Self-convergence of post-erase threshold voltages in a flash memory cell using transient response

Abstract

An erase method provides for self-converging erase on a flash memory cell by rapidly switching a bias on a control gate while a lateral field is present in a channel region. Preferably, the lateral field is provided by differentially biasing the source and drain of the cell and the change in bias of the control gate is sufficiently fast to induce a transient response at the floating gate. The net transient vertical field formed across a tunneling oxide between the channel region and the floating gate causes moderate hot carrier injection between the channel region and the floating gate. This method is self-converging, since carrier injection to the floating gate will not happen unless a sufficient number of carriers are removed from the floating gate during the array step. Since the bulk of the self-converging effect occurs as the control gate voltage is transitioning and shortly thereafter, very little time is needed at the end of an erase pulse to effect this response.