Huilin Ren

Date of Award

2001

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science in Electrical and Computer Engineering (MSECE)

Department

Electrical and Computer Engineering

Advisor

John F. Vetelino

Second Committee Member

Brian G. Frederick

Third Committee Member

David E. Kotecki

Abstract

The current versus voltage (IN) characteristics of undoped and gold doped W03 thin film sensors were studied both experimentally and theoretically. A simplified equivalent circuit was formulated for the W03 sensor. It includes forward and reverse biased Schottky diodes which represent the contact region between the electrodes and the film and a resistor which represent the sum of crystallite resistance and the intercrystallite resistance between the individual crystallites. According to the electron transport mechanisms through the Metal Semiconductor (MIS) boundaries and intercrystallite boundaries, two kinds of yV and corresponding ResistanceNoltage(RN)characteristics are possible, one based on thermionic emission theory and the other based on tunneling theory. A one channel gas delivery system was built to experimentally determine the IN characteristics. The experimental results were obtained for both gold-doped W03 and undoped W03 sensors. The W03 films were R-F sputtered onto a sapphire substrate and annealed in compressed air at 400°C for 24 hours. The voltage range for the tests was fiom -20V to 20V and the temperature range was fiom room temperature to 400°C. The experimental results for the IN characteristics were taken in compressed air, 30 PPM H2S and in ethylene. The IN experimental results are approximately linear except in the lower voltage region where they are nonlinear. The RN results are more sensitive and nonlinear. Besides, the sensor performance was also examined in the AC case. The preliminary results on real and imaginary components of the impedance were also presented as a function of frequency and voltage. Comparing the experimental results of the IN characteristics and the corresponding RN characteristics taken in compressed air or target gases such as H2S with the theoretical curve predicted by themionic emission and tunneling theories, it is obvious that the tunneling effect is the dominant electron transport mechanism in the W03 thin film sensor. The depletion width is the key parameter which determines the appropriate electron transport mechanism associated with MIS and intercrystallite boundaries. Since the depletion width is related to the argon oxygen ratio in the R-F magnetron sputtering system one can apriori design a sensor which will exhibit either themionic emission or tunneling at the MIS and intercrystallite boundaries.

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