Patent ID: 6525361
Filing Date: 2003-02-25
Classification: G11C,H01L

Abstract:
An integrated circuit comprising:an array of memory cells, the memory cells comprising transistors having channels in channel regions of a substrate, selected asymmetric multilevel memory cells in the array storing multiple bits; a word line and a bit line coupled respectively with rows and columns of memory cells in the array by which to read data stored in the array, the bit line comprising a first patterned layer of the integrated circuit, the word line comprising a second patterned layer of the integrated circuit; each of the selected asymmetric multilevel memory cells having a source formed in the substrate, a gate, a gate oxide disposed between the substrate and the gate; each of the selected asymmetric multilevel memory cells further comprising: an asymmetric drain formed in the substrate, the asymmetric drain having a width; a channel formed in the substrate, the channel in contact with the gate oxide and aligned with the gate, the channel extending from the source towards the asymmetric drain, the channel separated from the asymmetric drain by an offset, the channel adapted to store multiple bits, the channel adapted to form a depletion layer in the channel proximal to the gate oxide in response to a gate voltage; and an offset region disposed in the substrate and disposed between the channel and the asymmetric drain, the offset region having an initial conduction state, the offset region adapted to maintain the initial conduction state proximal to the gate oxide in response to the gate voltage; each of the selected asymmetric multilevel memory cells having a first asymmetric drain read current corresponding to a drain voltage and a source read current corresponding to a source voltage, the source voltage equal to the drain voltage, the drain read current having a different value than the source read current; the offset of each of the selected memory cell transistors sufficiently large to ensure that the selected memory cell transistors can only be accurately read by the corresponding word lines and corresponding bit lines in one direction, wherein the gate of each of the selected asymmetric multilevel memory cells is addressable by a corresponding word line, and the integrated circuit further comprises: a sense amplifier having a sense amplifier conductor, the sense amplifier conductor having a first side and a second side; a ground having a conductor, the ground conductor having a first side and a second side; the bit line disposed proximal to the ground conductor, the bit line disposed on the first side of the ground conductor, the bit line disposed proximal to the sense amplifier conductor, the bit line disposed on the second side of the sense amplifier conductor; a first selected memory cell disposed between the bit line and the second side of the sense amplifier conductor, the asymmetric drain of the first selected memory cell in communication with the sense amplifier conductor; and a second selected memory cell disposed between the bit line and the first side of the ground conductor, the asymmetric drain of the second selected memory cell in communication with the ground conductor; the first selected memory cell and the second selected memory cell having a common source, the bit line in communication with the common source of the first and second selected memory cells; wherein, the first memory cell can only be accurately read by the corresponding word line and the corresponding bit line in a first direction, the second memory cell can only be accurately read by the corresponding word line and the corresponding bit line in a second direction, the second direction opposite the first direction.