I. INTRODUCTION

Nitrogen dioxide (NO$_{2}$), reddish-brown with a very harsh smell at high concentrations, is one of the most prominent toxic gases in the atmosphere. Exposure to NO$_{2}$ can cause serious harm to human health, such as asthma, chest tightness, obstructive lung disease, chronic obstructive pulmonary disease (COPD) and sometimes acute exacerbation and, in fatal cases, deaths ^(1-3). The major sources of nitrogen oxide emission are fossil fuel combustion, the exhaust of motor engines and power plants. Therefore, the requirement for the development of NO$_{2}$ gas sensors in a harsh environment is becoming more and more urgent.

Gas sensors based on semiconductors have been extensively developed in recent decades ^(4-6). Generally, semiconductor-based gas sensors often require a high temperature to induce adsorption and desorption of the target gas molecules ^(7-9). Although Si-based semiconductor manufacturing technologies are now mature, due to the narrow bandgap at 1.1 eV, Si-based gas sensors cannot withstand temperatures above 200 $^{\circ}$C. To overcome that problem, gas sensors based on wide bandgap semiconductors (GaN, SiC, Ga$_{2}$O$_{3}$) are attracting great interest recently ^(10-15). However, these sensors typically operate in a temperature range of about 300 $^{\circ}$C, which is unsatisfactory in extreme situations, such as automobile exhaust pipes or power plants.

In this paper, we studied NO$_{2}$ gas sensors based on Pd-AlGaN/GaN HEMT operating at very high temperatures up to 500 $^{\circ}$C, thereby illustrating the outstanding advantage of semiconductor sensors based on GaN over Si-based ones.

II. EXPERIMENTAL

III. RESULT AND DISCUSSION

Firstly, we performed the Technology Computer-Aided Design (TCAD) simulation on Silvaco Deckbuild 5.0.10.R to investigate the relationship between the sensitivity of the HEMT sensor and the gate bias voltage. The simulation was based on physical models of concentration-dependent lifetime (CONSRH), Auger recombination (AUGER) and Fermi Statistics (FERMI). The NO₂ gas introduction was simulated by the variation of gate work function, since the oxygen ions, generated after adsorption of NO₂ molecules on Pd catalyst layer, provided a negative charge, leading to an increase of the Pd work function. The sensitivity curve was extracted by the transfer characteristics. The simulation parameters were shown in Table 1. The simulation results showed that the sensitivity tended to increase as the gate voltage decreased (Fig. 3), illustrating the trade-off between sensitivity and base current level, when the sensitivity was defined by formula (1). This result is entirely consistent with the results reported in ⁽¹²⁾.

Fig. 4 showed the sensor’s response at gate voltage of 0 V at different temperatures up to 500 $^{\circ}$C under 100~ppm of NO$_{2}$. The sensor showed stable operation with 21, 18.3 and 9.5 % of sensitivity at 300, 400, and 500~$^{\circ}$C, respectively. The sensor’s response time drastically decreased at higher temperatures, representing an acceleration of adsorption of NO$_{2}$ molecules under the catalytic effect of Pd.

Fig. 5 showed the sensor’s response at different temperatures under 1 ppm of NO$_{2}$. For concentrations as low as 1 ppm, we measured at gate voltage of -1 V to employ the possibility of sensitivity optimization by gate modulation of HEMTs, which was shown in Fig. 3. A similar result was observed when a very fast response time at 500~$^{\circ}$C of 6 s was recognized. The high sensitivity of 8.1 % for 1~ppm NO$_{2}$ showed a considerable improvement when compared with other studies using HEMT structure (Table 2).

IV. CONCLUSIONS

This study demonstrated the capability to operate up to 500 $^{\circ}$C of NO$_{2}$ gas sensors based on Pd-AlGaN/GaN HEMTs, illustrating the outstanding advantage of wide bandgap GaN semiconductor. Along with the ability of sensitivity optimization by gate modulation, the gas sensor based on AlGaN/GaN HEMTs is an excellent choice for micro-system in harsh environments.