Investigation of Temperature Sensing Capabilities of GaN/SiC and GaN/Sapphire Surface Acoustic Wave Devices

Abstract

This paper proposes high sensitivity temperature sensors based on single port surface acoustic wave (SAW) devices with GHz resonance frequencies, developed on GaN/SiC and GaN/Sapphire, which permit wide range, accurate temperature determinations. In contrast with GaN/Si SAW based temperature sensors, SiC and Sapphire substrates enable the proper functionality of these devices up to 500°C (773 K), as the high resistivity Si substrate becomes conductive at temperatures exceeding 250°C (523 K) due to the relative low bandgap (and high intrinsic carrier concentrations). Low temperature measurements were carried out using a cryostat between -266°C (7 K) and room temperature (RT) while the high temperature measurements are made on a modified RF probe station. A polynomial fit was used below RT and a linear approximation was evidenced between RT and 500°C (773 K). The structures were simulated at different selected temperatures based on a method that couples Finite Element Method (FEM) and Coupling of Modes (COM). The measured temperature coefficient of frequency (TCF) is about 46 ppm/°C for GaN/SiC SAWs and reaches values of 96 ppm/°C for GaN/Sapphire SAW in the temperature range 25 – 500°C (298 K – 773 K).