Nonvolatile semiconductor memory and method of operating the same

A method of reading a nonvolatile semiconductor memory having a memory cell comprising:

• a semiconductor substrate having a pair of trenches formed on a surface thereof;
• first electrodes formed in a pair of trenches through the intervention of a first insulating film to form selective transistors, respectively;
• a second electrode formed on the semiconductor substrate between the trenches through the intervention of a second insulating film;
• and a third electrode formed on the second electrode through the intervention of a third insulating film,
• wherein a voltage higher than a threshold value of the selective transistors is applied to the first electrodes, respectively, to change the two selective transistors into the ON state, thereby reading the memory cell.

A method of writing a nonvolatile semiconductor memory having a memory cell comprising:

• a semiconductor substrate having a pair of trenches formed on a surface thereof;
• first electrodes formed in a pair of trenches through the intervention of a first insulating film to form selective transistors, respectively;
• a second electrode formed on the semiconductor substrate between the trenches through the intervention of a second insulating film;
• and a third electrode formed on the second electrode through the intervention of a third insulating film,
• wherein a voltage is applied to one of the first electrodes, the voltage being higher than a threshold value of the selective transistor, to change the selective transistor into the ON state and a voltage is applied to another first electrode, the voltage being lower than a threshold value of the selective transistor, to change the selective transistor into the OFF state, thereby writing the memory cell.

A method of writing a nonvolatile semiconductor memory comprising a semiconductor substrate having a pair of trenches formed on a surface thereof;

• first electrodes formed in a pair of trenches through the intervention of a first insulating film, respectively;
• a second electrode formed on the semiconductor substrate between the trenches through the intervention of a second insulating film;
• a third electrode formed on the second electrode through the intervention of a third insulating film;
• wherein a plurality of the memory cells are arranged in matrix, the memory cells arranged in the X direction are connected with the third electrode lying in the X direction;
• the memory cells arranged in the Y direction are connected with the first electrode lying in the Y direction;
• the memory cells are connected to a pair of source/drain regions via selective transistors comprised of the first electrodes, respectively, to share the source/drain regions;
• wherein the first electrodes of the selective transistors connected to the drain regions are electrically isolated such that different voltages can be applied thereto, wherein a writing operation in the method comprises:
• (1) a first step of applying a voltage to the first electrode of the selective transistor connected to the source region, the voltage being lower than a threshold value of the selective transistor,
• applying a predetermined voltage to the drain regions and
• applying a voltage to all the first electrodes of the selective transistors connected to the drain regions, the voltage being higher than a threshold value of the selective transistors, to charge a channel region immediately below the second electrode of the memory cell with a drain voltage;
• and
• (2) a second step of applying a voltage to the first electrode of the selective transistor connected to the drain region of an unselected memory cell, the voltage being lower than a threshold value of the selective transistor,
• grounding a drain region connected to a selected memory cell and
• applying a voltage to the third electrode comprising the selected memory cell, the voltage being higher than a threshold value of the selected memory cell.