In existing substrate current I-sub models for short channel MOSFETs, the new model of the characteristic ionization length I or the velocity saturation region length L-d has been developed by using the polynomial fitting method in order to represent the variation of maximum electric field E-m with bias voltages in channel. This work proposes a bias-voltage- and gate-length-dependent parameter eta which was previously treated as a process-dependent constant, aimed at obtaining an accurate expression of E-m to increase the accuracy of I-sub model for ultra-deep submicron devices with ultra-thin gate oxides. This new method overcomes the complicated modeling of characteristic ionization length l, and avoids the extractions of different fitting constant eta corresponding to different devices. It also warrants the unique extraction of impact ionization coefficients A(i) and B-i. Compared with some existing I-sub models, the improved one presents more excellent agreements with the experiments of ultra-thin gate oxide (t(ox) = 1.24 nm) LDD NMOSFETs on 90 nm CMOS technology, especially in the high electric field region. Meanwhile, the new I-sub model accurately simulates the shift of the peak of substrate current along the gate bias axis with the shortening of gate length which usually occurs in ultra-thin gate oxide devices, helpful to the lifetime prediction of sub-100 nm devices. (c) 2006 Elsevier Ltd. All rights reserved.