AlGaN/GaN HEMTs have been shown to be efficient sensors for a broad range of physical parameters, in either liquid or dry condition, such as pressure sensor [1,2], gas detection [3,4], pH sensor [5], and more recently used as biosensors for the rapid detection of viruses [6]. These achievements could pave the way for the use of these HEMT transistors in electronic nose development particularly useful for volatile organic compound (VOC) detection. Among VOCs, acetone is one of the most important elements because it can be used as a biomarker for early disease, such as lung cancer, detection. For this purpose, sensors with responsivity in the range below 1 ppm are desired [7]. Up to now, only few works have been reported on acetone detection with rather contradictory observations between its detection at room temperature (RT) [8] or high temperature [9]. In this work, we report on AlGaN/GaN HEMT sensors for acetone concentration below 100 ppm and in a broad range of the sensor temperature varying from (RT) to 300°C.
At RT, in presence of acetone, a smooth and monotonic decrease of the current is observed with a rather large responsivity of 15µA/ppm and with large response time (several minutes) and memory effect. This decrease of the current can be explained by the electrostatic interaction between the 2D gas in the HEMT with the dipolar moment of the acetone molecules as described by Neuberger et al. [8]. This decrease is also in agreement with the work of Neuberger. et al [8].
At high temperature (300°C), in contrary to what has been reported in [9], a current decrease is first observed just after the acetone injection and then followed by an increase which saturates and stabilizes at a constant value. In order to clarify this unexpected behavior, a detailed study of the sensors response versus the temperature and acetone injection mode has been carried out. The output of this investigation is that a competition between the current variations induced by both the sensor and gas flow temperature difference from one side and acetone dipolar moment from the other side can explain this transient. At high temperature, the gas flow (especially for high acetone concentration) tends to cool down the sensor inducing an increase of the current, whereas the true acetone effect leads to a decrease of the current.
AlGaN/GaN HEMTs – based sensors are shown to allow for very sensitive acetone detection at both room and high temperature. Nevertheless, care must be taken during the characterization and operation of such sensors especially at high operating temperature. Increasing the latter, can help to improve the sensor response suppress the memory effect, but requires the control or the cancellation of the current transient due to the temperature difference between the gas flow and the transistor gate.
References
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