Non-volatile flash memory device

1. A non-volatile memory device having a unit cell comprising:

a transistor including a gate oxide layer on a substrate, a polysilicon gate contacting the gate oxide layer, sidewall floating gates, a tunnel oxide layer between the sidewall floating gates and the substrate, block oxide layers, a sidewall spacer on a side of the polysilicon gate and a side of the sidewall floating gates, a polysilicon oxide layer which covers a top and the side of the polysilicon gate and the side of the sidewall floating gates, and source and drain regions;

a word line vertically placed on the substrate and connected to the polysilicon gate; and

a pair of bit lines orthogonally placed to the word line and connected to the source and drain regions.

2. The device as defined by claim 1 , wherein the non-volatile memory device has no bit line contact.

3. The device as defined by claim 1 , wherein the polysilicon gate is a ‘T’ shaped gate, which means that the upper part of the polysilicon gate is wider than the lower part of the polysilicon gate.

4. The device as defined by claim 1 , wherein the block oxide layers comprise first block oxide layers and second block oxide layers.

5. The device as defined by claim 4 , wherein the first block oxide layers have a thickness ranging between 50 Å and 250 Å and are made of a material selected from a group consisting of Al 2 O 3 and Y 2 O 3 .

6. The device as defined by claim 4 , wherein the second block oxide layers have a thickness ranging between 20 Å and 150 Å and are made of SiO 2 .

7. The device as defined by claim 1 , wherein a threshold voltage of the sidewall floating gates decreases as an erase operation time increases and converges to a predetermined threshold voltage at the end of the erase operation.