A new room temperature FET-ammonia sensor

We report a new FET sensor which can detect ammonia at room temperature. We discovered a new application of high T/sub c/ cuprates, commonly known as high temperature superconducting copper oxides. The gas sensing properties of thin films of these materials have been examined through work function measurements. Thin films of YBCO and BSCCO are prepared on oxidized silicon substrates. These are then used in a capacitively controlled field effect transistor. One plate of the capacitor is a gas sensitive film and the other one is a reference electrode which is connected to the FET gate electrode. The operating principle of this sensor is based on the change in work function of the sensitive film due to gas adsorption on its surface. This is measured as change in transistor gate voltage. Measurements show that the nonamplified sensor signal is more than 10 mV for only 5 ppm of ammonia. Typical rise time of 24 sec and fall time of 250 sec are measured at 18°C. The sensor is selectively sensitive to ammonia and shows little or no response to hydrogen and hydrocarbon based gases. Measurements at different temperatures reveal that the sensor performs optimally in the 15-25°C temperature range. Device sensitivity to carbon monoxide, carbon dioxide, hydrogen and hydrogen gases has been found to be negligibly low. Thus a new FET sensor capable of sensing ammonia with selectivity around room temperature is reported in this paper.