Hydrogen plasmas generated by electron cyclotron resonance currently provide the most efficient plasma exposure technique available for passivating the grain boundaries of polycrystalline silicon. In this report, we show that careful optimization may be required when using this passivation approach on polycrystalline silicon gated, polycrystalline silicon thin film transistors fabricated using low temperature oxides. Optimization is found to be necessary for thin film transistors (TFTs) with polycrystalline Si gates in order to prevent the onset of high leakage currents which can develop when exposures are too long. The effects of exposure time, substrate temperature, microwave power level, pressure, and plasma dilution with an inert gas are examined to determine the conditions for optimal improvement in electrical performance. A model is also presented to explain this need for optimization of the electron cyclotron resonance hydrogen plasma passivation of poly‐Si TFTs.