Stacked memory cell utilizing negative differential resistance devices

Abstract

A memory cell includes two negative differential resistance (NDR) field effect transistors (FETs) forming a bistable latch, and an access transistor for allowing data to be passed to and from the storage node formed by the bistable latch. By stacking the NDR-FETs and the access transistor in two or more layers, area requirements for the memory cell can be reduced, thereby enabling increased circuit density in an integrated circuit (IC) incorporating the memory cell.

Description

FIELD OF THE INVENTION

This invention generally relates to semiconductor memory devices and technology, and in particular to static random access memory (SRAM) devices.

BACKGROUND OF THE INVENTION

The rapid growth of the semiconductor industry over the past four decades has largely been enabled by continual advancements in manufacturing technology which have allowed the size of the transistor, the basic building block in integrated circuits (ICs), to be steadily reduced with each new generation of technology. As the transistor size is scaled down, the chip area required for a given circuit is reduced, so that more chips can be manufactured on a single silicon wafer substrate, resulting in lower manufacturing cost per chip; circuit operation speed also improves, because of reduced capacitance and higher transistor current density. State-of-the-art fabrication facilities presently manufacture ICs with minimum transistor feature size smaller than 100 nm, so that microprocessor products with transistor counts approaching 1 billion transistors per chip can be manufactured cost-effectively. High-density semiconductor memory devices have already reached the gigabit scale, led by dynamic random access memory (DRAM) technology. The DRAM memory cell consists of a single pass transistor and a capacitor (1T/1C), wherein information is stored in the form of charge on the capacitor. Although the DRAM cell provides the most compact layout (with area ranging between 4F² and 8F², where F is the minimum feature half-pitch defined by lithography), it requires frequent refreshing (typically on the order of once per millisecond) because the charge on the capacitor leaks away at a rate of approximately 10⁻¹⁵ Amperes per cell. This problem is exacerbated by technology scaling, because the transistor leakage current increases with decreasing channel length, and also because a reduction in cell capacitance results in a smaller number of stored charge carriers, so that more frequent refreshing is necessary. Thus, scaling of DRAM technology to much higher densities presents significant technological challenges.

Static RAM (SRAM) does not require refreshing and is generally faster than DRAM (approaching 1 ns access times as compared to tens of ns for DRAM). However, the SRAM cell is more complex, requiring either four n-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) and two p-channel MOSFETs, or four n-channel MOSFETs and two polycrystalline-silicon (poly-Si) load resistors, resulting in significantly larger cell size (typically greater than >80 F²). Innovations which provide significant reductions in SRAM cell size while allowing the SRAM cell to retain its favorable operating characteristics are therefore highly desirable.

Negative differential resistance (NDR) devices have previously been proposed for compact static memory applications. E. Goto in IRE Trans. Electronic Computers, March 1960, p. 25 disclosed an SRAM cell consisting of two resonant tunneling diodes (RTDs) and a pass transistor. For a variety of NDR devices including RTDs, the current first increases with increasing applied voltage, reaching a peak value, then decreases with increasing applied voltage over a range of applied voltages, exhibiting negative differential resistance over this range of applied voltages and reaching a minimum (“valley”) value. At yet higher applied voltages, the current again increases with increasing applied voltage. Thus, the current-vs.-voltage characteristic is shaped like the letter “N”. A key figure of merit for NDR devices is the ratio of the peak current to the valley current (PVCR). The higher the value of the PVCR, the more useful the NDR device is for variety of circuit applications. The PVCR of RTDs is generally not high enough to make it practical for low-power SRAM application, because in order for the RTDs in a Goto cell to have sufficient current drive, the valley current is too large, causing large static power dissipation. In addition, RTDs require specialized fabrication process sequences so that the complexity of an integrated RTD/MOSFET SRAM process would be substantially higher than that of a conventional complementary MOS (CMOS) SRAM process, resulting in higher manufacturing cost.

Accordingly, there exists a significant need for NDR devices with very high (>10⁶) PVCR which can be easily integrated into a conventional CMOS technology, for compact, low-power, low-cost SRAM.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a static random access memory (SRAM) cell of significantly smaller size as compared to a conventional six-transistor SRAM cell, while retaining the desirable operating characteristics of the conventional SRAM cell without significant increase in manufacturing cost.

For achieving the object, the invention provides a semiconductor device comprising an n-channel insulated-gate field-effect transistor (IGFET) including a gate and source/drain electrodes, and two (preferably n-channel) NDR-FETs each including gate and source/drain electrodes, wherein the IGFET and NDR-FET elements are formed on a common substrate, with one of the source/drain electrodes of the IGFET semiconductor element connected to the drain electrode of a first NDR-FET and also to the source electrode of a second NDR-FET, the gate electrode of the IGFET connected to a first control terminal, the other one of the source/drain electrodes of the IGFET connected to a second control terminal, the drain electrode of the first NDR-FET connected to a power-supply terminal, the source electrode of the second NDR-FET connected to a grounded or negatively-biased terminal, and the gate electrodes of the NDR-FETs each biased at a constant voltage. The point of connection between the drain electrode of the first NDR-FET and the source electrode of the second NDR-FET is the data storage node. This semiconductor device can function as a bistable memory cell, with access to the data storage node provided via the IGFET.

In various embodiments, the first NDR-FET, the second NDR-FET, and the IGFET access transistor that make up the SRAM cell can be formed in two or more semiconductor layers in a stacked configuration, thereby reducing the layout area requirements of the SRAM cell. In one embodiment, the first NDR-FET, the second NDR-FET, and the IGFET access transistor can be formed in two different semiconductor layers, such that one of the first and second NDR-FETs and the IGFET access transistor overlies another of the first and second NDR-FETs and the IGFET access transistor. In another embodiment, the first and second NDR-FETs and the IGFET access transistor can each be formed in a different semiconductor layer, such that the three transistors are arranged one above another (e.g., the first NDR-FET overlies the IGFET access transistor, and the second NDR-FET overlies the first NDR-FET).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a static random access memory (SRAM) cell consisting of the combination of two NDR-FET elements which form a bistable latch and one n-channel enhancement-mode IGFET access element;

FIG. 2 is a plot of the current vs. storage node voltage characteristic of the bistable latch formed by the combination of two NDR-FETs as shown in FIG. 1;

FIG. 3 is a schematic cross-sectional view of an NDR-FET element connected to an IGFET, showing the various layers shared by the two elements which are co-fabricated using a single process flow.

FIGS. 4A and 4B are cross-sectional views of SRAM cells consisting of the combination of two NDR-FET elements and one n-channel enhancement-mode IGFET access element formed in multiple stacked semiconductor layers.

DETAILED DESCRIPTION OF THE INVENTION

A semiconductor device according a preferred embodiment of the invention will now be described with reference to FIGS. 1 and 2. FIG. 1 is a circuit diagram of a preferred embodiment of a static memory (SRAM) cell 100 consisting of two NDR-FET elements 120, 130 which form a bistable latch 140 and one enhancement-mode IGFET access element 110. FIG. 2 is a current vs. storage node voltage plot illustrating the operational characteristics of the static memory cell of FIG. 1. The NDR-FET element of the present invention is preferably of the type disclosed in the following King et al. applications: Ser. No. 09/603,101 entitled “A CMOS-PROCESS COMPATIBLE, TUNABLE NDR (NEGATIVE DIFFERENTIAL RESISTANCE) DEVICE AND METHOD OF OPERATING SAME” now U.S. Pat. No. 6,512,274; and Ser. No. 09/603,102 entitled “CHARGE TRAPPING DEVICE AND METHOD FOR IMPLEMENTING A TRANSISTOR HAVING A NEGATIVE DIFFERENTIAL RESISTANCE MODE” now U.S. Pat. No. 6,479,862; and Ser. No. 09/602,658 entitled “CMOS COMPATIBLE PROCESS FOR MAKING A TUNABLE NEGATIVE DIFFERENTIAL RESISTANCE (NDR) DEVICE” now U.S. Pat. No. 6,596,617 all of which were filed Jun. 22, 2000 and which are hereby incorporated by reference as if fully set forth herein.

As is shown in FIG. 1, IGFET 110 is configured as a transfer gate, allowing a BIT line to be connected to a storage node under the control of a WORD line. One of the source/drain electrodes of IGFET 110 is connected to the storage node at potential V_SN, the other source/drain electrode of IGFET 110 is connected to the BIT line, and the gate electrode of IGFET 110 is connected to the WORD line.

The source electrode of first NDR-FET 120 is connected to a ground terminal, the gate electrode of first NDR-FET 120 is supplied with first bias voltage V_BIAS1, the drain electrode of the NDR-FET 120 is connected together with the source electrode of a second NDR-FET 130 to the storage node, the gate electrode of second NDR-FET 130 is supplied with a second bias voltage V_BIAS2 and the drain electrode of second NDR-FET 130 is supplied with a positive voltage V_DD. The current flowing in the first NDR-FET, I_NDR1, is dependent on the difference between its drain electrode potential and its source electrode potential, V_SN, at first increasing rapidly as V_SN increases, reaching a peak value when V_SN is equal to a critical voltage V_NDR1, and rapidly decreasing to nearly zero as V_SN increases beyond the critical voltage V_NDR1. The bias voltage V_BIAS1 is sufficiently high so as to ensure that first NDR-FET 120 is turned on for values of V_SN ranging from 0 V (ground potential) to V_NDR1. The current flowing in the second NDR-FET, I_NDR2 is dependent on the difference between its drain electrode potential and its source electrode potential, V_DD−V_SN, at first increasing rapidly as V_DD−V_SN increases, reaching a peak value when V_DD−V_SN is equal to a critical voltage V_NDR2, and rapidly decreasing to nearly zero as V_DD−V_SN increases beyond the critical voltage V_NDR2. The bias voltage V_BIAS2 is ideally sufficiently high so as to ensure that second NDR-FET 130 is turned on for values of V_DD−V_SN ranging from 0 V (ground potential) to V_NDR2.

Next the preferred operation of bistable latch 140 in SRAM cell 100 of FIG. 1 will be described. FIG. 2 shows the current I_NDR1 vs. storage node voltage V_SN characteristic curve of first NDR-FET 120 obtained by changing the storage node voltage V_SN in a range between 0 and V_DD, superimposed with the current I_NDR2 VS. storage node voltage V_SN characteristic curve of second NDR-FET 130. A stable operating point of circuit 140 is a point where the I_NDR1 vs. V_SN characteristic curve of the first NDR-FET crosses the the I_NDR2 vs. V_SN characteristic curve of the second NDR-FET and additionally the characteristic curves I_NDR1 and I_NDR2 have the same gradient sign (positive or negative). (The crossing point where the characteristic curves I_NDR1 and I_NDR2 have opposite gradient is not a stable operating point.)

Therefore it is understood that circuit 140 is stable when the potential V_SN at the storage node is one of two values 0 and V_DD as shown in FIG. 2. Accordingly, the circuit can be used as a bistable memory cell by applying a potential of one of the two values 0 and V_DD to the BIT line as a write voltage. If the value of V_SN increases slightly above that of the low (0 V) stable operating point, current I_NDR1 flowing in first NDR-FET 120 becomes higher than the current I_NDR2 flowing in second NDR-FET 130, causing the value of V_SN to be decreased toward 0 V (ground potential), to restore it to that of the stable operating point. Thus first NDR-FET 120 serves as a “pull-down” device. If the value of V_SN falls slightly below that of the high (V_DD) stable operating point, the current I_NDR2 flowing in second NDR-FET 130 becomes higher than the current I_NDR1 flowing in first NDR-FET 120, causing the value of V_SN to be increased toward V_DD, to restore it to that of the stable operating point. Thus second NDR-FET 130 serves as a “pull-up” device.

IGFET 110 is controlled by the WORD line as follows: when the WORD line potential is sufficiently high, IGFET 110 is turned on, connecting the BIT line to the storage node to allow data transfer (reading data from the storage node, or writing data to the storage node); when the WORD line potential is low, IGFET 110 is turned off, so that the storage node is electrically isolated from the BIT line. In this manner, a bistable latch 140 is realized with two series-connected NDR-FET elements, and a compact static memory cell is obtained by integrating latch 140 with a IGFET pass transistor 110.

It should be noted that in order to achieve low standby current in the SRAM cell, the valley currents of the NDR-FETs (i.e. I_NDR1 at V_SN=V_DD and I_NDR2 at V_SN=0V) are preferably minimized, while in order to achieve a fast read access time, the peak currents of the NDR-FETs are preferably maximized. Since the NDR-FET peak current and valley current are controlled by the gate bias voltage applied to the NDR-FET, it is possible to achieve a very low valley current by using a lower gate bias voltage when the SRAM cell is in storage mode to achieve low static power dissipation, and to achieve a very high peak current by using a higher gate bias voltage when the SRAM cell is in read mode to achieve fast read access time. In this aspect, the NDR-FET PVCR can effectively be enhanced by several orders of magnitude.

As previously stated, the bias voltage V_BIAS2 should ideally be sufficiently high so as to ensure that second (pull-up) NDR-FET 130 is turned on for values of V_DD−V_SN ranging from 0 V (ground potential) to V_NDR2. Accordingly, V_BIAS2 should ideally be greater than or equal to V_DD+V_T, where V_T. is the threshold voltage of second NDR-FET 130. If second NDR-FET 130 is substantially an enhancement-mode device (i.e. V_T>0 V), then V_BIAS2 should be greater than V_DD. Thus, a separate power supply voltage or a boosted supply (such as that provided by a charge pump circuit) would be needed. It should be noted that the charge pump circuit would not consume much power, as it would only supply a high voltage, with negligible current.

As previously stated, the bias voltage V_BIAS1 should be sufficiently high so as to ensure that first (pull-down) NDR-FET 120 is turned on for values of V_SN ranging from 0 V (ground potential) to V_NDR1. Therefore, V_BIAS1 can be tied or coupled to V_DD if desired to reduce constraints on the aforementioned charge pump circuit. Alternatively, V_BIAS1 can be tied to V_BIAS2 to simplify the cell architecture and layout.

FIG. 3 is a schematic cross-sectional view of an NDR-FET element connected to an IGFET, such as would exist in the preferred embodiment. The NDR-FET and IGFET are formed to include and share many common layers, including at least a portion of the gate insulating film, gate film, interlayer insulator and metal, and hence can be readily fabricated together on a single substrate using a single process flow. For example, a common substrate 300, a common isolation area 310 and common interlayer insulation layers 380 (380′) are used by NDR-FETs and IGFETs respectively. Furthermore, a single gate electrode layer is used for gates 360, 360′ and a single metal/contact layer 390, 390′. Source/drain regions 370, 370′ are formed at the same time, and a common source/drain region 375′ is shared by the NDR-FET and IGFET. This latter region can serve as a storage node for example in the above embodiments. An NDR charge trapping layer 330 is included only within an NDR-FET region, for the reasons set forth in the aforementioned referenced applications. Finally, both devices can also share a gate insulation film 340, 340′ in some implementations.

FIG. 4A is a schematic cross-sectional view of an SRAM cell 400A consisting of two NDR-FET elements 411 and 412, which form a bistable latch, and one enhancement-mode IGFET access element (“transfer element”) 420. The circuit implemented by SRAM cell 400A is described above with respect to SRAM cell 100 in FIG. 1. SRAM cell 400A depicts an exemplary implementation of SRAM cell 100 in which the devices forming SRAM cell 400A are formed in a stacked configuration to reduce layout area consumed by SRAM cell 400A in an actual integrated circuit (IC). Specifically, IGFET access element 420 is formed in a first semiconductor layer 401-1, and NDR-FET elements 411 and 412 are formed in a second semiconductor layer 401-2 (separated from first semiconductor layer 401-1 by an insulating layer 402-1 (e.g., oxide layer)), such that NDR-FET element 411 overlies IGFET access element 420.

IGFET access element 420 includes source/drain regions R5 and R6 that are formed in first semiconductor layer 401-1, with a dielectric layer D3 formed on first semiconductor layer 401-1 between source/drain regions R5 and R6, and with a gate G3 formed on dielectric layer D3. Note that IGFET access element 420 is considered to be formed “in” first semiconductor layer 401-1 because source/drain regions R5 and R6 are formed in first semiconductor layer 401-1 (even through dielectric layer D3 and gate G3 are actually formed “on” first semiconductor layer 401-1). NDR-FET element 411 includes a source/drain region R1 and a source/drain region R2 that is shared with NDR-FET element 412. NDR-FET element 411 further includes a dielectric layer D1 formed on second semiconductor layer 401-2 between source/drain regions R1 and R2, and a gate G1 formed on dielectric layer D1. Similarly, NDR-FET element 412 includes source/drain regions R2 and R3, a dielectric layer D2 formed on second semiconductor layer 401-2 between source/drain regions R2 and R3, and a gate G2 formed on dielectric layer D2. Dielectric layers D1 and D2 include charge trapping layers C1 and C2, respectively, that provide the NDR characteristics for NDR-FET elements 411 and 412 described above. Finally, a vertical interconnect (plug) 405A connects source/drain region R6 of IGFET access element 420 with source/drain region R2 of NDR-FET elements 411 and 412 and forms a storage node for SRAM cell 400A.

As described above with respect to FIG. 1, supply voltages V_S1 and V_S2 (e.g., ground potential and V_DD, respectively) are connected across the series-connected NDR-FET elements 411 and 412, and appropriate bias voltages V_BIAS1 and V_BIAS2 are supplied to gates G1 and G2, respectively, to cause NDR-FET elements 411 and 412 to exhibit the desired bi-stable latch behavior. As further described above with respect to FIG. 1, gate G3 and source/drain region R5 of IGFET access element 420 are coupled to word (read/write) line WORD and a bit (data) line BIT, respectively, to control access and data communications with SRAM cell 400A.

In this manner, SRAM cell 400A provides a compact implementation of an SRAM cell. Because NDR-FET element 411 overlies (i.e., is positioned above) IGFET access element 420, the chip area (i.e., plan view area looking down at the chip) consumed by SRAM cell 400A is essentially equivalent to a 2T (two transistor) cell. Note that although both NDR-FET elements 411 and 412 are depicted as being formed in the same semiconductor layer 401-2 for exemplary purposes (and to simplify manufacturing), any distribution of devices between semiconductor layers 401-1 and 401-2 can be used to achieve the benefit of the stacked configuration. For example, IGFET access element 420 could be formed in second semiconductor layer 401-2 and both NDR-FET elements 411 and 412 could be formed in first semiconductor layer 401-1. Alternatively, IGFET access element 420 could be formed with one of NDR-FET elements 411 and 412 in one of semiconductor layers 401-1 and 401-2, with the other NDR-FET element being formed by itself in the other semiconductor layer. Various other configurations will be readily apparent.

Note further that additional area reduction for a 3T SRAM cell can be achived via stacking of all three devices in the cell (i.e., arranging the three transistors one above another). FIG. 4B is a schematic cross-sectional view of an SRAM cell 400B consisting of the two NDR-FET elements 411 and 412 and the one enhancement-mode IGFET access element 420 described with respect to SRAM cell 400A in FIG. 4A. However, unlike SRAM cell 400A, which is formed in two semiconductor layers, SRAM cell 400B is formed in three semiconductor layers 401-1, 401-2, and 401-3 (which are separated by insulating layers 402-1 and 402-2). Therefore, IGFET access element 420 and NDR-FET elements 411 and 412 can be formed over one another so that SRAM cell 400A effectively occupies the area of a 1T (one transistor) cell.

For exemplary purposes, IGFET access element 420 (which includes source/drain regions R5 and R6, dielectric layer D3, and gate G3) is formed in first semiconductor layer 401-1, NDR-FET element 411 (which includes source/drain regions R1 and R2, dielectric layer D1 (including charge trapping layer C1), and gate GI) is formed in second semiconductor layer 401-2, and NDR-FET element 412 (which includes source/drain regions R3 and R4, dielectric layer D2 (including charge trapping layer C2), and gate G2) is formed in third semiconductor layer 401-3. Note, however, that in various other embodiments, SRAM cell 400B can include any distribution of NDR-FET elements 411 and 412 and IGFET access element 420 among semiconductor layers 401-1, 401-2, and 401-3. A vertical interconnect 405B connects the source/drain regions R2, R4, and R6 of NDR-FET element 412, NDR-FET element 411, and IGFET access element 420, respectively, and forms storage node for SRAM cell 400B. Because NDR-FET elements 411 and 412 and IGFET access element 420 all overlie one another (i.e., are formed one over the other in a single stack), SRAM cell 400B implements the circuit of FIG. 1 in an extremely space-efficient manner.

While the invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. It will be clearly understood by those skilled in the art that foregoing description is merely by way of example and is not a limitation on the scope of the invention, which may be utilized in many types of integrated circuits made with conventional processing technologies. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. Such modifications and combinations, of course, may use other features that are already known in lieu of or in addition to what is disclosed herein. It is therefore intended that the appended claims encompass any such modifications or embodiments. While such claims have been formulated based on the particular embodiments described herein, it should be apparent the scope of the disclosure herein also applies to any novel and non-obvious feature (or combination thereof) disclosed explicitly or implicitly to one of skill in the art, regardless of whether such relates to the claims as provided below, and whether or not it solves and/or mitigates all of the same technical problems described above. Finally, the applicants further reserve the right to pursue new and/or additional claims directed to any such novel and non-obvious features during the prosecution of the present application (and/or any related applications).

Claims

1. A memory cell comprising: a first negative differential resistance field effect transistor (NDR-FET); a second NDR-FET connected in series with the first NDR-FET; and an access transistor connected between a data line and a junction between the first NDR-FET and the second NDR-FET, wherein at least one of the first NDR-FET, the second NDR-FET, and the access transistor overlies at least another of the first NDR-FET, the second NDR-FET, and the access transistor.

2. The memory cell of claim 1, wherein the access transistor is formed in a first semiconductor layer, wherein the first NDR-FET and the second NDR-FET are formed in a second semiconductor layer, the second semiconductor layer overlying the first semiconductor layer, and wherein one of the first NDR-FET and the second NDR-FET overlies the access transistor.

3. The memory cell of claim 2, wherein the first NDR-FET and the second NDR-FET share a common source/drain region.

4. The memory cell of claim 1, wherein the first NDR-FET and the second NDR-FET are formed in a first semiconductor layer, wherein the access transistor is formed in a second semiconductor layer, the second semiconductor layer overlying the first semiconductor layer, and wherein the access transistor overlies one of the first NDR-FET and the second NDR-FET.

5. The memory cell of claim 4, wherein the first NDR-FET and the second NDR-FET share a common source/drain region.

6. The memory cell of claim 1, wherein the first NDR-FET and the access transistor are formed in a first semiconductor layer, wherein the second NDR-FET is formed in a second semiconductor layer, the second semiconductor layer overlying the first semiconductor layer, and wherein the second NDR-FET overlies one of the first NDR-FET and the access transistor.

7. The memory cell of claim 1, wherein the first NDR-FET is formed in a first semiconductor layer, wherein the second NDR-FET and the access transistor are formed in a second semiconductor layer, the second semiconductor layer overlying the first semiconductor layer, and wherein one of the second NDR-FET and the access transistor overlies the first NDR-FET.

8. The memory cell of claim 1, wherein the first NDR-FET, the second NDR-FET, and the access transistor are arranged one above another.

9. The memory cell of claim 8, wherein the first NDR-FET is formed in a first semiconductor layer, wherein the second NDR-FET is formed in a second semiconductor layer, wherein the access transistor is formed in a third semiconductor layer, wherein the first semiconductor layer overlies the second semiconductor layer, and wherein the second semiconductor layer overlies the third semiconductor layer.

10. The memory cell of claim 8, wherein the first NDR-FET is formed in a first semiconductor layer, wherein the second NDR-FET is formed in a second semiconductor layer, wherein the access transistor is formed in a third semiconductor layer, and wherein the third semiconductor layer overlies the first semiconductor layer and the second semiconductor layer overlies the third semiconductor layer.

11. The memory cell of claim 8, wherein the first NDR-FET is formed in a first semiconductor layer, wherein the second NDR-FET is formed in a second semiconductor layer, wherein the access transistor is formed in a third semiconductor layer, and wherein the third semiconductor layer overlies the first semiconductor layer and the second semiconductor layer.

12. A method for making a memory cell, the method comprising: forming a first negative differential resistance (NDR) field effect transistor (FET) in series with a second NDR-FET; forming an access transistor for connecting a data line to a junction between the first NDR-FET and the second NDR-FET, wherein at least one of the first NDR-FET, the second NDR-FET, and the access transistor overlies at least another of the first NDR-FET, the second NDR-FET, and the access transistor.

13. The method of claim 12, wherein forming the access transistor comprises forming the access transistor in a first semiconductor layer, the method further comprising: forming an insulating layer over the access transistor; forming a second semiconductor layer over the insulating layer, wherein forming the first NDR-FET in series with the second NDR-FET comprises forming the first NDR-FET and the second NDR-FET in the second semiconductor layer such that one of the first NDR-FET and the second NDR-FET overlies the access transistor; and forming a vertical interconnect between a source/drain region of the access transistor and the junction between the first NDR-FET and the second NDR-FET.

14. The method of claim 13, wherein the junction between the first NDR-FET and the second NDR-FET comprises a shared source/drain region.

15. The method of claim 12, wherein forming the forming the first NDR-FET in series with the second NDR-FET comprises forming the first NDR-FET and the second NDR-FET in a first semiconductor layer, the method further comprising: creating an insulating layer over the first NDR-FET and the second NDR-FET; creating a second semiconductor layer over the insulating layer, wherein forming the access transistor comprises forming the access transistor in the second semiconductor layer such that the access transistor overlies one of the first NDR-FET and the second NDR-FET; and forming a vertical interconnect between a source/drain region of the access transistor and the junction between the first NDR-FET and the second NDR-FET.

16. The method of claim 15, wherein the junction between the first NDR-FET and the second NDR-FET comprises a shared source/drain region.

17. The method of claim 12, wherein forming the access transistor comprises forming the access transistor in a first semiconductor layer, and wherein forming the first NDR-FET in series with the second NDR-FET comprises: forming the first NDR-FET in the first semiconductor layer; creating an insulating layer over the access transistor and the first NDR-FET; creating a second semiconductor layer over the insulating layer; and forming the second NDR-FET in the second semiconductor layer, wherein the second NDR-FET overlies one of the first NDR-FET and the access transistor.

18. The method of claim 12, wherein forming the access transistor comprises forming the access transistor in a first semiconductor layer, and wherein forming the first NDR-FET in series with the second NDR-FET comprises: creating a first insulating layer over the access transistor; creating a second semiconductor layer over the first insulating layer; forming the first NDR-FET in the second semiconductor layer such that the first NDR-FET overlies the access transistor; creating a second insulating layer over the first NDR-FET; creating a third semiconductor layer over the second insulating layer; forming the second NDR-FET in the third semiconductor layer such that the second NDR-FET overlies the first NDR-FET; and forming a vertical interconnect connecting a source/drain region of the second NDR-FET, a source/drain region of the first NDR-FET, and a source/drain region of the access transistor.

19. The method of claim 12, wherein forming the first NDR-FET in series with the second NDR-FET comprises: forming the first NDR-FET in a first semiconductor layer; creating a first insulating layer over the first NDR-FET; creating a second semiconductor layer over the first insulating layer; forming the second NDR-FET in the second semiconductor layer such that the second NDR-FET overlies the first NDR-FET; creating a second insulating layer over the second NDR-FET; creating a third semiconductor layer over the second insulating layer, wherein forming the access transistor comprises forming the access transistor in the third semiconductor layer such that the access transistor overlies the second NDR-FET; and forming a vertical interconnect connecting a source/drain region of the access transistor, a source/drain region of the second NDR-FET, and a source/drain region of the first NDR-FET.