Bi-directional floating gate nonvolatile memory

Abstract

A memory transistor has a pair of separate floating gates overlying end regions of a channel and a control gate that overlies the floating gates and a central region of the channel. The memory transistor effectively operates as a pair of floating gate transistors with an intervening select transistor. Each floating gate can be charged to store a distinct binary, analog, or multi-bit value. The direction of the channel current controls which floating gate receives channel hot electron injection during programming and which floating gate state is sensed during reading. A read operation biases the word line higher that the threshold voltage used to store data and compares the resulting channel to reference currents to identify a stored binary, analog, or multi-bit value. The threshold voltage range can include negative threshold voltages, which increases the available range for multi-bit-per-floating gate storage. The memory transistors can be integrated into a contactless array architecture having approximately one global bit/virtual ground line for every four floating gates along a row.