The fabrication and enhanced performance of a polycrystalline silicon high voltage thin film transistor structure, which incorporates a semi-insulating field plate, are reported. For comparison, the performance of conventional offset drain and metal field plate structures, fabricated on the same wafer using the same low temperature(⩽610 °C)polycrystalline silicon process, is described. Electrical characterization of the high voltage thin film transistor structures demonstrates that the new three terminal device offers the highest blocking capability(>200 V)without sacrificing on state performance. ©1997 American Institute of Physics.