B.S.T.J. Briefs: A Floating Gate and its Application to Memory Devices

(Manuscript received May 16, 1967) A structure has been proposed and fabricated in which semipermanent charge storage is possible. A floating gate is placcd a small distance from an electron source. When an appropriately high field is applied through an outer gate, the floating gate charges up. T h e charges are stored even after the removal of the charging field due to much lower back transport probability. Stored-charge density of the order of lO^/cm* has been achieved and detected by a structure similar to an metal-insulator-semiconductor ( M I S ) field effect transistor. Such a device functions as a bistable memory with nondestructive read-out features. T h e memory holding time observed was longer than one hour. These preliminary results are in fair agreement with a simple analysis. It has been recognized for some time t h a t a field-effect device, such as t h a t described by Shockley and Pearson, 1 can be made bistable utilizing switchable permanent displacement charges on ferroelectric material.* Subsequent studies of ferroelectric material h a v e revealed, 1 however, t h a t the inherent speed capability of a device incorporating a ferroelectric material is limited by domain motion, whose highest speed is limited by the acoustic velocity. In the absence of highly ordered, near-ideal thin film ferroelectric material, the speed capability of a bistable device, therefore, is in the microsecond range at best. 4 In addition, m a n y ferroelectric materials suffer from irreversible mechanical disorder after m a n y cycles of polarization switching, 1 rendering some uncertainty on the long term device reliability aspect.