A NEW CHARGE-PUMPING MODEL CONSIDERING BULK TRAP STATES IN POLYSILICON THIN-FILM-TRANSISTOR

This paper proposes a new charge pumping model (bulk state model) in a polysilicon thin film transistor. The charge pumping current is calcu lated both from the bulk state model and the conventional model (inter face 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 a pplying this definition to each depth, the threshold voltage can be ca lculated as a function of depth. The threshold voltage shift is also t aken into account to calculate the threshold voltage. From the thresho ld voltage, two associated emission energy levels are calculated. On t he basis of the data from the field-effect conductance method, we assu me that the trap distribution function is composed of two tails and tw o Gaussian functions. For each depth, the charge pumping current densi ty is calculated by integrating the trap distribution function between two emission energy levels in silicon band gap. The charge pumping cu rrent is calculated by integrating the charge pumping current density with respect to depth. It shows that the calculated current of the bul k 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.