers will be accounted for.The threshold voltage instability can be problematic for advanced power electronic systems. This is because the transient threshold voltage change can be large. Most importantly, this can lead to device damage in error conditions, for example in overload or short circuit, because the device will not turn off safely if the transient threshold voltage is much smaller than expected. Studies on reliability of these devices indicate that important degradation mechanisms and device robustness are accompanied by a shift of the threshold voltage. On the other hand, the fabrication spread related threshold voltage mismatch of monolithically integrated GaN circuits could further be complicated by the transient change of the threshold voltage. Therefore, health monitoring, predictive maintenance as well as the predictability of transient changes in characteristics are conceivable for GaN devices, which could increase their acceptance especially in environments where unplanned downtime is not acceptable.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2312: Energy Efficient Power Electronics "GaNius"