New microhumidity field-effect transistor sensor in ppmv level

Abstract

An integrated humidity sensor able to detect and measure a very low humidity concentration and higher relative humidity (RH) levels as well with the possibility of industrial application has been designed and fabricated using very large scale integration technology where thin porous Al2O3 film acts as humidity sensing material. The porous structure of Al2O3 has been obtained by anodic oxidation of a thin film of Al in sulfuric acid. During anodic oxidation, a few top atomic layers of Ta are also oxidized into Ta2O5 whose properties affect the normal sensing properties of Al2O3 and a better quality of sensor with higher sensitivity and speed is obtained. The gate insulator structure of the field-effect transistor is SiO2/Si3N4/Ta/Ta2O5/Al2O3. The device is basically an enhancement mode metal–oxide–semiconductor field-effect transistor. The drain current ID of the device is measured at constant gate and drain voltages at 25 °C and found to be sensitive up to less than ∼1 ppmv moisture concentration. The response time of the sensor in ppmv level is less than 1 s. The device is also found to be very sensitive at higher RH levels and shows a linear dependency on RH. The response and recovery times of the sensor are 2 and 6 s, respectively.