Abstract: The incorporation of high voltage transistors into the advanced VLSI chips has been limited by the reliability of the manufactured integrated circuits.As a monitor of hot-carrier-injection reliability,the substrate current (ISUB) usually increases in high voltage transistors,but has only one peak in standard low voltage transistors.Correspondingly,the mechanisms of the hot-carrier-injection effect in high voltage N-channel transistors should also be investigated.Based on the Poisson’s equation,and simulation and experimental results,a second impact ionization region is responsible for the second increase of ISUB.An explanation for the appearance of this second impact ionization region that differs from the prevalent theory,the Kirk-effect,is proposed.The Kirk-effect predicts that the typical high-electric field region widens from the gate edge to the n+ drain edge.However,two separate high-electric-field regions with fixed locations coexist instead.The second high-field region is not the expansion of the conventional region.An improved equation for ISUB is proposed according to the two-high-field-region model.This two-high-field-region model is also consistent with the phenomena observed under the various HCI stress conditions with and without back bias.Back bias reduces the supplied voltage for high-voltage transistors by half without degrading their performance and reliability.

Key words: substrate current, two-high-field-region model, substrate current equation, hot-carrier-injection