Branched Polyethylenimine-Based Field Effect Transistor for Low Humidity Detection at Room Temperature

In this paper, electrically insulating branched polyethylenimine (PEI) is applied to a field effect transistor (FET) sensor platform for low relative humidity (RH) detection (< 20%) at room temperature. The platform contains horizontally aligned control gate and floating gate, so that PEI is readily formed by simple ink-jet printing process. The electrical properties of the sensor in both dry and humid N ₂ are measured. The subthreshold swing, gate leakage current, and off-state drain current are increased by water vapor adsorptions. The mechanism behind the phenomenon is deeply investigated by building an equivalent circuit model of the sensor and molecular simulations of water-vapor adsorptions on PEI. It could be due to the formation of the dipoles on the surface of the PEI layer by the adsorptions of water vapors via hydrogen bonding and the polarization of those dipoles by gate biases. The drain current increases with the rise of relative humidity levels and a relative high response of 137.6% is obtained corresponding to 17.8% RH. The novel sensing principle of the proposed sensor lead a high sensitivity, linearity and resolution to low RH below 20%. This research demonstrates a promising low humidity-level sensing capability of the proposed sensor and the study on the sensing principle is very useful for more gas-sensing applications of polymers based on FET platforms.