A method of forming a device is disclosed. The method includes providing a substrate prepared with a cell area separated by other active areas by isolation regions. First and second gates of first and second transistors in the cell area are formed. The first gate includes first and second sub-gates separated by a first intergate dielectric layer. The second gate includes a second sub-gate surrounding a first sub-gate. The first and second sub-gates of the second gate are separated by a second intergate dielectric layer. First and second junctions of the first and second transistors are formed. The method also includes forming a first gate terminal coupled to the second sub-gate of the first transistor and a second gate terminal coupled to at least the first sub-gate of the second transistor.
1. A device comprising:
a cell having first and second transistors coupled in series, wherein the first and second transistors are disposed between first and second cell terminals, wherein
the first transistor includes a first gate with first and second sub-gates separated by a first intergate dielectric layer, and
the second transistor includes a second gate with first and second sub-gates separated by a second intergate dielectric layer, the second sub-gate surrounds the first sub-gate of the second transistor;
a first gate terminal coupled to the second sub-gate of the first gate; and
a second gate terminal coupled to at least the first sub-gate of the second gate.
2. The device of claim 1 wherein:
the first transistor serves as a control gate;
the second transistor serves as a select gate;
a first junction of the second transistor serves as the second cell terminal, wherein the second cell terminal is coupled to a bitline; and
the second gate terminal is coupled to a wordline.
3. The device of claim 1 comprising:
a first gate dielectric layer below the first gate, wherein the first gate dielectric layer includes a tunneling window under the first sub-gate of the first gate; and
a second gate dielectric layer below the second gate.
4. The device of claim 1 wherein the first intergate dielectric layer comprises multiple dielectric layers.
5. The device of claim 1 wherein the second sub-gate that surrounds the first sub-gate in the second gate increases the distance from an edge of the first sub-gate to a heavily doped portion of a first junction of the second transistor to improve junction breakdown of the first junction due to high voltages.
6. The device of claim 1 wherein the second gate terminal is coupled to the first and second sub-gates of the second gate.
7. The device of claim 1 wherein the second gate terminal is coupled to the first sub-gate of the second gate while the second sub-gate of the second gate is floated.
8. A device comprising:
a substrate with a cell area having first and second transistors coupled in series, wherein
the first transistor includes a first gate with first and second sub-gates separated by a first intergate dielectric layer, and
the second transistor includes a second gate with first and second sub-gates separated by a second intergate dielectric layer, the second sub-gate surrounds the first sub-gate of the second transistor;
a first gate terminal coupled to the second sub-gate of the first gate; and
a second gate terminal coupled to at least the first sub-gate of the second gate.
9. The device of claim 8 wherein the first and second transistors comprise first and second doped regions adjacent to the first and second gates.
10. The device of claim 9 wherein the second doped regions of the first and second transistors are a common second doped region.
11. The device of claim 9 wherein:
the first doped region of the first transistor serves as a first cell terminal; and
the first doped region of the second transistor serves as a second cell terminal.
12. The device of claim 9 wherein the first and second doped regions of the first and second transistors comprise a heavily doped portion and a lightly doped extension portion.
13. The device of claim 9 comprising first and second buried doped regions adjacent to the first gate of the first transistor.
14. The device of claim 13 wherein the first and second buried doped regions overlap with the first and second doped regions of the first transistor, and extend beneath the first gate.
15. The device of claim 13 comprising:
a gate dielectric layer separating the first sub-gates of the first and second gates from the substrate, wherein the gate dielectric layer includes a tunneling window under the first sub-gate of the first gate.
16. The device of claim 15 wherein the first buried doped region extends beneath the tunneling window in the gate dielectric layer.
17. The device of claim 8 wherein the second gate terminal is coupled to the first and second sub-gates of the second gate.
18. The device of claim 8 wherein the second gate terminal is coupled to the first sub-gate of the second gate while the second sub-gate of the second gate is floated.
19. A device comprising:
a cell having first and second transistors, wherein
the first transistor includes a first gate with first and second sub-gates separated by a first intergate dielectric layer, and
the second transistor includes a second gate with first and second sub-gates separated by a second intergate dielectric layer, the second sub-gate surrounds the first sub-gate of the second transistor and increases a distance from an edge of the first sub-gate to a heavily doped portion in a doped region of the second transistor; and
a gate terminal coupled to at least the first sub-gate of the second gate.
20. The device of claim 19 comprising first and second buried doped regions adjacent to the first gate of the first transistor.