Screen-printed varistors: New strategy for high non-linear coefficient

Screen-printed thick film varistors (TFV) based on ZnO powder were developed with the aim to protect IGBT power transistors against overvoltages. The electrical characteristic of TFV was shown to depend on the electrode composition. Furthermore, characterization of the microassembly [electrodes/ZnO/Al2O3 substrate] proved that in the case of ZnO layers fired at 1200 °C, the low non-linearity coefficients (α < 10) were correlated to the bismuth oxide evaporation and/or migration to the substrate. Higher non-linear coefficients were obtained only with electrode inks containing small amounts of Bi2O3. The obtained values of α larger than 40 are the highest ones so far recorded for ZnO TFV and compare well to those of ceramics.