A new charge pumping model considering bulk trap states in polysilicon thin film transistor

Abstract

This paper proposes a new charge pumping model (bulk state model) in a polysilicon thin film transistor. The charge pumping current is calculated both from the bulk state model and the conventional model (interface state model) and each of them is compared with the measured data. In the bulk state model, the threshold voltage is defined as the gate voltage when the trapping time constant, which is a function of free electron concentration, is equal to the applied gate-pulse width. By applying this definition to each depth, the threshold voltage can be calculated as a function of depth. The threshold voltage shift is also taken into account to calculate the threshold voltage. From the threshold voltage, two associated emission energy levels are calculated. On the basis of the data from the field-effect conductance method, we assume that the trap distribution function is composed of two tails and two Gaussian functions. For each depth, the charge pumping current density is calculated by integrating the trap distribution function between two emission energy levels in silicon band gap. The charge pumping current is calculated by integrating the charge pumping current density with respect to depth. It shows that the calculated current of the bulk state model is more consistent with that of the measured rather than that of the interface state model. (C) 1998 Elsevier Science Ltd. All rights reserved.