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In existing impact-ionization current (Jsub) models for short-channel MOSFETs, various models for the characteristic ionization length (I) or the velocity-saturation region length (lsat) have been developed by using the polynomial-fitting method in order to model the bias dependence of the maximum electric field (Em) in the channel. This paper proposes a bias-voltage- and gate-length-dependent effective maximum electric field (Em,eff) based on energy-balance equation, aimed at obtaining an accurate expression of Em to increase the accuracy of the Isub model for deep submicrometer devices. This new method overcomes the complicated modeling of I, avoids the extraction of different fitting constants for different devices, and enables unique extraction of the impact-ionization coefficients (A and B) for different devices. This improved model demonstrates excellent agreements with the numerical data of nMOSFETs from a 90-nm-technology wafer file. Only one unique set of parameters is needed to fit the data from devices with different biases and lengths for the same technology node. Moreover, since the lattice temperature (Tl) is built in the formulation of Em,eff, a compact Isub model with self-lattice-heating is developed, which also accounts for the excess substrate current observed in the SOI devices due to carrier heating in the channel.