Robert B. Richart

A convergence scheme for over-erased flash EEPROM's using substrate-bias-enhanced hot electron injection

A novel convergence scheme using substrate-bias-enhanced hot electron injection is proposed to tighten the cell threshold voltage distribution after erasure for stacked gate flash EEPROMs. By lowering the drain voltage and increasing the magnitude of the negative substrate bias voltage, the substrate current is reduced but the hot electron gate current is enhanced significantly, and the convergence time is shown to be more than a hundred times shorter than the previous scheme. With the convergence operation performed near the ON-OFF transition region of the cells, the total drain current for all the converged cells is reduced and low power consumption is achieved.<<ETX>>

A multilevel approach toward quadrupling the density of flash memory

A multilevel scheme is presented that explores the possibility of quadrupling flash EEPROM storage density. Sixteen levels (4 bits/cell) of charge are stored in existing NOR stacked gate devices. A distinction is made between logical threshold voltages (as seen by the sense amplifier) and transistor threshold voltages (as defined by the gate characteristics), and precise programming gives distinct logical threshold voltage distributions, whereas transistor threshold voltage distributions are contained in a small 2.5 V range and kept low so that logical distributions survive a ten-year equivalent data retention bake.