Modelling of the humidity effect on the barrier height in SnO2 varistors

Tin dioxide based SnO2–Bi2O3–Co3O4–Nb2O5–Cr2O3 ceramic device with combined varistor and humidity-sensitive properties is studied. This varistor–sensor has high humidity sensitivity coefficient of about 420 at low electric field and high non-linearity coefficient of about 50 at electric field 3500 V cm−1. Current–voltage characteristics of the samples with separate electrodes for the central and the peripheral parts are studied in air with different relative humidity. It is shown that current through the peripheral part of a sample is increased much stronger than through the central part. The observed increase of low-field current on relative humidity is explained by the decrease of the barrier height. The estimations of the lowering of the barrier height in a humid air is performed using the suggested models.