The thermal effects of substrate removal on GaN HEMTs using Raman Thermometry

Abstract

The ability to fabricate AlGaN/GaN high electron mobility transistors (HEMTs) on Si substrates has enabled the production of low cost high power electronics. To further enhance the performance of GaN electronics for high power conversion, the ability to maintain high off-state breakdown voltages with large electron densities is necessary. The use of Si substrates, however, limits the device's capabilities due to its weak electrical field strength. This limitation has been identified as the main cause for breakdown in HEMTs when the high electric field reaches the silicon substrate underneath the region between the gate and drain. To overcome this obstacle, removal of the Si substrate between the gate and drain region has shown to increase the device's breakdown voltage up to 3000 V. While removing the Si substrate extends the capabilities of GaN HEMTs for high voltage applications, the effects of the Si removal on the thermal performance during operation has not yet been investigated. Raman Thermometry, a well-developed technique, is used to compare the maximum temperature rise between a Local Substrate Removed (LSR) device and a non-LSR device. The application of nanoparticles (TiO2 and ZnO) for measuring surface temperatures via Raman spectroscopy is also investigated and applied to determine a more accurate temperature of the gate junction temperature. The LSR device was found to have a much higher thermal resistance than its non-LSR device counterpart limiting the maximum power dissipation the LSR device can achieve before severe degradation. Volumetric averaged residual stress mapping was also measured via Raman Spectroscopy and suggests the removal of the Si relaxes the stress in the GaN buffer layer and AlGaN barrier which can be exploited in designs to improve reliability. Methods to improve the thermal reliability of LSR devices are key to implementing such devices as future power switches.