Grain junction properties of ZnO varistors

Abstract

Grain-grain junctions in ZnO varistor ceramics were electrically contacted by conventional photoresist- and contact deposition techniques and investigated in terms of the statistical distribution of the breakdown voltage. Its maximum appears at 3.4 V for constant current of 5 μA. The I–V-responses for three measured individual junctions have a pre-breakdown characteristic of space charge limited current. They are interpreted to originate in a bypass branch of intergranular material which is not responsible for the true varistor effect, but rather masks its pre- breakdown characteristic. The breakdown region is examined for the temperature coefficient of the breakdown voltage, which yields -2 × 10 -4/K. The measured temperature dependence of I–V-responses comply with calculated data for band-band-tunnelling in a n-i-n-homojunction model. Dielectric properties show a complicated dependence on bias voltage and temperature which, in accordance with pulse current measurements, is tentatively interpreted in terms of the bypass branch of intergranular material with ionic loss mechanism.