Simultaneous Measurement of Temperature and Salinity Based on GaN Monolithic Devices

Precise measurement of temperature and salinity is crucial for marine engineering, resource exploitation, and environmental protection. However, developing a compact device capable of simultaneously measuring both parameters remains a considerable challenge. This study presents a GaN monolithic device that integrates a light-emitting diode (LED) and a photodetector (PD) on a single GaN-on-sapphire chip. Increased salinity reduces the light transmitted to the PD through the sapphire waveguide, decreasing the photocurrent of the PD. Rising temperatures narrow the space charge region, lowering the forward voltage of the PD. Through a signal decoupling of the photocurrent and forward voltage using a linear matrix, salinity with a sensitivity of 0.634 μA/% within a range of 0–7 % and temperature with a sensitivity of 1.53 mV/°C within a range of 5–40 °C can be extracted. The potential applications of this device in practical seawater monitoring are also explored.