1. Field of the Invention
This invention relates generally to the art of microelectronic electronic circuits. More specifically, this invention relates to the architecture of flash memory devices. Even more specifically, this invention relates to the architecture of flash memory devices that utilize 1-transistor memory cells that allow the erasure of a bit or byte or word.
2. Discussion of the Related Art
A microelectronic flash or block erase Electrically Erasable Programmable Read-Only Memory (Flash EEPROM) includes an array of cells that can be independently programmed and read. The size of each cell and thereby the memory are made small by omitting transistors known as select transistors that enable the cells to be erased independently. As a result, all of the cells are erased together as a block.
A memory of this type includes individual Metal-Oxide-Semiconductor (MOS) field effect transistor memory cells, each of which includes a source, a drain, a floating gate and a control gate to which various voltages are applied to program the cell with a binary 1 or 0, or to erase all of the cells as a block.
The cells are connected in an array of rows and columns, with the control gates of the cells in a row being connected to a respective wordline and the drains of the cells in a column being connected to a respective bitline. The sources of the cells are connected together. This arrangement is known as a NOR memory configuration.
A cell is programmed by applying a voltage, typically 9 volts to the control gate, applying a voltage of approximately 5 volts to the drain and grounding the source, which causes hot electrons to be injected from a drain depletion region into the floating gate. Upon removal of the programming voltages, the injected electrons are trapped in the floating gate and create a negative change therein which increases the threshold voltage of the cell to a value in excess of approximately 4 volts.
A cell is read by applying typically 5 volts to the control gate, applying 1 volt to the bitline to which the drain is connected, grounding the source, and sensing the bitline current. If the cell is programmed and the threshold voltage is relatively high (4 volts), the bitline current will be zero or at least relatively low. If the cell is not programmed or erased, the threshold voltage will be relatively low (2 volts), the control gate voltage will enhance the channel, and the bitline current will be relatively high.
A cell can be erased in several ways. In one arrangement, a cell is erased by applying a relatively high voltage, typically 12 volts, to the source, grounding the control gate and allowing the drain to float. This causes the electrons that were injected into the floating gate during programming to undergo Fowler-Nordheim tunneling from the floating gate through the thin tunnel oxide layer to the source. A cell can also be erased by applying a negative voltage on the order of -10 volts to the control gate, applying 5 volts to the source and allowing the drain to float. Another method of erasing is by applying 5V to the P-well and -10V to the control gate while allowing the source/drain to float.
The erase procedure causes electron trapping to occur in the tunnel oxide. In addition, the undererase and overerase procedures cause electron trapping to occur in the tunnel oxide. Although each programming/erase cycle adds only a small number of electron trapping, the cumulative electron trapping increases as each programming/erase cycle is completed which, in turn, increasingly degrades the erase time.
Because prior art flash memory devices must be erased as a block, it is necessary to erase the entire memory device if only one memory cell must be changed from a programmed state to an unprogrammed state. Each time the memory device is erased, the cumulative electron trapping increases as each erase/programming cycle is completed and, as discussed above, the operation of the memory cell is degraded.
Therefore, what is needed are memory devices made up of 1 transistor memory cells that allow the memory devices to be erased one bit at a time, one byte at a time or 1 word at a time.