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We have developed a technique to completely extract trap densities at front- and back-insulator interfaces and grain boundaries in poly-Si thin-film transistors. First, the trap densities at the front- and back-insulator interfaces are extracted from the front and back low-frequency capacitance-voltage characteristics using the Poisson and carrier-density equations. Next, the trap density at the grain boundaries is extracted from a current-voltage characteristic using 2-D device simulation. Particularly, in this paper, actual trap densities are extracted to evaluate oxygen and hydrogen plasma treatments. First, it is found that the energy profiles of these trap densities are quite different. Next, the oxygen plasma treatment has the effect to reduce trap densities but cannot diffuse to the back-insulator interface. Finally, it is concluded that this extraction technique is useful to evaluate film properties, diagnose fabrication processes, and improve transistor characteristics.