Abstract
An early paper [1] proposed the virtual pore widening model to explain the temporal drift in the characteristics of porous aluminum oxide humidity sensor. It is shown here that this model does not provide a plausible mechanism for the decrease in sensitivity caused by aging. Instead, it predicts the contradictory behavior of an increase in sensitivity with aging, which is not true. Alternatively, hydration of the aluminum oxide on exposure to moisture provides a more cogent explanation of the aging process, as observed in an alumina-gate ion-sensitive field-effect transistor (ISFET). In this paper, the monotonic decrease in capacitance of Al2O3 humidity sensor with the extent of hydration has been established, both by equivalent circuit approach and by modeling the sensor as a heterogeneous dielectric. Capacitance calibration characteristics of the humidity sensor have been simulated by employing the theory of alumina humidity sensor. Similarities and dissimilarities of aging of this sensor with ISFET drift have been elucidated. Whereas in ISFET, a thin hydrated layer is formed only at the surface of the gate dielectric, in Al2O3 sensor, hydration of inner layers in the porous film is highly likely. Unification of aging mechanisms in sensors is briefly discussed.
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