Current/voltage characteristics of a semiconductor metal oxide gas sensor

The current/voltage (I/V) characteristics of undoped and gold-doped tungsten trioxide (WO3) thin film sensors were studied experimentally and theoretically. An equivalent circuit, which includes forward and reverse-biased Schottky diodes representing the contact region between the electrodes and film, and an equivalent resistor which represents the resistance of the individual WO3 film crystallites and intercrystallite boundaries was formulated. Depending upon the electron transport mechanisms through the metal–semiconductor (M/S) interfaces and the WO3 film, two different I/V and therefore resistance/voltage (R/V) characteristics are possible, one based on thermionic emission theory and the other on tunneling theory. In order to determine the dominant electron transport mechanism several WO3 films were tested. Amorphous and polycrystalline WO3 and WO3: Au films were RF-sputtered onto a sapphire substrate. A dc voltage ranging from −20 to 20 V was applied to the films in compressed air with and without ethylene gas present. The experimental R/V results are approximately constant except in the low voltage region where they are nonlinear. Comparing the experimental results of the R/V characteristics, with the theoretical curves predicted by thermionic emission and tunneling theories, some interesting conclusions can be made regarding the dominant electron transport mechanism in the WO3 films. The depletion width between the M/S interface and the intercrystallite boundaries is the key parameter, which determines the appropriate electron transport mechanism. Since the depletion width is related to the oxygen vacancies in the film and therefore the argon/oxygen ratio in the sputtering process, one can a priori design a sensor, in which the electron transport will exhibit either thermionic emission or tunneling.