AC Excitation to Mitigate Drift in AlGaN/GaN HEMT-Based Sensors

Aluminum gallium nitride/gallium nitride (AlGaN/GaN) high electron mobility transistor (HEMT)-based sensors hold promise as small solid-state physical and chemical sensors because they can operate without a reference electrode and can be integrated into miniaturized sensor arrays. However, overextended time periods, low-frequency noise causes anomalous variations (drift) in sensor signal, especially in liquid environments. These effects occur despite electromagnetic interference mitigation. To understand the low-frequency noise, 1/ f noise measurements between 0.1 and 100 kHz were undertaken, in both air and water, under constant pH and normal laboratory pressure conditions. The 1/ fγ noise for the device in water was larger in magnitude than in air, and estimates for the γ -parameter in air and water were approximately 1 and 1.5, respectively. The corner frequency was observed between 100 and 1000 Hz. Based on this analysis, alternating current (ac) excitation at 1 kHz was applied to the conduction channel to compare the sensor stability in deionized water with dc operation. In this controlled test, the introduction of ac excitation resulted in a strong correlation of sensor signal with the ambient temperature variations over nearly 90 h of testing (effectively acting as a temperature sensor with a high degree of stability) while operation in dc mode resulted in largely no correlation with temperature. This indicates that ac excitation above the corner frequency is a potentially effective method to mitigate long-term sensor instability, a critical limitation for any AlGaN/GaN transistor-based physical or chemical sensors in aqueous environments.