A particulate matter micro-sensor for automotive exhaust systems based on a gateless wide-bandgap AlGaN/GaN high electron mobility transistor was developed and tested. Soot particles were generated by a laminar diesel flame and characterized with Raman spectroscopy, thermogravimetric analysis and scanning electron microscopy. Particle adsorption at the rate of 0.25 µg/min on the sensor surface resulted in 5.52% sensing response after 20 s and large signal variation of 4.44 mA, indicating fast response time. Saturated response of 34.72% (27.94 mA) was obtained after 10 min of deposition. The sensitivity towards soot is attributed to the modulation of the two-dimensional electron gas density by charged particles on the sensing surface. After soot deposition, the sensor was successfully regenerated by thermal oxidation of the carbonaceous particles at 600 °C. The sensing response remained unchanged post-regeneration indicating high temperature stability and harsh environment operation compatibility of the demonstrated GaN-based sensor. Nevertheless, interconnect metal optimization is still required to mitigate high-temperature interdiffusion.

The present work reports on the hydrogen gas detection properties of Pt-AlGaN/GaN high electron mobility transistor (HEMT) sensors with recessed gate structure. Devices with gate recess depths from 5 to 15 nm were fabricated using a precision cyclic etching method, examined with AFM, STEM and EDS, and tested towards H2 response at high temperature. With increasing recess depth, the threshold voltage ( VTH ) shifted from -1.57 to 1.49 V. A shallow recess (5 nm) resulted in a 1.03 mA increase in signal variation ( Δ IDS ), while a deep recess (15 nm) resulted in the highest sensing response ( S ) of 145.8% towards 300 ppm H2 as compared to reference sensors without gate recess. Transient measurements demonstrated reversible H2 response for all tested devices. The response and recovery time towards 250 ppm gradually decreased from 7.3 to 2.5 min and from 29.2 to 8.85 min going from 0 nm to 15 nm recess depth. The power consumption of the sensors reduced with increasing recess depth from 146.6 to 2.95 mW.

In this work, a thin-film transistor gas sensor based on the p-N heterojunction is fabricated by stacking chemical vapor deposition-grown tungsten disulfide (WS₂) with a sputtered indium-gallium-zinc-oxide (IGZO) film. To the best of our knowledge, the present device has the best NO₂ gas sensor response compared to all the gas sensors based on transition-metal dichalcogenide materials. The gas-sensing response is investigated under different NO₂ concentrations, adopting heterojunction device mode and transistor mode. High sensing response is obtained of p-N diode in the range of 1-300 ppm with values of 230% for 5 ppm and 18 170% for 300 ppm. On the transistor mode, the gas-sensing response can be modulated by the gate bias, and the transistor shows an ultrahigh response after exposure to NO₂, with sensitivity values of 6820% for 5 ppm and 499 400% for 300 ppm. Interestingly, the transistor has a typical ambipolar behavior under dry air, while the transistor becomes p-type as the amount of NO₂ increases. The assembly of these results demonstrates that the WS₂/IGZO device is a promising platform for the NO₂-gas detection, and its gas-modulated transistor properties show a potential application in tunable engineering for two-dimensional material heterojunction-based transistor device.