Charge transfer storage systems.

摘要

Storage systems are provided with storage cells made of devices having different threshold voltages for storing information permanently or semipermanently. The devices are arranged adjacent to each other and communicating with a diffusion region (12,14,16) in a semiconductor substrate (10). Information is sensed by detecting the charge transferred from a selected cell to the diffusion region (12, 14, 16). In an embodiment of the invention, a P type substrate (10) has an N + diffusion region (12, 14, 16)formed therein with a plurality of adjacent and parallelly arranged word lines (26 to 48) insulated from the substrate (10) and disposed adjacent to the N + diffusion region (12, 14, 16). A P + region (18, 20, 22), preferably implanted into the substrate (10), is disposed under selected segments of the word lines (26 to 48) to provide devices having a first or high threshold voltage. The remaining devices which are not associated with a P + region (18,20, 22) have a second or low threshold voltage. By applying a voltage of the same magnitudeto each of the word lines (26 to 48), potential wells are formed which are filled by charges from the diffusion region (12,14,16). Since the potential wells associated with the high threshold devices hold less charge than the low threshold devices, a charge or voltage sensing circuit connected to the N+ diffusion region (12, 14, 16) is used to detect the amount of charge flowing between the wells and the diffusion region (12, 14, 16) to thus identify the high and low threshold devices when the voltage on the selected word line (26 to 48) is decreased. By eliminating the P + regions (18, 20, 22) and storing charge, e.g., electrons, at selected locations under the word lines in dual insulating layers, the cells may be electrically programmable.