Multiple layer floating gate non-volatile memory device
The disclosed systems and methods relate to floating gate non-volatile memory cells, with a floating gate comprising at least two layers constructed in different conductive or semiconductive materials. At least two of the layers of the floating gate are separated by an intermediate dielectric layer having a predetermined thickness enabling direct tunneling current between the layers.
1. Non-volatile memory device comprising at least one memory cell on a substrate in a semiconductor material, each memory cell comprising:
a channel region in between two doped regions of said substrate;
a floating gate above said channel region, said floating gate comprising at least two layers constructed in different conductive or semiconductive materials;
a first dielectric separating the floating gate from the substrate, said first dielectric being constructed in one or more dielectric materials and having predetermined properties enabling charge transfer by tunnelling between the channel region and the floating gate upon programming and erasing the cell;
a control gate above said floating gate, said control gate being constructed in one or more conductive or semiconductive materials;
a second dielectric separating the control gate from the floating gate, said second dielectric being constructed in one or more dielectric materials and having predetermined properties for suppressing charge transfer between the floating gate and the control gate;
a first interface being defined as the contact surface between the channel region and the first dielectric;
a second interface being defined as the contact surface between the first dielectric and the floating gate;
a third interface being defined as the contact surface between the floating gate and the second dielectric;
a fourth interface being defined as the contact surface between the second dielectric and the control gate;
wherein at least two of the layers of the floating gate are separated by an intermediate dielectric layer having a predetermined thickness enabling direct tunnelling current between the layers; and
wherein an overlap of the control and floating gates is larger than an overlap of the floating gate and the channel.
2. The non-volatile memory device of claim 1 , wherein the intermediate layer has a thickness of 0.5 to 1.5 nm.
3. The non-volatile memory device of claim 1 , wherein the semiconductor material of the substrate and the materials of the floating and control gates are chosen such that during programming the maximum electron affinity (χ S ) along the first interface is by a first predetermined amount (D 2 ) smaller than the minimum work function (φ FG,top ) along the third interface and that during erasing the maximum work function (φ FG,bottom ) at the second interface is by a second predetermined amount (D 1 ) smaller than the minimum work function (φ CG ) at the fourth interface, said first and second predetermined amounts (D 1 , D 2 ) being chosen for suppressing charge transfer by tunnelling between the floating gate and the control gate relative to the charge transfer by tunneling between the floating gate and the substrate upon programming and erasing the cell.
4. The non-volatile memory device of claim 1 , wherein the materials of the floating gate are chosen such that the minimum work function (φ FG,top ) at the third interface is larger than the maximum work function (φ FG,bottom ) at the second interface.
5. The non-volatile memory device of claim 4 , wherein the floating gate comprises a first layer contacting the first dielectric and a second layer contacting the second dielectric, the material of the first layer being n-type semiconductor material and the material of the second layer being p-type semiconductor material.
6. The non-volatile memory device of claim 1 , wherein the material of the floating gate at the second interface is a semiconductor material with a bandgap equal to or larger than the bandgap of the semiconductor substrate.
7. The non-volatile memory device of claim 1 , wherein the material of the floating gate at the second interface is a semiconductor material with an electron affinity equal to or larger than the electron affinity of the semiconductor substrate.
8. The non-volatile memory device of claim 1 , wherein the floating gate comprises a first layer contacting the first dielectric and a second contacting the second dielectric, and wherein the material of the floating gate at the second interface and material of the semiconductor substrate are selected such that the barrier for injection of carriers from the first layer to the semiconductor substrate is equal to or lower than the barrier for injection of carriers from the semiconductor substrate to the first layer.
9. The non-volatile memory device according to claim 8 , wherein the material of the floating gate at the second interface is the semiconductor material of the substrate.
10. The non-volatile memory device of claim 1 , wherein the material of the control gate at the fourth interface is p-type semiconductor material.
11. The non-volatile memory device of claim 1 , wherein the third interface has a larger area than the second interface.
12. The non-volatile memory device of claim 11 , wherein the floating gate is T-shaped.
13. The non-volatile memory device of claim 1 , wherein at least one of said dielectrics comprises a stack of different dielectric materials.