Dopant and defect interactions in polycrystalline silicon thin-film transistors

The influence of channel doping on the threshold voltage of polycrystalline silicon thin-film
transistors sTFTsd has been investigated using both boron and phosphorus implants. For each dopant
type, asymmetric TFT behavior 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 an asymmetric density of states, in
which the deep trapping state density in the lower half of the band gap increased with increasing
boron concentration, and the effect of the dopant on threshold voltage was itself determined by this
local defect concentration.