Nonvolatile semiconductor memory and method of operating the same

1. A method of reading a nonvolatile semiconductor memory having a memory cell, the method comprising:

providing a semiconductor substrate having a pair of trenches formed on a surface thereof; first selective gate electrodes formed adjacent each other in said pair of trenches through the intervention of a first insulating film to form selective transistors, respectively; a second electrode that is a floating gate formed on the semiconductor substrate between the trenches through the intervention of a second insulating film; and a third electrode formed on the floating gate through the intervention of a third insulating film, and wherein the floating gate of said memory cell is centrally located between a pair of the first selective gate electrodes; and

applying a voltage higher than a threshold value of the selective transistors to both of the first selective gate electrodes at the same time so that the same voltage is applied to both of the first selective gate electrodes at the same time, respectively, to change the two selective transistors into the ON state so that the two selective transistors are in the ON state at the same time, thereby reading the memory cell.

2. A method of writing a nonvolatile semiconductor memory including at least one memory cell, the method comprising:

providing a semiconductor substrate having a pair of trenches formed on a surface thereof; first selective gate electrodes for the memory cell formed in said pair of trenches through the intervention of a first insulating film to form selective transistors for the memory cell, respectively; a second electrode that is a floating gate formed on the semiconductor substrate between the trenches through the intervention of a second insulating film; and a third electrode formed on the floating gate through the intervention of a third insulating film, and wherein the floating gate of said memory cell is centrally located between a pair of the first selective gate electrodes;

simultaneously applying (a) a first voltage to one of the first selective gate electrodes of the memory cell, the first voltage being higher than a threshold value of the selective transistor, to change the selective transistor into the ON state and (b) a second voltage to another first selective gate electrode of the memory cell, the second voltage applied to the another first selective gate electrode of the memory cell being lower than a threshold value of the selective transistor, to change the selective transistor into the OFF state, thereby writing the memory cell, wherein the first and second voltages are simultaneously applied, and wherein the first selective gate electrodes of the memory cell are arranged so that both of the first selective gate electrodes of the memory cell are located between a source and drain of the memory cell.

3. A method of writing a nonvolatile semiconductor memory, the method comprising

providing for a first memory cell a semiconductor substrate having a pair of trenches formed on a surface thereof first electrodes formed in said pair of trenches through the intervention of a first insulating film, respectively; a second electrode that is a floating gate formed on the semiconductor substrate between the trenches through the intervention of a second insulating film; a third electrode formed on the floating gate through the intervention of a third insulating film, wherein the first electrodes of the first of the memory cell are arranged so that both of the first electrodes of the memory cell are located between a source and drain of the first memory cell, and wherein the floating gate of said memory cell is centrally located between a pair of the first electrodes;

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 for said first memory cell in the array in comprises:

(1) a first step of simultaneously: (a) applying a voltage to the first electrode of the selective transistor connected to the source region of the memory cell, the voltage being lower than a threshold value of the selective transistor, (b) applying a predetermined voltage to the drain regions, and (c) 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.