Gas sensing setup diagram. 

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... fabricated hexagon Au/ZnO/Si Schottky diode was probe in a sealed gas chamber connected to a gas vapor system. Initially, the chamber was filled with air and left to stabilize for 15 minutes. In order to calibrate the device error, five run tests were conducted to investigate the stability of the sensor towards the same gas, i.e. air in closed chamber. The sensor was then exposed to two types of targeted gaseous in a sealed chamber which is 96 % ethanol vapor and 100 % acetone vapor at constant room temperature 25 °C. Fig. 2 shows the setup diagram of gas sensing system. Fig. 3 shows the FESEM of the ZnO thin film. There are visible cracks and grain which suspected formed during the annealing process. The cracks are inhomogeneous and have gap range from 100 nm to 300 nm. The hexagon Au contact area is 23.38 mm 2 with thickness of approximately 1 µm. The stability test was conducted by taking five separated measurements with the same air sample. The result shows the average standard deviation for this sensor is 5.7 %. Small standard deviation shows that the sensor output is stable. Fig. 5 presents the I-V characteristic of the fabricated sensor measured using Agilent U2723A source meter in forward biased mode. It shows a similar pattern with typical Schottky diode characteristic (Fig. 4) which starts to rectify, approximately at 0.3 V. Fig. 6 shows the I-V characteristic of the hexagon Au/ZnO/Si Schottky diode in reverse biased mode. The I-V characteristics in Fig. 5 and Fig. 6 show distinct responses based on the I-V curve shift in forward biased and reverse biased upon exposing to the targeted gaseous (Air, Ethanol and Acetone). Table 1 and Table 2 depict a comparison of voltage shift in forward and reverse biased with respect to the air in order to investigate which bias mode give larger shift. Results in the tables show that reverse biased has a larger shift compared to forward bias by a difference of 450 -900 mV. It was observed at the reverse biased, the voltage difference between the target respects to the air at 10 µA give 550 mV compared to forward biased which shows 100 mV of voltage shift. Another measurement was taken at 80 µA and it showed reverse biased characteristic with larger voltage shift which was 1200 mV compared to forward biased, which only 300 mV. This result suggests with the addition of edges to the contact, it will enhance the electric field and thus resulting in better shift at reverse biased mode. As done by previous research group, reverse biased gave larger shift due to high electric field [24]. On the other hand, there is no phase difference between ethanol and acetone sample due to the lower current setup configuration. The experimental can be enhanced by utilizing high current range, where response on the ethanol and acetone is significant. Table 1. Comparison of voltage shift in forward and reverse biased at 10 µA with respect to ...

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... high electric field produced by the metal contact layer assisted by the performance of thin film device to the level almost similar with nanostructure effect. Although this device utilizing thin film metal oxide, as shown in Fig. 2, it still produce performance at the reverse biased, similar to the nanostructure based device due to the high electric field initiated by sharp ...