We studied a-Si: H thin film transistors (TFTs) with gate metal/substrate combinations, including Cr or Al/Cr gate metal on plain Kapton, and Cr/Al/Cr on SiNx passivated Kapton, to identify effects of roughness on TFT performance. TFTs with Cr/Al/Cr on SiNx passivated Kapton are superior in the essential characteristics: low threshold voltage, low gate leakage current I-gs, high on-current, high field-effect mobility, and steep subthreshold slope. The off-current is comparable to those of TFTs on plain Kapton. The Igs of all TFTs rises with increasing surface roughness of the gate metal. The major contributor to roughness is the metal film itself, not the substrate. (c) 2007 The Electrochemical Society.