Investigation on anomalous leakage currents in poly-TFTs including dynamic effects

Field-enhanced off-currents are an important limiting factor of polycrystalline-silicon thin-film transistors (TFTs) which still prevents a wider use of such devices in active-matrix liquid crystal displays (AMLCDs) as switching elements within the pixel matrix. The purpose of this work is that of investigating the anomalously-large leakage currents in poly-TFTs by numerical simulation, taking into account for the effects of energy distributed traps and field-enhanced generation mechanisms. The investigation is carried out both in steady-state and in transient conditions in accordance with the typical timing of the driving circuitry, and accounts for the kinetics of trapped carriers. Furthermore, the influence of material quality and device geometry is investigated. This study shows that the electric field increases by 30% during the off-transition with respect to steady-state. However, drain engineering using either LDD or active-gate structures allows for a substantial decrease of the peak electric-field value in dynamic conditions.