A NOVEL HUMIDITY-DETECTION MECHANISM FOR ZNO DENSE PELLETS

The humidity sensitivity of ZnO dense pellets with two, one, or no ohmic contacts is reported. Sintered ZnO pellets are prepared by traditional ceramic processing. Three different materials are used for the ohmic contacts (Zn-containing Ag, In- and Ga-containing Ag and At pastes). Non-ohmic contacts are made by mechanically pressing Au sheets on the uncoated sides of sintered pellets. The electrical characterization of the ZnO pellets is performed by measuring dark-current-voltage (I-V) characteristics at different relative humidity (rh) values and at room temperature. The I-V curves for pellets with two ohmic contacts are linear in both dry and wet environments, showing ohmic behaviour and no rh sensitivity. ZnO with two non-ohmic contacts shows I-V curves typical of a back-to-back diode and no rh sensitivity. The I-V curves for pellets with one ohmic contact show a diode behaviour with a rectifying character attributed to the presence of a Schottky barrier. The electrical response of these pellets is sensitive to rh, with a nearly linear current versus rh variation. The I-V curves in the presence of high rh values continue to show rectifying characteristics. The rh sensitivity is bias dependent. It depends on the material used as an ohmic contact, and is the highest with Al. Also the response time depends on the ohmic electrode materials. The rh sensitivity of dense ZnO pellets with a Schottky barrier cannot be explained in terms of ionic or electronic conduction, but it may be related to the variation of the barrier height due to the presence of surface states. Their behaviour seems to be closer to that of p-n junctions, with a sensing mechanism different from those already proposed for ceramic materials.