This paper provides a comprehensive review of recent progress in GaN-based gas sensors with a focus on their dynamic-related responses, mechanisms and applications, which are important for sensing performance, efficiency, and signal processing.

The concentration of the two-dimensional electron gas (2DEG) at the AlGaN/GaN heterojunction is dominated by the stress developed in AlGaN layer. In this work, the output performance of AlGaN/GaN high electron mobility transistors (HEMTs) was improved by depositing a stressed SiN _x as a passivation layer, and the external strain applied onto AlGaN was measured using Raman spectroscopy. As a result, an improvement in on-state resistance and drive current of the device was attained due to the extra 2DEG concentration induced by the external strain. Also, to determine the strain-induced threshold-voltage shift, the stress distribution over the gate region was analyzed.

A Pt-gated AlGaN/GaN high electron mobility transistor (HEMT) was fabricated and used for sensing hydrogen (H₂) gas in air ambient at extreme temperatures. Compared to previous studies, this work extends the investigation of GaN HEMT device H₂ detection capabilities to temperatures as high as 450 °C in air background. The performance of the device was comprehensively demonstrated and analyzed over a wide temperature range from 200 to 450 °C. To begin with, an optimum operating drain-source (V _ds ) and gate-source voltages (V _gs ) were found as 4.5 and 0 V, respectively. At this bias, the sensor demonstrated the highest sensitivity (28.19%) in conjunction with fast response (48.62 s), and recovery time (109.48 s) at 450 °C. The impact of temperature on sensitivity, response, and recovery time was also discussed, revealing that sensitivity ranged from 24.9 to 28.2% between 250 and 450 °C, which is significantly higher than the sensitivity at 200 °C (18.8%). Additionally, response times under 50 s were obtained from 350 to 450 °C, with the lowest being 39.54 s at 400 °C. Furthermore, the signal-to-noise ratio consistently exceeded 30 dB across the temperature range of 250 to 450 °C. Finally, the first-derivative method, utilized for assessing the triggering capability of the sensor, revealed peak performance at 300 °C.

A novel AlGaN/GaN Schottky barrier diode (SBD) thermal sensor featuring a recessed anode and a thin Al_0.25Ga_0.75N barrier layer is fabricated, which is demonstrated to have ultralow power, excellent linearity and high sensitivity for thermal sensing. The AlGaN layer thickness is employed to trade off the tunneling current proportion against the two dimensional electron gas (2DEG) density for the thermionic emission (TE) dominated electron transportation with better linearity and high sensitivity of thermal sensing at ultralow power. A sensitivity of 1.39 mV/K with a linearity of 0.995 operating at 0.082 - 0.324 V for a constant current of 10^-7 A is revealed. The proposed SBD sensor shows the great potential application for on-chip, low power, and high performance thermal sensing in monolithic AlGaN/GaN heterostructure based power ICs.

In this work, the DC performance and RF characteristics of GaN-based high-electron-mobility transistors (HEMTs) using the SiN_x stress-engineered technique were systematically investigated. It was observed that a significant reduction in the peak electric field and an increase in the effective barrier thickness in the devices with compressive SiN_x passivation contributed to the suppression of Fowler–Nordheim (FN) tunneling. As a result, the gate leakage decreased by more than an order of magnitude, and the breakdown voltage (BV) increased from 44 V to 84 V. Moreover, benefiting from enhanced gate control capability, the devices with compressive stress SiN_x passivation showed improved peak transconductance from 315 mS/mm to 366 mS/mm, along with a higher cutoff frequency (f_t) and maximum oscillation frequency (f_max) of 21.15 GHz and 35.66 GHz, respectively. Due to its enhanced frequency performance and improved pinch-off characteristics, the power performance of the devices with compressive stress SiN_x passivation was markedly superior to that of the devices with stress-free SiN_x passivation. These results confirm the substantial potential of the SiN_x stress-engineered technique for high-frequency and high-output power applications, which are crucial for future communication systems.