ANALYSIS OF DRAIN FIELD AND HOT-CARRIER STABILITY OF POLY-SI THIN-FILM TRANSISTORS

Hot carrier instabilities in poly-Si thin film transistors (TFTs) are caused by high electric fields at the drain. These high fields are det ermined mainly by the abruptness of the lateral n(+) doping profile in the drain and the two-dimensional (2D) coupling of the x and y compon ents of the electric field between the gate and drain. The density of trapping states in the poly-Si film. however, has a much less signific ant impact on the field. Further. it is shown that improving the prope rties of the poly-Si film tends to have an adverse affect on hot carri er stability. Consequently, it is concluded that drain field relief is essential for hot carrier stability of n-channel poly-Si TFTs. it is shown that gate overlapped lightly doped drain (GOLDD) architectures c an be used to relieve the drain field without introducing series resis tance. Stable TFTs have been fabricated with GOLDD, consistent with ci rcuit operation up to drain biases of 20 V. GOLDD is also effective in reducing the field enhanced leakage current in the off-state.