CHARACTERIZATION OF LOW-TEMPERATURE POLY-SI THIN-FILM TRANSISTORS

There is developing interest in using thin film transistors as active elements in a range of large area electronics applications. The characteristics of poly-Si thin film transistors (TFTs), processed at glass compatible temperatures, have been investigated. The particular features examined were the leakage current, hydrogenation mechanism and mobility. The hydrogenation was found to proceed by a lateral penetration through the gate oxide around the edges of the poly-Si gate finger. This led to a channel length dependence of sub-threshold slope in partially hydrogenated devices. In contrast, the leakage current, which was shown to be a generation current at the drain junction, did not require hydrogen penetration into the centre of the channel and hence passivation of the generation centres was channel length independent. The hydrogen diffusion coefficient in fine grain poly-Si was estimated at 350-degrees-C to be approximately 1-10 x 10(-14) cm2/s depending upon the detailed material properties. Thermal crystallisation of LPCVD and PECVD amorphous silicon was found to be comparable with both leading to large dendritic grains and enhanced carrier mobility.