Non-volatile memory having a reference transistor
View Patent ↗A non-volatile memory ( 30 ) comprises nanocrystal memory cells ( 50, 51, 53 ). The program and erase threshold voltage of the memory cell transistors ( 50, 51, 53 ) increase as a function of the number of program/erase operations. During a read operation, a reference transistor ( 46 ) provides a reference current for comparing with a cell current. The reference transistor ( 46 ) is made from a process similar to that used to make the memory cell transistors ( 50, 51, 53 ), except that the reference transistor ( 46 ) does not include nanocrystals. By using a similar process to make both the reference transistor ( 46 ) and the memory cell transistors ( 50, 51, 53 ), a threshold voltage of the reference transistor ( 46 ) will track the threshold voltage shift of the memory cell transistor ( 50, 51, 53 ). A read control circuit ( 42 ) is provided to bias the gate of the reference transistor ( 46 ). The read control circuit ( 42 ) senses a drain current of the reference transistor ( 46 ) and adjusts the gate bias voltage to maintain the reference current at a substantially constant value relative to the cell current.
1. A nonvolatile memory comprising:
a substrate, wherein a first area of the substrate is for a memory array and a second area of the substrate is for a reference transistor;
gate stack structures overlying the first area of the substrate, each of the gate stack structures comprising a first dielectric layer overlying the substrate, a storage material layer overlying the first dielectric layer, a second dielectric layer overlying and surrounding the storage material layer, and a gate material overlying the second dielectric layer; and
the reference transistor having a reference transistor gate stack structure comprising a third dielectric layer and a fourth dielectric layer, the third dielectric and the fourth dielectric collectively permitting a threshold voltage of the reference transistor to vary with respect to program and erase operations of the nonvolatile memory over time, the reference transistor further comprising the gate material overlying the fourth dielectric layer.
2. The nonvolatile memory of claim 1 wherein the first dielectric, the second dielectric, the third dielectric and the fourth dielectric are comprised of differing compositions.
3. The nonvolatile memory of claim 1 wherein the storage material layer further comprises nanoclusters of at least one of silicon, nitride, germanium, silver, platinum, gold, tungsten, and tantalum.
4. The nonvolatile memory of claim 1 further comprising:
a third area of the substrate, the third area containing other circuitry for use with the nonvolatile memory.
5. The nonvolatile memory of claim 1 further comprising:
diffusion regions formed in the first area of the substrate for forming source and drain regions for the gate stack structures.
6. The nonvolatile memory of claim 1 wherein the third dielectric layer and the fourth dielectric layer of the reference transistor permit the reference transistor to have a variable threshold voltage as a function of a number of times the reference transistor is biased.
7. A nonvolatile memory array comprising:
a substrate comprising a first area for a memory array and a second area for a reference transistor;
a first dielectric layer overlying the first area of the substrate;
a storage material layer overlying the first dielectric layer and the first area of the substrate;
a second dielectric layer overlying and surrounding the storage material layer and overlying the first dielectric and the first area of the substrate;
a third dielectric layer overlying only the second area of the substrate;
a fourth dielectric layer overlying the third dielectric layer above the second area of the substrate;
a conductive gate layer overlying the second dielectric in the first area of the substrate and overlying the fourth dielectric in the second area of the substrate;
a plurality of gate stacks of transistors in the first area of the substrate and a reference gate stack in the second area of the substrate;
current electrode regions in the first area of the substrate and the second area of the substrate that form memory cells in the first area of the substrate and a reference transistor in the second area of the substrate for providing a reference current to be compared with a memory cell current of a predetermined one of the memory cells in the first area of the substrate; and
control circuitry coupled to the plurality of gate stacks of transistors in the first area of the substrate and to the reference gate stack in the second area of the substrate for providing a substantially same program or erase voltage for substantially a same length of time each time said memory array is programmed or erased.
8. The nonvolatile memory array of claim 7 wherein the storage material layer further comprises a layer of nanoclusters.
9. The nonvolatile memory array of claim 7 wherein said nanoclusters further comprise dots comprising at least one of silicon, nitride, germanium, silver, platinum, gold, tungsten, and tantalum that are spaced apart at least in some regions so as to not be in direct contact.
10. The nonvolatile memory of claim 7 further comprising:
a third area of the substrate containing the control circuitry.
11. The nonvolatile memory of claim 7 wherein the first area of the substrate is a first well region having a first doping concentration and the second area of the substrate is a second well region having a second doping concentration, the first doping concentration being substantially equal to the second doping concentration.
12. The nonvolatile memory of claim 7 wherein the first area of the substrate is a first well region having a first doping concentration and the second area of the substrate is a second well region having a second doping concentration, the first doping concentration being different from the second doping concentration.
13. A semiconductor comprising:
a substrate having a first region and a second region;
a plurality of memory cell transistors in the first region of the substrate, each of the plurality of memory cell transistors comprising a gate stack structure comprising a gate dielectric and a layer of storage material having a predetermined height; and
a reference transistor in the second region of the substrate, the reference transistor having a reference transistor gate stack structure comprising a first dielectric layer and a second dielectric layer, the first dielectric layer and the second dielectric layer collectively permitting the threshold voltage of the reference transistor to vary with respect to program and erase operations of the plurality of memory cell transistors over time.
14. The semiconductor of claim 13 wherein the first dielectric layer and the second dielectric layer comprise a material of substantially a same composition.
15. The semiconductor of claim 13 wherein the first dielectric layer and the second dielectric layer comprise materials of differing composition.
16. The semiconductor of claim 13 wherein the layer of storage material within the plurality of memory cell transistors comprises nanocrystals, the layer of storage material having at least some regions where the nanocrystals are physically spaced apart.
17. The semiconductor of claim 13 wherein the first dielectric layer and the second dielectric layer of the reference transistor collectively have a height substantially equal to the predetermined height.