Improvement in Sensitivity of InN Resitive Gas Sensor By Thickness Modulation for Liver Malfunction Application
Authors: Agarwal Ashish , Kumar Rai Sujeet , Avinash Kumar , Yeh J. Andrew 
Published: February 2020

The detection of Volatile Organic Compounds (VOCs) biomarkers from the exhaled breath has opened the new techniques for medical diagnostics because of its noninvasive and inexpensive way of disease detection. Exhaled breath contains more than 1840 VOCs 1 which concentration may vary in ppb or in ppt range. The signature of some VOCs present in breath abruptly change with the specific disease, called biomarkers. Ammonia is an important biomarker for liver disease 2. There are many conventional techniques to detect ammonia in sub-ppm range including, proton transfer reaction mass spectrometry (PTR-MS), selected ion flow tube coupled mass spectrometry (SIFT-MS) 3, but these instruments are bulky and very costly. Metal Oxide (MOS) based gas sensors are very promising sensors for detection of breath biomarkers, but it also has a disadvantage of high operation temperature (more than 300ºC). Extensive attention has been paid recently to the indium nitride (InN) epitaxial layers and devices for gas sensing application because of its exceptional electronic properties, including excellent electron mobility, high electron density and comparatively low operation temperature ~200ºC. Moreover, an unusual phenomenon of strong electron accumulation at the surface of InN makes it highly sensitive gas sensor for VOCs detection 4.
In this work, two InN based gas sensors of different thickness of the sensing film T1 (40nm) and T2 (60nm) are used respectively, for gas sensing. When the various concentration (0.3ppm, 0.5ppm, 1 ppm and 2ppm) of NH3 gas are exposed on the sensors, the current response is obtained at different thickness shown in (Figure 1a and 1b) respectively. The current variation response of 2 ppm NH3 gas on T1 is approx. 10 fold higher than T2 as shown in (Figure 1c). The current variation response for 0.5ppm on T1 is 0.04% while T2 showed negligible response as shown in (Figure 1d). The change at the surface of InN epilayer will be more significance on the total conductivity of thin layer as compare to thick layer 5. Hence, the lower thickness of the InN epilayer has higher response as compare to higher thickness. Therefore, lower thickness (40 nm) is suitable for diagnosis of liver malfunction.

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Figure 1:
(a) current response at T1 (40 nm thickness), (b) current response at T2 (60 nm thickness), (c) current variation response at 2 ppm NH3 gas at T1 and T2, (d) current variation response at 0.5 ppm NH3 gas at T1 and T2.

Figure 1