Surface effect humidity sensor based on alumina and porous silicon materials: some electrical parameters, sensitivity and internal noises in comparison

For comparison, two types of materials: porous alumina and porous silicon have been prepared by anodization method and used for fabrication of humidity sensors in the same size and sensor structure which operate on the base of conductivity versus relative humidity (RH%). The material structures of porous alumina and porous silicon were investigated by transmission electron microscope (TEM), atomic force microscope (AFM). The porous structures of two materials are shown. The conductance, capacitance and admittance vs. humidity and frequency of sensors have been also measured by impedance analysis spectroscopy. The relative permittivity of two porous materials has been determined. The internal noise sources as well as noise equivalent circuits of two kinds of sensors have been established and measured. The experimental results showed that conductance vs. humidity curve of porous silicon is better than that of alumina, especially the conductance magnitude of annealed porous silicon sensor is larger by 2 - 3 orders in comparison with that of the alumina sensor or nonannealed silicon sensor. The porous silicon sensor gives shorter response time, smaller hysteresis, more stability, longer life in comparison with that of alumina sensor. The differences of the properties of two kinds of sensors may be explained by different porous structures as well as conductive mechanisms.