Humidity Sensitive Electrical Properties of Dense ZnO with Non-Ohmic Electrode

The humidity sensitive electrical properties of ZnO dense sintered compacts with two, one, or no ohmic contact are reported. Sintered ZnO discs were prepared by traditional ceramic processing. Ohmic contacts were obtained with Al electrodes. Non-ohmic contacts were made by mechanically pressing Au sheets on the uncoated sides of sintered discs. Dark current-voltage (I-V) characteristics of ZnO pellets were measured at different relative humidity (rh) values and at room temperature. The I-V curves for pellets with one ohmic contact showed a diode behaviour, with a rectifying character, attributed to the presence of a Schottky barrier. The rectifying characteristics of ZnO pellets were enhanced in the presence of humidity, and the rh-sensitivity was bias-dependent. The rh-sensitivity of dense ZnO pellets with a Schottky barrier can be related to the variation of the barrier height due to the presence of surface states. Their behaviour resembles that of p-n junctions, with a sensing mechanism which is different from those already proposed in the literature for ceramic materials.