We demonstrate that it is possible to extract the defect density distribution within the entire energy gap of a poly-Si layer by using a novel Schottky barrier thin-film transistor test structure. Our methodology, albeit based on the well-known field-effect conductance method, does not require two separate n- and p-channel test devices necessary in the conventional methods. This unique and much simplified single-test-device feature is made possible thanks to the ambipolar characteristics and the suppression of subthreshold leakage current in the new test scheme. Effects of process treatments such as plasma hydrogenation could also be clearly resolved using the new test structure. (c) 2005 The Electrochemical Society. All rights reserved.