Method and circuit for tunnel-effect programming of floating gate mosfet

Abstract

The present programming method exploits the close dependence of tunneling current on the voltage drop across the tunnel oxide layer. A bootstrapped capacitor connected to the drain terminal of the transistor is employed. The charge state of the capacitor determines the bias of the tunnel oxide and is in turn determined by the charge state in the floating gate. Biasing by the bootstrapped capacitor is such as to permit passage of the tunneling current and, hence, a change in the threshold voltage of the transistor until the floating gate reaches the desired charge level, and to prevent passage of the tunneling current upon the transistor reaching the desired threshold. Programming is thus performed automatically and to a predetermined degree of accuracy, with no need for a special circuit for arresting the programming operation when the desired threshold is reached.