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 upto 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).

Thin AlN piezoelectric layers have been deposited on high resistivity Si and glass substrates by reactive RF magnetron sputtering, in order to manufacture one port GHz operating SAW type resonators to be used as temperature sensors. The growth morphology surface topography, crystallographic structure and crystalline quality of the AlN layers have been analysed. Advanced nano-lithographic techniques have been used to manufacture structures having interdigitated transducers with fingers and finger interdigit spacing width in the 250 – 170 nm range. High resonance frequency ensures the increase of the sensitivity, but also of its normalized value, the temperature coefficient of frequency (TCF). The resonance frequency shift vs. temperature has been measured in the -267 – +150 °C temperature range, using a cryostat set-up adapted for on wafer microwave measurements up to 50 GHz. The sensitivity and the TCF were determined in the 23 – 150 °C temperature range for all measured structures. High values for the sensitivity as well as for the TCF have been obtained. The performances are compared with previous results obtained for GaN/Si GHz operating sensors. For the first time, a numerical method based on finite element technique and coupling of modes was implemented in order to simulate the variation of the resonance frequency of the envisaged AlN/Si and AlN/glass SAW structures within the 25 – 150 °C temperature range.