An improved substrate current model for ultra-thin gate oxide MOSFETs

摘要

In existing substrate current I_(sub) models for short channel MOSFETs, the new model of the characteristic ionization length l 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 η 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 η 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.