CT-NOR differential bitline sensing architecture
View Patent ↗Providing for a non-volatile semiconductor memory architecture that achieves high read performance is described herein. In one aspect, an array of memory transistors arranged electrically in serial is configured to control a gate voltage of a pass transistor. The pass transistor, in turn, enables current flow between two metal bitlines of the semiconductor memory architecture. Accordingly, a relative voltage or relative current of the two metal bitlines can be measured and utilized to determine a program or erase state of a transistor of the serial array of transistors. In a particular aspect, a transistor with small capacitance is chosen for the pass transistor, resulting in a fast correspondence of the pass transistor gate voltage/current relative to transistor array current. This can equate to fast read times for the transistor array, based on differential sensing of the two metal bitlines.
1. A semiconductor memory device, comprising:
a first array of transistors arranged electrically in serial from source to drain, one end of the first array is electrically coupled to a gate of a pass transistor and an opposite end of the first array is electrically coupled to a metal bitline of the semiconductor memory device; and
a second metal bitline coupled to a first end of a channel region and the metal bitline coupled to a second end of the channel region of the pass transistor, wherein a state of a transistor of the first array of transistors is determined from a difference in an electrical characteristic of the metal bitline relative to the second metal bitline.
2. The semiconductor memory device of claim 1 , further comprising a sense amplifier circuit coupled to both the metal bitline and the second metal bitline that measures the difference in the electrical characteristic.
3. The semiconductor memory device of claim 1 , wherein the electrical characteristic is a relative voltage, relative current, relative transient current or voltage of the metal bitline with respect to the second metal bitline.
4. The semiconductor memory device of claim 1 , wherein the electrical characteristic is based at least in part on a voltage polarity or a direction of current flow for the metal bitline relative the second metal bitline.
5. The semiconductor memory device of claim 1 , further comprising a second array of transistors arranged electrically in serial from source to drain, one end of the second array is electrically coupled to the gate of the pass transistor.
6. The semiconductor memory device of claim 5 , wherein a second end of the second array is electrically coupled to the second metal bitline.
7. The semiconductor memory device of claim 5 , wherein the first array of transistors and the second array of transistors are electrically coupled to the pass transistor and are both programmed and read based on voltages applied on the WLs, D-select, S-select, gate of the pass transistor or based on a relative voltage or current of the metal bitline and the second metal bitline, or a combination thereof.
8. The semiconductor memory device of claim 5 , further comprising a first string select wordline configured to toggle current flow on or off for the second array of transistors, and configured to conduct current flow for the first array of transistors, wherein the first string select wordline can isolate the second array of transistors from the gate of the pass transistor enabling programming or reading the first array of transistors.
9. The semiconductor memory device of claim 8 , further comprising a second string select wordline configured to toggle current flow on or off for the first array of transistors, and configured to conduct current flow for the second array of transistors, wherein the second string select wordline can isolate the first array of transistors from the gate of the pass transistor enabling programming or reading the second array of transistors.
10. The semiconductor memory device of claim 9 , further comprising a third array of transistors arranged electrically in serial from source to drain, one end of the third array is electrically coupled to the gate of the pass transistor.
11. The semiconductor memory device of claim 10 , further comprising a third string select wordline configured to conduct current flow for the third array of transistors and configured to connect or isolate at least one additional array of transistors to or from, respectively, the gate of the pass transistor, wherein the third array of transistors is programmed or read when the first array, the second array and the at least one additional array are isolated from the gate of the pass transistor.
12. The semiconductor memory device of claim 9 , wherein the at least one additional array of transistors is a fourth array of transistors arranged electrically in serial from source to drain, one end of the fourth array is electrically coupled to the gate of the pass transistor.
13. The semiconductor memory device of claim 12 , further comprising a fourth string select wordline configured to conduct current flow for the fourth array of transistors and configured to isolate the third array of transistors from the gate of the pass transistor, wherein the fourth array of transistors is programmed or read when:
the first array, the second array and the third array are isolated from the gate of the pass transistor by the second string select wordline, the first string select wordline and the fourth string select wordline, respectively; and
the third string select wordline electrically connects the fourth array to the gate of the pass transistor.
14. A method of fabricating a semiconductor memory device, comprising:
connecting an array of transistors that are arranged electrically in serial to a gate of a toggle transistor at one end of the array;
connecting a second end of the array to a metal bitline of the semiconductor memory device; and
connecting the metal bitline and a second metal bitline of the semiconductor memory device to opposite ends of a channel region of the toggle transistor, wherein a change in current or voltage of the metal bitline relative the second metal bitline is indicative of a program or erase state of one or more transistors of the array.
15. The method of claim 14 , further comprising electrically isolating or connecting the metal bitline and the second metal bitline via a gate voltage of the toggle transistor.
16. The method of claim 15 , further comprising:
selecting a transistor of the array;
electrically connecting a channel of the transistor to the gate of the toggle transistor;
measuring the change in current or voltage of the metal bitline relative the second metal bitline; and
determining whether the transistor is in the program state or the erase state based on the change.
17. The method of claim 14 , further comprising:
connecting a second array of transistors to the gate of the toggle transistor at one end of the second array;
connecting another end of the second array to the second metal bitline; and
selectively connecting or isolating the array or the second array from the gate of the toggle transistor to facilitate programming, erasing or reading a transistor of the array or of the second array, respectively.
18. The method of claim 17 , further comprising:
forming a plurality of wordlines on the array of transistors and the second array of transistors, wherein at least one transistor formed by the plurality of wordlines on the array of transistors and on the second array of transistors, respectively, is a depletion transistor; and
selectively connecting or isolating the array or the second array from the gate of the toggle transistor by activating different ones of the plurality of wordlines.