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A new two-dimensional device simulator is developed to investigate the effects of velocity overshoot on Si MOSFET's. An electron temperature-dependent mobility model, in which mobility is determined as a function of electron-gas temperature, is used in the simulator. Marked velocity overshoot occurs in the vicinity of the drain edge of MOSFET's and makes the potential barrier height at the source edge lower for ultrashort-channel MOSFET's. Therefore, velocity overshoot effects appear not only as degradation of electron transit time but also as increased drain current as compared with the case in which drift velocity does not overshoot. The increase in drain current depends strongly upon low-field mobility and bias conditions and appears for channel lengths shorter than 1000 nm. When low-field mobility is higher than 500 cm2/V. s and channel length is 100 nm, the increase in drain current is more than 1.5 times for bias conditions of strong inversion and a lateral electric field of more than 105V/cm in the vicinity of the drain edge.