A set up for the ethanol sensing characteristic measurement was shown in Fig. 7. The sensing response and recovery characteristics of MOS sensors were studied in a gas flow chamber under a dynamic equilibrium. Electrical measurements were performed using a volt-amperometric technique. A constant bias voltage of 5 V was applied to the sensor. Another voltage was applied to the heater coil used for heating the sensor and maintaining at a desired operating temperature. A thermocouple was employed to monitor the sensor temperature. The ammeter, voltmeter, and thermocouple signals were monitored and recorded via an interfaced personal computer. The ethanol sensing properties of the device were observed by the resistance change under an ethanol vapor atmosphere at different operating temperatures. Since our goal was to apply as an alcohol breath analyzer, the ethanol vapor at various concentrations was generated from ethanol solutions using alcohol simulator (GUTH Laboratories, Inc., Harrisburg, PA). The function of this alcohol simulator was to simulate alcohol concentration at conditions similar to exhaled human breath, being varied for ethanol concentration of 50–1,000 ppm for this experiment. A set up for the ethanol sensing characteristic measurement was illustrated in Fig. 7. The sensor sensitivity is defined by the ratio of the electrical resistance of the sensor in air and the electrical resistance of the sensor in ethanol vapor.