Thermal Resistance Of GaN Wafers And Electronic Devices

In recent years,wide-bandgap semiconductors,represented by GaN and SiC,have been extensively studied and developed for a wide range of applications.Benefiting from the large band gap and breakdown field strength,GaN-based high electron mobility transistors?HEMTs?are good candidates for high-power,high-frequency amplifiers.However,localized device-level self-heating limits the peak power density and degrades the device reliability.For GaN HEMTs,the effective thermal boundary resistance(TBR_eff)between the GaN epilayer and the substrate dominates the thermal diffusion process within devices,and it is therefore critically important to characterize the TBR_eff experimentally.Currently available methods for measuring the TBR_eff include time-domain thermoreflectance?TDTR?,Raman thermography,etc.that require metal transducer coating or device processing,which increase the cost and time for the TBR_eff characterization and in the meantime introduce additional uncertainties in the measurement.In this work,we have developed a rapid and non-destructive optical characterization method to measure the TBR_eff between the GaN epilayer and the substrate on a wafer level,with no additional processing required.The method is based on transient thermoreflectance?TTR?using above-bandgap ultraviolet lasers.We have successfully measured the thermal transport properties of GaN heteroepitaxial structures including GaN-on-SiC,GaN-on-Si,GaN-on-diamond,as well as a novel nanocrystalline diamond thin film grown on GaN.In addition,we have used Raman spectroscopy to measure the temperature distribution of industrial-grade GaN HEMTs in operational DC conditions,and to determine the overall thermal resistance as well as the interfacial thermal resistance in each device.We have compared the thermal resistances of GaN HEMTs with different gate widths or with different packaging methods.We have demonstrated that Raman thermography combined with finite element device thermal simulation is an effective tool to study the thermal resistance of GaN electronic devices.