Relative-air humidity sensing element based on a micromachined floating polysilicon resistor

This paper presents a new type of air-humidity sensing element based on heat transfer mechanisms such as conduction and convection. The sensing element is a single floating resistor of doped polysilicon and operates in two phases: thermal actuation and thermal sensing. As actuator, a convenient biasing current is applied to the resistor leading to self-heating by Joule effect. As sensor, the resistor biasing current is reduced and the time constant of the cool-down process is measured. This thermal-time constant (TCC) is close related to the relative humidity of the air (%RH) surrounding the floating resistor. Characterization results show that TCC was 16 s, 5.1 s and 1.4 s for 30, 50 and 70 %RH, respectively. The sensing element operated at room temperature and presented a maximum power consumption of 5 mW.