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 paper, a method to extend the detection range of hydrogen sulfide (H₂S) gas sensor is demonstrated. The sensor is based on AlGaN/GaN high electron mobility transistors (HEMTs) with Pt gate. It is observed that the as-fabricated devices exhibited sensing signal saturation at 30 ppm H₂S exposure in dry air. A pre-treatment using H₂ pulses in dry air ambient at 250 °C was applied to extend the detection range of the sensor. The H₂ treated H₂S gas sensor was able to detect a higher H₂S concentration up to 90 ppm at 250 °C without complete saturation.

Digital etching is an effective method to lower dry etch damages in A1GaN/GaN HEMTs. This work systematically investigated O₂-plasma-based digital etching of AlGaN and p-GaN. AlN layers were used as the etch stop layers in the AlGaN etch. Important process aspects such as the use of the AlN layers, the RF power, the oxygen flow rate, the oxidation time and the resulting roughness were studied. These are technically relevant to obtain controllable, uniform etch surfaces with low surface damages for better HEMT performance.

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.

O₂ plasma-based digital etching of Al_0.25Ga_0.75N with a 0.8 nm AlN spacer on GaN was investigated using an inductively coupled plasma etcher. Silicon oxide layer was used as the hard mask. At 40 W RF bias power and 40 sccm oxygen flow, the etch depth of Al_0.25Ga_0.75N was 5.7 nm per cycle. The 0.8 nm AlN spacer layer acted as an etch-stop layer in three cycles. The surface roughness improved from 0.66 to 0.33 nm after the three and seven digital etch cycles. Compared to the dry etch only approach, this technique smoothed the surface instead of causing surface roughening. Compared to the selective thermal oxidation with a wet etch approach, this method is less demanding on the epitaxial growth and saves the oxidation process. It was shown to be effective in precisely controlling the AlGaN etch depth required for recessed-AlGaN HEMTs.

We developed an AlGaN/GaN high electron mobility transistor (HEMT) sensor with a tungsten trioxide (WO₃) nano-film modified gate for nitrogen dioxide (NO₂) detection. The device has a suspended circular membrane structure and an integrated micro-heater. The thermal characteristic of the Platinum (Pt) micro-heater and the HEMT self-heating are studied and modeled. A significant detection is observed for exposure to a low concentration of 100 ppb NO₂ /N₂ at ∼300 °C. For a 1 ppm NO₂ gas, a high sensitivity of 1.1% with a response (recovery) time of 88 second (132 second) is obtained. The effects of relative humidity and temperature on the gas sensor response properties in air are also studied. Based on the excellent sensing performance and inherent advantages of low power consumption, the investigated sensor provides a viable alternative high performance NO₂ sensing applications. It is suitable for continuous environmental monitoring system or high temperature applications.

Ohmic contacts to AlGaN/GaN with different metal stacks on Si or Sapphire substrate are fabricated and compared in this paper. For Au-capped ohmic contacts, the lowest contact resistances of 0.7 Ω·mm and 1.3 Ω·mm are achieved by Ti/Al/Ti/Au (20/110/40/50 nm) and Ti/Al/Ni/Au (20/110/40/50 nm) stacks, respectively. It also shows that the substrate material and epitaxial structure play an important role in ohmic contact engineering. For CMOS compatible Au-free structures, the Ti/Al/W (20/100/30 nm), Ti/Al/Ni/W (20/100/20/10 nm) and (20/100/10/20 nm) are demonstrated with the minimum contact resistance values of 0.45, 1.3, and 1.6 Ω·mm, respectively. The three metal stacks of Au-free ohmic contact are compared and obtained results are explained.

Wide bandgap gallium nitride material has highly favorable electronic properties for next generation power and high frequency electronic devices. A less widely studied application is highly miniaturized chemical and gas sensors capable of operating in harsh environment conditions. In this work we present our recent developments on design, fabrication and testing of AlGaN/GaN high electron mobility transistor (HEMT) based sensors for detection of various gases. First, the method of as-fabricated device baseline value stabilization is demonstrated. Secondly, the impact of sensor design is discussed with the emphasis on gate electrode geometry optimizations to enhance sensing performance. Then we present the sensing characteristics of Pt-HEMTs towards H ₂ S and compare them to H ₂ and NO ₂ . Finally we demonstrate recent results of NO ₂ detection with Ti/Au based HEMT sensors, which are superior to those using Pt based devices.

This paper presents a hybrid-powered wireless sensor node using enhanced triboelectric nanogenerator (TENG) as both energy harvester and air-flow sensor and two 11 cm2 solar panels as extra power supply. A low budget commercial RF microcontroller is included for data conversion, signal processing and wireless transmission. The method of flow-rate detection depends on the vibration frequency of the film inside the triboelectric generator. Experiment results show that this flow sensor is capable of detecting flow rate from 7.6 m/s to 17.1 m/s, with a standard deviation of 3.4, 3s setup time and 30s charge time. With a Raspberry Pi, the wireless signal can be received and delivered to Internet and therefore, can be monitored easily from any portable terminal with internet-access.

This paper reports on the layout optimization of Pt-AlGaN/GaN HEMT-sensors for enhancing hydrogen sensor performance. Sensors with gate width and length ratios W_g/L_g from 0.25 to 10 were designed, fabricated and tested for the detection of hydrogen gas at 200 °C. Sensitivity, sensing current variation and transient response are directly related to the sensor gate electrode W_g/L_g ratio. The obtained results demonstrated a 217 % increase in sensitivity and 4630 % increase in sensing current variation at 500 ppm H2 for a W_g/L_g from 0.25 to 10. In addition, the detection limit was lowered to 5 ppm. Transient characteristics demonstrated faster sensor response to H2, but slower recovery rates with increasing ratio.

A method for highly controllable etching of AlGaN/GaN for the fabrication of high sensitivity HEMT based sensors is developed. The process consists of cyclic oxidation of nitride with O₂ plasma using ICP-RIE etcher followed by wet etching of the oxidized layer. Previously reported cyclic oxidation-based GaN etching obtained very slow etching rate (∼0.38nm/cycle), limited by oxidation depth. The proposed approach allows fine control of the oxidation enabling the formation of accurately controlled recess of very thin (20∼30nm) barrier layers. With optimized power settings, etch rates from ∼0.6 to ∼11nm/cycle were obtained. AFM results did not show any increase in surface roughness after etching, indicating that surface quality of the etched layer was not affected by the etching process.