For my undergraduate project on “Analysis of DOS in the Channel of Poly-silicon Thin Film Transistors” in ADTL lab, the goal was to provide an accurate and convenient methodology to monitor the defect density of state (DOS) in the channel. By a clever design of a cross-shaped TFT structure (As shown in attached file) which have NTFT and PTFT in each direction, the entire spectrum of DOS from the conduction band to the valance band can be extracted from the same device, which is not possible in all previous methods. So this method is more convenient and trust-worthy than conventional data extracted from two separate n-channel and p-channel TFTs. The monitoring method was successfully used to clarify the mechanism about how a plasma treatment modifies the channel properties of a TFT, and the process can be divided into two stages. During the first stage, both deep and tail states are modified. While during the second stage, only tail states continue to be modified. Furthermore, the proposed method can also be applied to analyze the quality of films fabricated by different crystallization methods. It clearly shows the smaller defect DOS of SPC TFTs over the as-deposited TFTs. This new monitoring method provides an unambiguous measurement of defect density, and it could be a useful tool for further improvement of TFTs’ quality.