Moisture sensitive field effect transistors using gate structure

Abstract

A humidity sensing device, named a moisture sensitive field effect transistor (HUMIFET) has been fabricated on an n-type Si wafer using conventional VLSI technology. A series of sensors, each having area approximately 3.0 mm2 have been designed on a single Si wafer. The FET device is an n-channel MISFET with a duplex gate structure. A humidity sensitive layer of an anodically oxidized film of Al covers the under-gate electrode area and serves as a humidity sensitive film. The structure of the HUMIFET is Si/SiO2/Si3N4/Al2O3 and a thin porous gold (Au) layer on Al2O3 acts as the upper gate electrode. The device shows an enhancement mode characteristics with a threshold voltage of approximately 2.9 V at 33% relative humidity (RH) level which increases with decreasing RH level. The capacitance of the sensing film is measured and its temperature dependence is also observed. The drain current of the device is measured with RH at constant gate and drain voltages and the hysteresis area is found. The sensitivity of the device is 4.8 μA/RH for the whole range of RH levels at constant gate and drain voltages. During a one month study on reproducibility, the device is found to be very stable. The response and recovery times of the device are approximately 6 and approximately 8 s respectively.