Enhanced channel mobility in polysilicon thin film transistors

Due to scattering by charged grain boundaries, carrier mobility μ in the channel of polysilicon thin film transistors (TFT) is usually much lower than the bulk silicon value. We have studied a series of /sub p/-channel TFT devices with varying gate oxide thicknesses d/sub ox/ and found that CL shows a strong increase when d/sub ox/ is reduced below 150 /spl Aring/. We attribute this effect to the screening of the charged grain boundary by the gate conductor. The screening becomes effective when the characteristic length associated with the potential barrier at charged grain boundaries becomes comparable to the optical distance between the grain boundary charge and its mirror image in the gate electrode. From the known structure parameters the onset of the strong screening is estimated to occur at oxide thicknesses of about 100 /spl Aring/.