Mechanism and device-to-device variation of leakage current in polysilicon thin film transistors

An experimental study of mechanisms responsible for off-stage leakage currents in n- and p-channel poly-TFTs (thin-film transistors) is presented. Unlike turn-on characteristics, leakage currents are not symmetrical with respect to source-drain voltage polarity. The conduction mechanism in the off-state is consistent with the model of thermionic-field emission near the drain. The spatial and energetic distribution of trap-states in the drain depletion region affects the tunneling probability and causes device-to-device variation and the asymmetry in leakage current. The temperature dependence of the leakage current arises from the thermally activated carrier population at the energy state of the maximum tunneling probability. Cumulative leakage currents follow a Poisson-type log-normal distribution which can be significantly affected by processing control. Less than 10-fA/ mu m TFT width in leakage current and more than nine orders of magnitude on/off current ratio at 10-V drain bias were achieved in n-channel poly-TFTs.<>