Channel doping effects in poly-Si thin film transistors

The influence of channel doping on the threshold voltage of poly-Si thin film transistors (TFTs) has been investigated using both boron and phosphorus implants. For each dopant type, asymmetric TFT behaviour was seen between enhancement and depletion mode TFTs, with the enhancement mode TFTs always showing a greater shift in threshold voltage for a given dose. From a detailed analysis of the boron-doped samples, using a two-dimensional device simulator, it was demonstrated that the results could be best explained in terms of a non-symmetric density of states, in which the deep trapping state density in the lower half of the band-gap increased with increasing boron concentration. This is the first reported observation in poly-Si of shallow level dopants also introducing deep levels in the majority carrier half band-gap.