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Flash EEPROM system cell array with more than two storage states per memory cell

  • US 5,642,312 A
  • Filed: 05/22/1996
  • Issued: 06/24/1997
  • Est. Priority Date: 06/08/1988
  • Status: Expired due to Term
First Claim
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1. For an array of electrically alterable memory cells divided into blocks of cells and having means for addressing individual cells within said blocks to read and alter their states, said memory cells individually including a field effect transistor with a floating gate and having a threshold voltage level that is a given level in the absence of net charge on said floating gate but which is variable in accordance with an amount of net charge carried by said floating gate, a method of operating the array, comprising:

  • establishing a plurality of effective threshold voltage levels in excess of two that correspond to a plurality of detectable programmed states of the individual cells in excess of two,setting the effective threshold voltage level of at least one addressed cell within one of said blocks from a starting level to one of the plurality of threshold voltage levels by altering the amount of charge on the floating gate of said addressed cell until the effective threshold voltage of said addressed cell is substantially equal to one of said plurality of effective threshold voltage levels, whereby the state of said addressed cell is set to one of said plurality of programmed states, setting the effective threshold voltage level including the steps of;

    applying a given voltage to said addressed cell for a predetermined time sufficient to move the effective threshold voltage level of the addressed cell from the starting level toward said one of the plurality of threshold voltage levels,thereafter reading an electrical parameter of the addressed cell to determine whether the effective threshold voltage of the addressed cell has reached said one of the plurality of threshold voltage levels, andrepeating the voltage applying and reading steps until it is detected by the reading step that the effective threshold voltage of the addressed cell has been set to said one of the plurality of threshold voltage levels.

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