A Standalone GaN HEMT‐Based pH Sensing System for Agricultural, Industrial, and Biomedical Applications

ABSTRACT For living things to function correctly, healthy soil conditions, water quality management, industrial product efficacy control, potential of hydrogen (pH) level regulation, and monitoring are critical and essential. In this work, we report a portable high‐resolution packaged Gallium Nitride High Electron Mobility Transistor (GaN HEMT)‐based sensor and standalone system for the detection and monitoring of pH and neurochemicals as a case study. The proposed sensing platform is standalone in nature and offers a sensitivity of 288 and 300 µA/pH for the conventional and meander‐gated sensors, with linearity of 0.887 and 0.981, respectively. The sensitivity was enhanced from 85 to 181 µA/pH when a dual‐capped sensor was used instead of the conventional sensor with no cap layer. The sensitivity was further enhanced to ∼1.50 mA/pH when sensing areas were increased from 300 to 750 µm 2 . The sensor offers a low average drift value of 1.92 µA/min for the repeated tests and 3.28 µA/min for the device‐to‐device variability analysis for a sensor with a source‐to‐drain distance ( L sd ) of 25 µm. The device design with a meander gate shows a better drift performance with drift values of 2.40 µA/min as compared to the conventional design with a value of 4.80 µA/min for sensors with a sensing area of 300 µm 2 . The long‐term stability analysis shows a low drift value of 0.02 µA/h for a test performed in an open environment for 6 h at room temperature. Similarly, the sensor drift values were found to be 0.55 nA/h for tests conducted in dark environments, demonstrating the sensor's robustness and the negligible impact of the test environment on sensor performance. A stable sensor drift performance was observed for the sensor input bias of 3.3 and 5.0 V, respectively. The sensor's capability to work without a reference electrode has been demonstrated, and the sensor drift values were found to be only 0.50 and 1.25 µA/min for tests conducted with and without reference electrodes. The sensors offer a sensitivity value of 123.33 and 126.67 µA/pH for the tests conducted with and without reference electrodes, clearly demonstrating the platform's suitability for reference electrode‐free operation. The sensors offer a peak voltage ( S V ) and current sensitivity ( S I ) of 58.33 mV/pH and 1.27 mA/pH, respectively, for the sensor with a source‐to‐drain distance ( L sd ) of 25 µm. Further, the sensor offers a high‐resolution detection of pH solutions in the range of 7.10–7.40 with a sensitivity value of 287 µA/pH, clearly demonstrating its suitability in biological fluids such as blood. The sensor shows hysteresis values as low as 1.0 and 3.0 µA for detecting pH and phosphate‐buffered saline (PBS) solutions. The sensors have been tested in different test environments and offer a sensitivity of 30.0, 26.67, and 23.33 µA/pH for the test conducted at 15°C, 20°C, and 25°C, respectively, clearly demonstrating their suitability and adaptability for variable test conditions. The sensor demonstrates a highly repeatable and reproducible performance for detecting biological grade solutions such as PBS with concentration ranges from 0.1 to 10×. Further, the sensor shows a sensitivity of 28.80 and 46.66 µA/min for detecting potassium chloride (KCl) and sodium chloride (NaCl), respectively. Thus, the key features of the developed platform include fast response time, high resolution, compact standalone system, negligible drift values, robustness in variable test environments, portable test setup, and high sensitivity, thereby demonstrating its suitability for multi‐sectoral application areas.