Quantum Dots-Sensitized High Electron Mobility Transistor (HEMT) for Room-Temperature Gas Detection
Authors: Hu Zhixiang , Liu Jingyao , Zhou Licheng , Li Long , Zhao Yunong , Li Hua-Yao , Zhang Yang , Liu Huan 
Published: May 2020

Introduction

Nowadays there is an increasing demand for low power and low-cost gas sensors for a wide range of applications. With the advantages of miniaturization, low power consumption and good compatibility with the standard CMOS circuit, gas sensing field effect transistors (FETs) are the most attractive approaches.
Since the first Pd gate field effect transistor gas sensor came into our insight [1], researchers have done a lot of work to materialize the device potential, starting with the choice of the sensing materials and sensor structure [2]. Due to the proximity of two-dimensional electron gas (2DEG) to the sensing surface and high sheet carrier concentration in the order of 1012~1013 cm−2, high electron mobility transistors (HEMTs) are extremely sensitive to the surface charge and variation in the environmental conditions, thereby enabling the sensing of the gases at low concentrations.
In this paper, we demonstrate ppm level NO2 detection using AlGaAs/GaAs HEMT-based gas sensor with controlling gate and a PbS quantum dots sensitization layer.

Experiment

The HEMT structures in this paper were grown by molecular beam epitaxy (MBE) on GaAs substrate [3]. It was composed of an undoped 300 nm thick GaAs buffer layer, 15 nm thick In0.2Ga0.8As channel layer, 4 nm thick Al0.3Ga0.7As spacer layer, Si δ-doping layer, 25 nm thick Al0.3Ga0.7As barrier layer, and 30 nm thick GaAs cap layer. The sheet electron concentration of the HEMT was 2.3 × 1012 cm−2 with a mobility of 5.0 × 103 cm2V−1s−1 at 300 K.
The sensing materials, PbS CQDs, were prepared by wet chemical synthesis technique according to previously paper [4], and the PbS quantum dots sensitization layer was formed as a sensing receptor on the exposed Au gate electrode using an electrohydrodynamic jet printer (HWK Co. Ltd., China).The CQDs capped with abundant oleic acid and oleylamine have stable dispersibility, a certain viscosity in the solvent and excellent solution processability, which makes it readily capable of being integrated with the electrohydrodynamic jet printing process at room temperature. The principle is to apply a high voltage power supply between the conductive nozzle (first electrode) and the conductive substrate (second electrode) and use a strong electric field force between nozzle and the substrate to pull the liquid out of the nozzle to form a droplet.
Gas sensing characteristics were measured in a vacuum chamber probe station. The gas mixed in the mixing chamber flowed through the inlet of the chamber and out through the outlet. The flow rates of the gases were controlled by mass flow controllers (MFCs). Gas pressure and humidity in the probe station chamber remained constant for stable gas measurement. A series of electrical measurements of the fabricated sensors were carried out using the probe station and a semiconductor parameter analyzer (B1500A, Agilent Technologies). And the sensor response was defined as R=|Ig-I0|/I0, the I0 and Ig is the source to drain current in air and target gas atmosphere, respectively.

Results and Conclusions

The HEMT-type sensor showed high responsibility and excellent repeatability towards ppm level NO2. In this paper, extremely low concentration of the NO2 (0~25 ppm) was introduced, which shows a high response of 36.1% towards 25 ppm NO2 at room temperature, much higher than reported results [5-7], which even operated at a high temperature over 300 oC.
In summary, through gate sensitization, the HEMT shows high sensitivity for ppm level NO2 detection at room temperature, the sensing performance can be both attributed to the receptor function of PbS quantum dots and the transducer function of HEMT. And the results of this paper will pave our future way for multiple target gas detection with in one chip.

References

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