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Multi-state flash EEPROM system using incremental programing and erasing methods

  • US 5,293,560 A
  • Filed: 11/03/1992
  • Issued: 03/08/1994
  • Est. Priority Date: 06/08/1988
  • Status: Expired due to Term
First Claim
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1. An electrically erasable programmable read only memory (EEPROM) system, comprising:

  • a semiconductor substrate including an array of a plurality of storage cells arranged in rows and columns across said substrate, each cell including;

    a source region and a drain region formed in said substrate with a channel therebetween,a floating gate positioned over a first portion of the channel in a manner that a level of charge on the floating gate establishes a threshold level of a first transistor, thereby affecting a level of conductance through the first channel portion between the source and drain regions,a control gate positioned over a second portion of the channel adjacent the first channel portion, thereby forming a second transistor connected in series with the first transistor, said control gate also extending across the floating gate in a manner that a level of electrical potential on the control gate affects the level of conductance of both of the first and second transistors through the respective first and second portions of the channel between the source and drain regions, andan erase gate positioned adjacent said floating gate;

    programming means connectable to a selected cell within the array of cells for applying voltages to the first transistor of the selected cell to increase an electron charge on its floating gate, thereby to alter the threshold level of the first transistor of the selected cell,reading means connectable to an addressed cell within the array of cells for detecting a level of conductance between the source and drain regions of the addressed cell in response to a predetermined voltage being applied across the source and drain regions of the addressed cell and a predetermined potential level being applied to the control gate of the addressed cell,erasing means connectable to erase gates of storage cells within one of a plurality of distinct blocks of cells of the array for simultaneously removing electrical charge from the floating gates of the cells within said one block that is sufficient to drive the threshold levels of the first transistors of said one block of cells to an erased level, each of said plurality of blocks including cells within multiple rows and multiple columns,wherein said reading means includes means for discriminating between a plurality of more than two predetermined threshold levels of the first transistor of the addressed cell, at least one of said more than two programmable threshold levels having a net positive charge on the floating gate of the first transistor of the addressed cell, each cell having a plurality of storage states more than two, andsaid programming means includes means for applying a sequence of programming voltage pulses to the first transistor of the selected cell to increase an electron charge on its floating gate from said erased level until at least either (a) a threshold level detected by said reading means in the first transistor of the selected cell is substantially equal to a desired one of the more than two threshold levels, or (b) a preset number of pulses has been applied.

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