Abstract: A humidity sensor having a sensing component and a reference component. Each component may indicate humidity by using a transistor that has an upper gate dielectric layer with a dielectric constant which varies according to exposure to a change of moisture. The sensing component may have its upper gate dielectric layer exposed to an ambient environment. The reference component may have its upper gate dielectric layer with an absence of exposure to any environment. Sensing and reference transistor outputs may be processed with differential electronics to provide an output to indicate humidity in the ambient environment. Differential processing may reflect an output having a high common mode rejection ratio. An example of an upper gate dielectric layer deposited over a thin gate dielectric layer may be a hydrophobic polymer material. Other materials may instead be used. The components and processing electronics may be transistor circuits fabricated with integrated circuit technology.