# Patent

## Title
Memory device comprising electrically floating body transistor

## Abstract

A semiconductor memory cell comprising an electrically floating body. A method of operating the memory cell is provided.

## Background
CROSS-REFERENCE
This application claims the benefit of U.S. Provisional Application No. 61/775,521, filed Mar. 9, 2013 and U.S. Provisional Application No. 61/816,153, filed Apr. 25, 2013, both of which applications are hereby incorporated herein, in their entireties, by reference thereto.


FIELD OF THE INVENTION
The present invention relates to semiconductor memory technology. More specifically, the present invention relates to a semiconductor memory device comprising of an electrically floating body transistor.
BACKGROUND OF THE INVENTION
Semiconductor memory devices are used extensively to store data. Memory devices can be characterized according to two general types: volatile and non-volatile. Volatile memory devices such as static random access memory (SRAM) and dynamic random access memory (DRAM) lose data that is stored therein when power is not continuously supplied thereto.
DRAM based on the electrically floating body effect has been proposed, both on silicon on insulator (SOI) substrate (see for example “A Capacitor-less 1T-DRAM Cell”, S. Okhonin et al., pp. 85-87, IEEE Electron Device Letters, vol. 23, no. 2, February 2002 (“Okhonin”) and “Memory Design Using One-Transistor Gain Cell on SOI”, T. Ohsawa et al., pp. 152-153, Tech. Digest, 2002 IEEE International Solid-State Circuits Conference, February 2002 (“Ohsawa”), which are hereby incorporated herein, in their entireties, by reference thereto) and on bulk silicon substrate (see, for example, “Further Insight Into the Physics and Modeling of Floating-Body Capacitorless DRAMs”, A. Villaret et al., pp. 2447-2454, IEEE Transactions on Electron Devices, vol. 52, no. 11, November 2005 (“Villaret”), “Scaled 1T-Bulk Devices Built with CMOS 90 nm Technology for Low-cost eDRAM Applications”, R. Ranica, et al., pp. 38-41, Tech. Digest, Symposium on VLSI Technology, 2005 (“Ranica”), and “Simulation of Intrinsic Bipolar Transistor Mechanisms for future capacitor-less eDRAM on bulk substrate”, R. Pulicani et al., pp. 966-969, 2010 17th IEEE International Conference on Electronics, Circuits, and Systems, December 2010 (“Pulicani”), which are hereby incorporated herein, in their entireties, by reference thereto). Such memory eliminates the capacitor used in the conventional 1T/1C memory cell, and thus is easier to scale to smaller feature size. In addition, such memory allows for a smaller cell size compared to the conventional 1T/1C memory cell. Similar to the 1T/1C DRAM memory cell, the floating body DRAM memory cell only has one stable state in a memory cell, and thus requires periodic refresh operations to restore the state of the memory cell.
A bi-stable floating body memory cell, where more than one stable state exists for each memory cell, has been described in U.S. Patent Application Publication No. 2010/00246284 to Widjaja et al., titled “Semiconductor Memory Having Floating Body Transistor and Method of Operating” (“Widjaja-1”) and U.S. Patent Application Publication No. 2010/0034041, “Method of Operating Semiconductor Memory Device with Floating Body Transistor Using Silicon Controlled Rectifier Principle” (“Widjaja-2”), which are both hereby incorporated herein, in their entireties, by reference thereto.
Previous disclosures on floating body memory cell formed on bulk silicon substrate requires a buried well layer and insulating layers to isolate a floating body region.
SUMMARY OF THE INVENTION
In one aspect of the present invention, a semiconductor memory cell includes: a floating body region configured to be charged to a level indicative of a state of the memory cell selected from at least first and second states; a first region in electrical contact with the floating body region; a second region in electrical contact with said floating body region and spaced apart from the first region; a gate positioned between the first and second regions; a first insulating region located above the floating body region; second insulating regions adjacent to the floating body region; a buried layer region located below the floating body region and the second insulating regions and spaced from the second insulating regions so as not to contact the second insulating regions, wherein the floating body region is bounded by the first insulating region above the floating body region, the second insulating regions adjacent to the floating body region, and a depletion region formed as a result of an application of a back bias to the buried layer region.
In at least one embodiment, the semiconductor memory cell further comprises a substrate region, wherein the buried layer region is positioned between the substrate region and the floating body region.
In at least one embodiment, the first region has a first conductivity type selected from a p-type conductivity type and an n-type conductivity type; the second region has a first conductivity type selected from a p-type conductivity type and an n-type conductivity type; the floating body region has a second conductivity type selected from a p-type conductivity type and an n-type conductivity type and different from the first conductivity type; and the buried layer region has the first conductivity type.
In at least one embodiment, the back bias is applied to the buried layer region as a constant positive voltage bias or pulsed positive bias.
In at least one embodiment, the buried layer region is configured to inject charge into or extract charge out of the floating body region to maintain the state of the memory cell.
In at least one embodiment, the floating body region, the first region, the second region, the gate, the first insulating region, the second insulating regions and the buried layer form a memory device, and the semiconductor memory cell further comprises an access transistor connected in series with the memory device.
In another aspect of the present invention, a semiconductor memory cell includes: a floating body region configured to be charged to a level indicative of a state of the memory cell selected from at least first and second states; a first region in electrical contact with the floating body region; a second region in electrical contact with the floating body region and spaced apart from the first region; a gate positioned between the first and second regions; a first insulating region located above the floating body region; second insulating regions adjacent to the floating body region; a buried layer region located below the floating body region and the second insulating regions and spaced from the second insulating regions so as not to contact the second insulating regions, wherein the floating body region is bounded by the first insulating region above the floating body region, the second insulating regions adjacent to the floating body region, and a top boundary of depletion region formed to the buried layer region as a result of an application of a back bias, and wherein application of the back bias results in at least two stable floating body charge levels.
In at least one embodiment, the semiconductor memory cell further includes a substrate region, wherein the buried layer region is positioned between the substrate region and the floating body region.
In at least one embodiment, the first region has a first conductivity type selected from a p-type conductivity type and an n-type conductivity type; the second region has the first conductivity type; the floating body region has a second conductivity type selected from a p-type conductivity type and an n-type conductivity type and different from the first conductivity type; and the buried layer region has the first conductivity type.
In at least one embodiment, the back bias is applied to the buried layer region as a constant positive voltage bias or pulsed positive voltage.
In at least one embodiment, the buried layer region is configured to inject charge into or extract charge out of the floating body region to maintain the state of the memory cell.
In at least one embodiment, the floating body region, the first region, the second region, the gate, the first insulating region, the second insulating regions and the buried layer form a memory device, and the semiconductor memory cell further includes an access transistor connected in series with the memory device.
In another aspect of the present invention, a semiconductor memory array includes at least two memory cells, wherein each memory cell includes: a floating body region configured to be charged to a level indicative of a state of the memory cell selected from at least first and second states; a first region in electrical contact with the floating body region; a second region in electrical contact with the floating body region and spaced apart from the first region; a gate positioned between the first and second regions; a first insulating region located above the floating body region; second insulating regions adjacent to the floating body region; a buried layer region located below the floating body region and the second insulating regions, wherein at least one of the second insulating regions adjacent to the floating body region insulates the memory cell from an adjacent memory cell.
In at least one embodiment, the semiconductor memory cells each further include a substrate region, wherein the buried layer region is positioned between the substrate region and the floating body region.
In at least one embodiment, the first region has a first conductivity type selected from a p-type conductivity type and an n-type conductivity type; the second region has the first conductivity type; the floating body region has a second conductivity type selected from a p-type conductivity type and an n-type conductivity type and different from the first conductivity type; and the buried layer region has the first conductivity type.
In at least one embodiment, the back bias is applied to the buried layer region as a constant positive voltage bias or pulsed positive voltage.
In at least one embodiment, the buried layer region is configured to inject charge into or extract charge out of the floating body region to maintain the state of the memory cell.
In at least one embodiment, the floating body region, the first region, the second region, the gate, the first insulating region, the second insulating regions and the buried layer form a memory device, and the semiconductor memory cell further comprises an access transistor connected in series with the memory device.
In another aspect of the present invention, a floating body memory cell is formed on bulk silicon substrate (or other semiconductor materials) which is isolated by a depletion region formed through application of a bias condition on a back bias region.
These and other features of the invention will become apparent to those persons skilled in the art upon reading the details of the devices and methods as more fully described below.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a schematic, cross-sectional illustration of a memory cell according to the present invention.
FIG. 1B is a schematic, cross-sectional illustration of a memory cell according to another embodiment of the present invention.
FIG. 2A is a schematic, cross-sectional illustration of a memory cell according to another embodiment of the present invention.
FIG. 2B is a schematic, cross-sectional illustration of a memory cell according to another embodiment of the present invention.
FIG. 3A is a schematic, cross-sectional illustration of a memory cell according to another embodiment of the present invention.
FIG. 3B is a schematic, top-view illustration of the memory cell shown in FIG. 3A.
FIG. 4A is a schematic, cross-sectional illustration of a memory cell according to another embodiment of the present invention.
FIG. 4B is a schematic, cross-sectional illustration of a memory cell according to another embodiment of the present invention.
FIG. 4C is a schematic, cross-sectional illustration of a memory cell according to another embodiment of the present invention.
FIGS. 5A and 5B illustrate the energy band diagram formed by the halo region and the drain region under equilibrium condition and when a positive bias is applied to the drain region, respectively.
FIG. 6 schematically illustrates a write logic-1 operation performed on a memory array according to an embodiment of the present invention.
FIG. 7 illustrates bias conditions applied on the terminals of a memory cell to perform a write logic-1 operation, according to an embodiment of the present invention.
FIGS. 8A and 8B illustrate a graph of the drain current as a function of the gate voltage for a memory cell with high tunneling current according to an embodiment of the present invention and for a memory cell with low tunneling current, respectively.
FIG. 9 is a schematic, cross-sectional illustration of a memory cell according to another embodiment of the present invention.
FIG. 10 is a schematic, cross-sectional illustration of a memory cell according to another embodiment of the present invention.
FIG. 11 schematically illustrates multiple memory cells joined to make a memory array.
FIG. 12 schematically illustrates a holding operation performed on a memory array according to an embodiment of the present invention.
FIG. 13 illustrates bias conditions applied on the terminals of a memory cell to perform a holding operation, according to an embodiment of the present invention.
FIG. 14A shows an energy band diagram characterizing an intrinsic bipolar device when a floating body region is positively charged and a positive bias is applied to a buried well region of a memory cell according to an embodiment of the present invention.
FIG. 14B shows an energy band diagram of an intrinsic bipolar device when a floating body region is neutrally charged and a positive bias is applied to a buried well region of a memory cell according to an embodiment of the present invention.
FIG. 15 shows a graph of the net current I flowing into or out of a floating body region as a function of the potential V of the floating body, according to an embodiment of the present invention.
FIG. 16A shows a schematic curve of a potential energy surface (PES) of a memory cell according to an embodiment of the present invention.
FIG. 16B illustrates a charge stored in a floating body region of a memory cell as a function of a potential applied to a buried well region, connected to a BW terminal, according to an embodiment of the present invention.
FIG. 17 schematically illustrates a read operation performed on a memory array according to an embodiment of the present invention.
FIG. 18 illustrates bias conditions applied on the terminals of a memory cell to perform a read operation, according to an embodiment of the present invention.
FIG. 19 schematically illustrates a write logic-0 operation performed on a memory array according to an embodiment of the present invention.
FIG. 20 illustrates bias conditions applied on the terminals of a memory cell to perform a write logic-0 operation, according to an embodiment of the present invention.
FIG. 21 schematically illustrates a write logic-1 operation performed on a memory array according to an embodiment of the present invention.
FIG. 22 illustrates bias conditions applied on the terminals of a memory cell to perform a write logic-1 operation, according to an embodiment of the present invention.
FIG. 23 schematically illustrates a write logic-1 operation using band-to-band tunneling mechanism performed on a memory array according to an embodiment of the present invention.
FIG. 24 illustrates bias conditions applied on the terminals of a memory cell to perform a write logic-1 operation using band-to-band tunneling mechanism, according to an embodiment of the present invention.
FIG. 25 illustrates voltages applied to the gate and drain regions to perform a write logic-1 operation according to another embodiment of the present invention.
FIG. 26 is a schematic, cross-sectional illustration of a memory cell according to an embodiment of the present invention.
FIG. 27 is a schematic, cross-sectional illustration of a plurality of memory cells connected to form a memory array, according to an embodiment of the present invention.
FIG. 28 is a schematic, cross-sectional illustration of a plurality of memory cells connected to form a memory array according to another embodiment of the present invention.
FIG. 29 schematically illustrates a depletion region that is formed as a result of a bias condition applied to the memory cell according to an embodiment of the present invention.
FIG. 30 is a schematic, cross-sectional illustration of a memory cell according to another embodiment of the present invention
FIG. 31 schematically illustrates a depletion region that is formed as a result of a bias condition applied to the memory cell illustrated in FIG. 31.
FIG. 32 illustrates a bias condition applied to the memory cell during a write logic-0 operation according to an embodiment of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
Before the present devices and methods are described, it is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.
Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described. All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited.
It must be noted that as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a memory cell” includes a plurality of such memory cells and reference to “the region” includes reference to one or more regions and equivalents thereof known to those skilled in the art, and so forth.
The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. The dates of publication provided may be different from the actual publication dates which may need to be independently confirmed.
A schematic cross-sectional view of a memory device 50, according to an embodiment of the present invention, is shown in FIG. 1A. Memory cell 50 includes a substrate 12 of a first conductivity type such as p-type, for example. Substrate 12 is typically made of silicon, but may additionally or alternatively comprise, for example, germanium, silicon germanium, gallium arsenide, carbon nanotubes, or other semiconductor materials. In some embodiments of the invention, substrate 12 can be the bulk material of the semiconductor wafer. In other embodiments, substrate 12A can be a well of the first conductivity type embedded in either a well 29 of the second conductivity type (e.g., n-type, when the first conductivity type is p-type, or vice versa) in the bulk of the semiconductor wafer 12B of the first conductivity type or, alternatively, in the bulk of the semiconductor wafer of the second conductivity type, such as n-type, for example as shown in FIG. 1B. To simplify the description, the substrate 12 will usually be drawn as the semiconductor bulk material as it is in FIG. 1A.
Memory cell 50 also comprises a buried layer region 22 of a second conductivity type, such as n-type, for example; a floating body region 24 of the first conductivity type, such as p-type, for example; and source/drain regions 16 and 18 of the second conductivity type, such as n-type, for example. Buried layer 22 may be formed by an ion implantation process on the material of substrate 12. Alternatively, buried layer 22 can be grown epitaxially on top of substrate 12.
The floating body region 24 of the first conductivity is bounded on top by surface 14, source line region 16, drain region 18, and insulating layer 62, and on the bottom by buried layer 22. Floating body 24 may be the portion of the original substrate 12 above buried layer 22 if buried layer 22 is implanted. Alternatively, floating body 24 may be epitaxially grown. Depending on how buried layer 22 and floating body 24 are formed, floating body 24 may have the same doping as substrate 12 in some embodiments or a different doping, if desired in other embodiments.
A source line region 16 having a second conductivity type, such as n-type, for example, is provided in floating body region 24, so as to bound a portion of the top of the floating body region in a manner discussed above, and is exposed at surface 14. Source line region 16 may be formed by an implantation process on the material making up substrate 12, according to any implantation process known and typically used in the art. Alternatively, a solid state diffusion or a selective epitaxial growth process could be used to form source line region 16.
A bit line region 18 having a second conductivity type, such as n-type, for example, is also provided in floating body region 24, so as to bound a portion of the top of the floating body region in a manner discussed above, and is exposed at cell surface 14. Bit line region 18 may be formed by an implantation process on the material making up substrate 12, according to any implantation process known and typically used in the art. Alternatively, a solid state diffusion or a selective epitaxial growth process could be used to form bit line region 18.
A gate 60 is positioned in between the source line region 16 and the drain region 18, above the floating body region 24. The gate 60 is insulated from the floating body region 24 by an insulating layer 62. Insulating layer 62 may be made of silicon oxide and/or other dielectric materials, including high-K dielectric materials, such as, but not limited to, tantalum peroxide, titanium oxide, zirconium oxide, hafnium oxide, and/or aluminum oxide. The gate 60 may be made of, for example, polysilicon material or metal gate electrode, such as tungsten, tantalum, titanium and/or their nitrides.
Insulating layers 26 (like, for example, shallow trench isolation (STI)), may be made of silicon oxide, for example, though other insulating materials may be used. The bottom of insulating layer 26 may reside inside the buried region 22 allowing buried region 22 to be continuous as shown in FIG. 1. Alternatively, the bottom of insulating layer 26 may reside below the buried region 22 as in FIGS. 3A and 3B (shown better in FIG. 3A). This requires a shallower insulating layer 28, which insulates the floating body region 24, but allows the buried layer 22 to be continuous in the perpendicular direction of the cross-sectional view shown in FIG. 3A. For simplicity, only memory cell 50 with continuous buried region 22 in all directions will be shown from hereon. Alternatively, the bottom of insulating layer 26 may reside above the buried layer region 22.
Memory cell 50 may comprise the regions 17 and 19 of the first conductivity type. The regions 17 and 19 of the first conductivity type can be pocket region as shown in FIGS. 1A and 1B. The regions located at an angle with respect to the surface 14 can be referred as halo regions 17 and 19 as shown in FIG. 2A. The regions 17 and 19 of the first conductivity type can also be formed under the spacer regions 61 as shown in FIG. 2B. Alternatively, the region 19 of the first conductivity type can be asymmetrically located near the bit line region 18, as shown in FIGS. 4A-4C. The regions 17 and 19 of the first conductivity type will be referred to as halo regions hereafter for simplicity.
Cell 50 includes several terminals: word line (WL) terminal 70 electrically connected to gate 60, bit line (BL) terminal 74 electrically connected to bit line region 18, source line (SL) terminal 72 electrically connected to source line region 16, buried well (BW) terminal 76 electrically connected to buried layer 22, and substrate terminal 78 electrically connected to the substrate 12. The doping concentration of the halo region 17 and/or 19 is such that the built-in electric field potential between the halo region 17 and the source region 16 and/or between the halo region 19 and the drain region 18 is high enough to result in lower potential for band-to-band tunneling across the halo region and the source/drain region. To harness the lower potential for band-to-band tunneling, the doping concentration of the first conductivity type region 17 and/or 19 should be high, and is preferentially greater than 5×1018 but not to exceed 5×1019/cm3.
FIGS. 5A and 5B illustrate the band diagram of the halo region 19 and the drain region 18 for the halo region 19 with the conventional doping concentration and the halo region 19 with the high doping concentration, respectively. FIGS. 5A and 5B illustrate the band diagram when the halo region 19 is of p-type conductivity and the drain region 18 is of n-type conductivity, and assumes the same voltages are applied to the source region 16. Under reverse bias conditions as shown in FIG. 5B, electrons may tunnel from the valence band of the halo region 19 to the conduction band of the drain region 18, leaving excess holes to the floating body region 24 connected to the halo region 19.
Several operations can be performed by memory cell 50 such as holding, read, write logic-1 and write logic-0 operations, and have been described for example in U.S. Patent Application Publication No. 2010/00246284 to Widjaja et al., titled “Semiconductor Memory Having Floating Body Transistor and Method of Operating” (“Widjaja-1”) and U.S. Patent Application Publication No. 2010/0034041, “Method of Operating Semiconductor Memory Device with Floating Body Transistor Using Silicon Controlled Rectifier Principle” (“Widjaja-2”), which are both hereby incorporated herein, in their entireties, by reference thereto. In prior art, the lower limit of the drain region 18 voltage for writing logic-1 operation using impact ionization is approximately 1.2 V, greater than the band-gap of silicon semiconductor. However, due to the inherent energy band-bending, the write logic-1 voltage of the memory cell 50 can be scaled to a voltage less than 1.2V through the formation of high concentration halo region 17 and/or 19, where band-to-band tunneling is intrinsically present at low voltage of the drain region 18.
FIGS. 6 and 7 illustrate the operating conditions for a write logic-1 operation, where the following bias conditions are applied: a positive voltage is applied to the BW terminal 76a, zero voltage is applied to the SL terminal 72a, a positive voltage is applied to the BL terminal 74a, zero voltage is applied to the WL terminal 70a, while zero voltage is applied to the substrate terminal 78. The positive voltage applied to the BL terminal 74 is sufficiently high to cause hole tunneling from the drain region 16 to the floating body 24. When cell 50 is in an array 80, the unselected BL terminals 74 (e.g. 74b, . . . , 74n) will be at zero voltage, and all SL terminals 72 will be at zero voltage, the unselected WL terminals 70 (e.g. 70n and any other WL terminals 70 not connected to selected cell 50a) will be at a slight positive voltage. The slight positive voltage applied on the unselected WL terminals 70 increases the potential of the floating bodies 24, and reduces the electric field between the drain regions 18 and the halo regions 19. As a result, no hole injection (or significantly less hole injection) is observed on the unselected cells 50. In one particular non-limiting embodiment, about +0.6 volts is applied to the selected BL terminal 74a, about 0.0 volts is applied to the selected SL terminal 72a, about 0.0 volts is applied to the selected WL terminal 70a, about +1.2 volts is applied to BW terminal 76a, and about 0.0 volts is applied to terminal 78, as illustrated in FIGS. 6 and 7.
The unselected BL terminals 74 are at 0.0 volts, the unselected SL terminals 72 are at 0.0 volts, the unselected WL terminals 70 are at +0.4 volts, about +1.2 volts is applied to the unselected BW terminals 76, and about 0.0 volts is applied to the unselected substrate terminals 78. However, these voltage levels may vary while maintaining the relative relationships between voltage levels as generally described above.
FIGS. 8A and 8B show a comparison between the drain current and gate voltage relationship of the memory cell 50 with a high tunneling current according to the present invention (FIG. 8A) and that of a conventional memory cell with a low tunneling current (FIG. 8B), respectively. With a high tunneling current cell 50 of the present invention, because the intrinsic energy band-bending is large enough to create band-to-band tunneling as explained with regard to FIG. 5B, the operating voltage where high hole injection current is observed is when lower than +1.2V is applied to the drain region and about zero voltage is applied to the gate region (connected to the WL terminal). For a conventional memory cell with a low tunneling current, because the intrinsic energy band-bending is not enough to cause the band-to-band tunneling, the high hole injection current occurs only when a negative voltage is applied to the gate electrode in order to electrically bend the energy band. Due to the higher hole injection current (as a result of the halo regions 17 and/or 19 having a high doping concentration that results in a high built-in electric field), the high tunneling current memory cell 50 does not require negative voltage for the write logic-1 operation, which simplifies the design of the periphery circuitry due to absence of (lack of requirement for) a negative voltage generator. In addition, the voltage applied to the drain region 18 of the high tunneling current memory cell 50 is also lower compared to that of the low tunneling current cell.
FIG. 9 illustrates memory cell 150 according to another embodiment of the present invention. Memory cell 150 is constructed as a bulk planar junctionless memory cell transistor, where the channel region 23 have the same conductivity type as the source and drain regions 16 and 18 as described for example in “Bulk Planar Junctionless Transistor (BPJLT): An Attractive Device Alternative for Scaling”, S. Gundapaneni et al., pp. 261-263, IEEE Electron Device Letters, vol. 32, no. 3, March 2011 (“Gundapaneni”), which is hereby incorporated herein, in its entirety, by reference thereto. The lateral doping concentration across the I-I′ cut line near the surface region 14 is uniform and of the same conductivity type. The thickness of the channel region 23 has to be thin enough for the gate to fully deplete its majority carrier during when zero voltage is applied to the gate region 60. In one particular embodiment, the thickness of the channel region 23 is about 10 nm. However, the thickness may vary depending on the thickness of the gate dielectric 62 and the work function of the gate 60 (see below).
Memory cell 150 includes a substrate 12 of a first conductivity type such as p-type, for example. Substrate 12 is typically made of silicon, but may additionally or alternatively comprise, for example, germanium, silicon germanium, gallium arsenide, carbon nanotubes, and/or other semiconductor materials. In some embodiments of the invention, substrate 12 can be the bulk material of the semiconductor wafer.
Memory cell 150 also comprises a buried layer region 22 of a second conductivity type, such as n-type, for example, and a floating body region 24 of the first conductivity type, such as p-type, for example. Buried layer 22 may be formed by an ion implantation process on the material of substrate 12. Alternatively, buried layer 22 can be grown epitaxially on top of substrate 12.
The floating body region 24 of the first conductivity type is bounded on top by channel region 23, source line region 16, and drain region 18, and on the bottom by buried layer 22. Floating body 24 may be the portion of the original substrate 12 above buried layer 22 if buried layer 22 is implanted. Alternatively, floating body 24 may be epitaxially grown. Depending on how buried layer 22 and floating body 24 are formed, floating body 24 may have the same doping as substrate 12 in some embodiments or a different doping, if desired in other embodiments.
A gate 60 is positioned in between the source line region 16 and the drain region 18, above the channel region 23. The gate 60 is insulated from the floating body region 24 by an insulating layer 62. Insulating layer 62 may be made of silicon oxide and/or other dielectric materials, including high-K dielectric materials, such as, but not limited to, tantalum peroxide, titanium oxide, zirconium oxide, hafnium oxide, and/or aluminum oxide. The gate 60 may be made of, for example, polysilicon material or metal gate electrode, such as tungsten, tantalum, titanium and their nitrides, where the gate electrode 60 has a high work function. Also, high work function gate materials such as valence-band edge metal or p-type polysilicon may be used to form the gate 60. The high work function of the gate electrode 60 will fully deplete the majority carrier of the channel region 23 near the surface when zero voltage is applied to the gate electrode 60, turning off the conduction path between the source region 16 and the drain region 18.
Insulating layers 26 (like, for example, shallow trench isolation (STI)), may be made of silicon oxide, for example, though other insulating materials may be used. The bottom of insulating layer 26 may reside inside the buried region 22 allowing buried region 22 to be continuous as shown in FIG. 9. Alternatively, the bottom of insulating layer 26 may reside below the buried region 22 (see for example FIGS. 3A and 3B). For simplicity, only memory cell 150 with continuous buried region 22 in all directions will be shown from hereon. Alternatively, the bottom of insulating layer 26 may reside above the buried layer region 22.
Cell 150 includes several terminals: word line (WL) terminal 70 electrically connected to gate 60, bit line (BL) terminal 74 electrically connected to bit line region 18, source line (SL) terminal 72 electrically connected to source line region 16, buried well (BW) terminal 76 electrically connected to buried layer 22, and substrate terminal 78 electrically connected to the substrate 12. The states of the memory cell 150 are determined from the charge stored in the floating body region 24. If the floating body region 24 is positively charged, the conductivity of the memory cell 150 (for example the current flow from the BL terminal 74 to the SL terminal 72) will be higher than if the floating body region is neutral. The high conductivity state will be referred to as the logic-1 state, while the low conductivity state will be referred to as the logic-0 state. The same operations performed on memory cell 50 can be performed on memory cell 150 such as holding, read, write logic-1 and write logic-0 operations. The drain voltage applied during write logic-1 state using impact ionization mechanism can be reduced on junctionless memory cell transistor 150, because of the higher impact ionization generation on junctionless transistor, for example as described in “Low subthreshold slope in junctionless multi gate transistors”, C.-W. Lee et al., pp. 102106, Applied Physics Letter 96, 2010 (“Lee”), which is hereby incorporated herein, in its entirety, by reference thereto. In one particular non-limiting embodiment, about +1.0 volts is applied to the selected BL terminal 74a, about 0.0 volts is applied to the selected SL terminal 72a, about 0.0 volts is applied to the selected WL terminal 70a, about +1.2 volts is applied to BW terminal 76a, and about 0.0 volts is applied to terminal 78a. The unselected BL terminals 74 are at 0.0 volts, the unselected SL terminals 72 are at 0.0 volts, the unselected WL terminals 70 are at 0.0 volts, about +1.2 volts is applied to the unselected BW terminals 76, and about 0.0 volts is applied to the unselected substrate terminals 78. However, these voltage levels may vary while maintaining the relative relationships between voltage levels as generally described above.
FIG. 10 illustrates memory cell 250 according to another embodiment of the present invention. Memory cell 250 also comprises substrate 12 of a first conductivity type such as p-type, for example, and a buried layer region 22 of a second conductivity type, such as n-type, for example. Memory cell 250 also comprises an intrinsic-type floating body region 24, where no significant amount of dopant species is present. The floating body region 24 is bounded on top by the surface 14, the insulating layer 62, and the source region 16, and drain region 18. The source region 16 has a first conductivity type such as p-type, for example, while the drain region 18 has a second conductivity type such as n-type, for example. As a result, near the surface region (along the I-I′ cut line shown in FIG. 10, a p-i-n diode is formed from the source region 16, the floating body region 24, and the drain region 18).
A gate 60 is positioned in between the source line region 16 and the drain region 18, but does not fully overlay the floating body region 24, where the gate region 60 is spaced from the source region 16, forming the gap region 25, for example as described in U.S. patent application Ser. No. 13/244,899 to Y. Widjaja, titled “Asymmetric Semiconductor Memory Device Having Electrically Floating Body Transistor” (“Widjaja-3”), which is hereby incorporated herein, in its entirety, by reference thereto.
Insulating layers 26 (like, for example, shallow trench isolation (STI)), may be made of silicon oxide, for example, though other insulating materials may be used. The bottom of insulating layer 26 may reside inside the buried region 22 allowing buried region 22 to be continuous as shown in FIG. 10. Alternatively, the bottom of insulating layer 26 may reside below the buried region 22 as in FIGS. 3A and 3B (shown better in FIG. 3A). This requires a shallower insulating layer 28, which insulates the floating body region 24, but allows the buried layer 22 to be continuous in the perpendicular direction of the cross-sectional view shown in FIG. 3A. For simplicity, only memory cell 250 with continuous buried region 22 in all directions will be shown from hereon. Alternatively, the bottom of insulating layer 26 may reside above the buried layer region 22.
Cell 250 includes several terminals: word line (WL) terminal 70 electrically connected to gate 60, bit line (BL) terminal 74 electrically connected to bit line region 18, source line (SL) terminal 72 electrically connected to source line region 16, buried well (BW) terminal 76 electrically connected to buried layer 22, and substrate terminal 78 electrically connected to the substrate 12.
Memory cell 250 conduction utilizes channel length modulation through the gate 60, as described for example in “Impact Ionization MOS (I-MOS)—Part I: Device and Circuit Simulations”, K. Gopalakrishnan, et al., pp. 69-76, IEEE Transactions on Electron Devices, vol. 52, no. 1, January 2005 (“Gopalakrishnan”), which is hereby incorporated herein, in its entirety, by reference thereto. At low gate bias, no inversion is formed in the channel area underneath the gate 60. As the potential of the gate 60 is increased, an inversion layer forms underneath the gate 60. A strong electric field is developed across the gap region 25, sufficient to generate impact ionization.
The states of the memory cell 250 are determined from the charge stored in the floating body region 24. If the floating body region 24 is positively charged, the gate voltage where an inversion layer is formed under the gate 60 (often referred to as the threshold voltage), is lower than that if the floating body region 24 is neutral. The memory state having a lower threshold voltage will be referred to as the logic-1 state, while the memory state having a higher threshold voltage will be referred to as the logic-0 state.
FIG. 11 schematically illustrates an exemplary embodiment of a memory array 280 of memory cells 250 (four exemplary instances of memory cell 250 being labeled as 250a, 250b, 250c, and 250d) arranged in rows and columns.
In many, but not necessarily all, of the figures where array 280 appears, representative memory cell 250a will be representative of a “selected” memory cell 250 when the operation being described has one (or more in some embodiments) selected memory cells 250. In such figures, representative memory cell 250b will be representative of an unselected memory cell 250 sharing the same row as selected representative memory cell 250a, representative memory cell 250c will be representative of an unselected memory cell 250 sharing the same column as selected representative memory cell 250a, and representative memory cell 250d will be representative of a memory cell 250 sharing neither a row nor a column with selected representative memory cell 250a. 
Present in FIG. 11 are word lines 70a through 70n, source lines 72a through 72n, bit lines 74a through 74p, buried well terminals 76a through 76n, and substrate terminal 78. Representation of the lines/terminal with letters a-n or a through p, includes not only embodiments which include literally twelve lines/terminals (i.e., a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p) or fourteen lines/terminals (i.e., a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p), but is meant to more generically represent a plurality of such line terminals, which can be less than twelve (i.e., as low as one or greater than twelve, thirteen or fourteen (much greater than fourteen up to any positive integer practical).
Each of the source lines 72a through 72n is associated with a single row of memory cells 250 and is coupled to the source line region 16 of each memory cell 250 in that row. Each of the bit lines 74a through 74p is associated with a single column of memory cells 250 and is coupled to the bit line region 18 of each memory cell 250 in that column
Several operations can be performed on memory cell 250 such as holding, read, write logic-1 and write logic-0 operations.
FIG. 12 schematically illustrates performance of a holding operation on memory array 280, while FIG. 13 illustrates the bias applied on the terminals of a memory cell 250 during the holding operation. The holding operation is performed by applying a positive back bias to the BW terminal 76, a slight positive bias to the SL terminal 72, zero bias on the WL terminal 70, BL terminal 74, and substrate terminal 78. The positive back bias applied to the buried layer region 22 connected to the BW terminal 76 will maintain the state of the memory cell 250. The positive bias applied to the BW terminal 76 needs to generate an electric field sufficient to trigger an impact ionization mechanism when the floating body region 24 is positively charged, as will be described through the band diagram shown in FIGS. 14A and 14B. The impact ionization rate as a function of the electric field is for example described in “Physics of Semiconductor Devices”, Sze S. M. and Ng K. K., which is hereby incorporated herein, in its entirety, by reference thereto.
In one embodiment the bias conditions for the holding operation on memory cell 250 are: about 0.0 volts is applied to WL terminal 70, about 0.0 volts is applied to BL terminal 74, about +0.4 volts is applied to SL terminal 72, a positive voltage, for example, about +1.0 volts is applied to BW terminal 76, and about 0.0 volts is applied to the substrate terminal 78. In other embodiments, different voltages may be applied to the various terminals of memory cell 250 and the exemplary voltages described are not limiting.
FIGS. 14A and 14B show band diagrams along the II-II′ cut line from FIG. 10 illustrating an n-type drain region 18, an intrinsic floating body region 24, and an n-type buried well region 22, when the floating body is positively charged and when the floating body is neutral, respectively. The horizontal dashed lines indicate the Fermi levels in the various regions. The Fermi level is located in the band gap between the solid line 27 indicating the top of the valence band (the bottom of the band gap) and the solid line 29 indicating the bottom of the conduction band (the top of the band gap) as is well known in the art. If floating body 24 is positively charged, electrons will flow from the drain region 18 to the buried well region 22 due the barrier lowering of the floating body region 24. As a result of the positive bias applied to the buried well region 22 (connected to the BW terminal 76), the electrons are accelerated and create additional hot carriers (hot hole and hot electron pairs) through an impact ionization mechanism. The resulting hot electrons flow into the BW terminal 76 while the resulting hot holes will subsequently flow into the floating body region 24. When the following condition is met: β×(M−1)≈1—where β is the forward common-emitter current gain and M is the impact ionization coefficient—the amount of holes injected into the floating body region 24 compensates for the charge lost due to forward bias current between the floating body region 24 and the source line region 16 or bit line region 18 and due to holes recombination. This process maintains the charge (i.e. holes) stored in the floating body region 24 which will keep the electron flow from the drain region 18 to the buried well region 22 for as long as a positive bias is applied to the buried well region 22 through BW terminal 76.
The region where the product β×(M−1) approaches 1 and is characterized by hole current moving into the floating body region is similar to the reverse base current region that has been described for example in “A New Static Memory Cell Based on Reverse Base Current (RBC) Effect of Bipolar Transistor”, K. Sakui et al., pp. 44-47, International Electron Devices Meeting, 1988 (“Sakui-1”), “A New Static Memory Cell Based on the Reverse Base Current Effect of Bipolar Transistors”, K. Sakui et al., pp. 1215-1217, IEEE Transactions on Electron Devices, vol. 36, no. 6, June 1989 (“Sakui-2”), “On Bistable Behavior and Open-Base Breakdown of Bipolar Transistors in the Avalanche Regime—Modeling and Applications”, M. Reisch, pp. 1398-1409, IEEE Transactions on Electron Devices, vol. 39, no. 6, June 1992 (“Reisch”), which are hereby incorporated herein, in their entireties, by reference thereto.
The latching behavior based on the reverse base current region has also been described in a biristor (i.e. bi-stable resistor) for example in “Bistable resistor (Biristor)—Gateless Silicon Nanowire Memory”, J.-W. Han and Y.-K. Choi, pp. 171-172, 2010 Symposium on VLSI Technology, Digest of Technical Papers, 2010 “(“J.-W. Han”), which is hereby incorporated herein, in its entirety, by reference thereto. In a two-terminal biristor device, a refresh operation is still required. J.-W. Han describes a 200 ms data retention for the silicon nanowire biristor memory. As will be described, the holding operation does not require any interruptions to the memory cell 250 access. As a result, the holding operation can be performed for as long as a positive bias is applied to the buried well region 22 through BW terminal 76.
If floating body 24 is neutrally charged, a state corresponding to logic-0, no (or low) electron flow will occur from the drain region 18 to the buried well region 22. As a result, no impact ionization occurs and memory cells in the logic-0 state will remain in the logic-0 state. FIG. 14B shows the corresponding energy band diagram when the floating body 24 is neutral and when zero voltage is applied to the drain region 18 and a positive bias is applied to the buried well region 22. In this state, an energy barrier, indicated by solid line 23, exists between the drain region 18 and the buried well region 22.
In the holding operation described with regards to FIGS. 12 and 13, there is no individually selected memory cell. Rather the holding operation will be performed at all cells connected to the same buried well terminal 76. In addition, the holding operation does not interrupt read or write access to the memory cell 250.
FIG. 15 shows a graph of the net current I flowing into or out of the floating body region 24 as a function of the potential VFB of the floating body 24, where a slight positive voltage is applied to the source region 16, zero voltage is applied to the drain region 18, and a positive voltage is applied to the buried well region 22. A negative current indicates a net current flowing into the floating body region 24, while a positive current indicates a net current flowing out of the floating body region 24. At low floating body 24 potential, between 0V and VFB0 indicated in FIG. 15, the net current is flowing into the floating body region 24 as a result of the junction between the floating body region 24 and the buried well region 22 being reverse biased. If the value of the floating body 24 potential is between VFB0 and VTS, the current will switch direction, resulting in a net current flowing out of the floating body region 24. This is because of the junction formed by the floating body region 24 and the drain region 18 is forward biased as the floating body region 24 becomes increasingly more positive. As a result, if the potential of the floating body region 24 is less than VTS, then at steady state the floating body region 24 will reach VFB0. If the potential of the floating body region 24 is higher than VTS, the current will switch direction, resulting in a net current flowing into the floating body region 24. This is as a result of the base current flowing into the floating body region 24 being greater than the junction leakage current. When the floating body 24 potential is higher than VFB1, the net current will be out of the floating body region 24. This is because the junction leakage current is once again greater than the base current flowing into the floating body region 24.
The holding operation results in the floating body memory cell having two stable states: the logic-0 state and the logic-1 state separated by an energy barrier, which are represented by VFB0, VFB1, and VTS, respectively. FIG. 16A shows a schematic curve of a potential energy surface (PES) of the memory cell 250, which shows another representation of the two stable states resulting from applying a back bias to the BW terminal 76 (connected to the buried well region 22).
The values of the floating body 24 potential where the current changes direction, i.e. VFB0, VFB1, and VTS, can be modulated by the potential applied to the BW terminal 76. These values are also temperature dependent.
The holding/standby operation also results in a larger memory window by increasing the amount of charge that can be stored in the floating body 24. Without the holding/standby operation, the maximum potential that can be stored in the floating body 24 is limited to the flat band voltage VFB as the junction leakage current to regions 16 and 18 increases exponentially at floating body potential greater than VFB. However, by applying a positive voltage to substrate terminal 78, the bipolar action results in a hole current flowing into the floating body 24, compensating for the junction leakage current between floating body 24 and regions 16 and 18. As a result, the maximum charge VMC stored in floating body 24 can be increased by applying a positive bias to the substrate terminal 78 as shown in FIG. 16B. The increase in the maximum charge stored in the floating body 24 results in a larger memory window.
Floating body DRAM cells described in Ranica-1, Ranica-2, Villaret, and Pulicani only exhibit one stable state, which is often assigned as logic-0 state. Villaret describes the intrinsic bipolar transistors enhance the data retention of logic-1 state, by drawing the electrons which otherwise would recombine with the holes stored in the floating body region. However, only one stable state is observed because there is no hole injection into the floating body region to compensate for the charge leakage and recombination.
FIGS. 17 and 18 illustrate the read operation performed on the memory array 280 and the bias conditions applied to the selected memory cell 250a, respectively. The read operation is performed by applying the following bias conditions: a positive voltage is applied to the selected WL terminal 70a, a positive voltage is applied to the selected BL terminal 74a, a slight positive voltage is applied to the selected SL terminal 72a, a positive voltage is applied to the selected BW terminal 76a, and zero voltage is applied to the substrate terminal 78. The following bias is applied to the unselected terminals: zero voltage is applied to the unselected WL terminals 70, zero voltage is applied to the unselected BL terminals 74, a slight positive voltage is applied to the unselected SL terminals 72, zero or positive voltage is applied to the unselected BW terminals 76, and zero voltage is applied to the unselected substrate terminals 78.
The charge stored in the floating body region 24 will modulate the threshold gate voltage to form an inversion layer in the channel region underneath the gate. The gate voltage during the read operation is selected such that inversion layer is formed when the memory cell 250 is in logic-1 state (i.e. when the floating body region 24 is positively charged) and no inversion layer is formed when the memory cell 250 is in logic-0 state (i.e. when the floating body region 24 is neutral). As Gopalakrishnan describes, memory cell 250 has a steep subthreshold slope, resulting in low leakage current from the unselected cells. As a result, the sensing operation of the state of the memory cell 250 may be performed faster.
In one particular non-limiting embodiment, about +0.4 volts is applied to the selected WL terminal 70a, about +1.0 volts is applied to the selected BL terminal 74a, about +0.4 volts is applied to the selected SL terminal 72a, about +1.0 volts is applied to the selected BW terminal 76a, and about 0.0 volts is applied to the selected substrate terminal 78a. However, these voltage levels may vary while maintaining the relative relationships between voltage levels as generally described above.
FIGS. 19 and 20 illustrate a row write logic-0 operation performed on the memory array 280 and the bias conditions applied to the terminals of the selected memory cell 250a, respectively. The following bias conditions are applied to the selected terminals to perform a row write logic-0 operation: zero voltage is applied to the selected WL terminal 70a, zero voltage is applied to the selected SL terminal 72a, zero voltage is applied to the selected BL terminal 74a, zero voltage or a positive voltage is applied to the selected BW terminal 76a, and zero voltage is applied to the selected substrate terminal 78a. The following bias conditions are applied to the unselected terminals: zero voltage is applied to the unselected WL terminals 70, a slight positive voltage is applied to the unselected SL terminals 72, zero voltage is applied to the unselected BL terminals 74, zero or positive voltage is applied to the unselected BW terminals, and zero voltage is applied to the unselected substrate terminals 78.
Removing the slight positive voltage that is applied to the selected SL terminal 72 removes the potential well that allows for positive charge storage in the floating body region 24. As a result, holes will be evacuated from the floating body region 24 and will flow to the source region 16.
In one particular non-limiting embodiment, about 0.0 volts is applied to the selected WL terminal 70a, about 0.0 volts is applied to the selected BL terminal 74a, about 0.0 volts is applied to the selected SL terminal 72a, about 0.0 volts is applied to the selected BW terminal 76a, and about 0.0 volts is applied to the selected substrate terminal 78a. However, these voltage levels may vary while maintaining the relative relationships between voltage levels as generally described above.
FIGS. 21 and 22 illustrate a write logic-1 operation performed on the memory array 280 and the bias conditions applied to the terminals of the selected memory cell 250a, respectively. The following bias conditions are applied to the selected terminals to perform a write logic-1 operation: a positive voltage is applied to the selected WL terminal 70a, a slight positive voltage is applied to the SL terminal 72a, a positive voltage is applied to the selected BL terminal 74a, a positive voltage is applied to the selected BW terminal 76a, and zero voltage is applied to the substrate terminal 78a. The following bias conditions are applied to the unselected terminals: zero voltage is applied to the unselected WL terminals 70, a slight positive voltage is applied to the unselected SL terminals 72, zero voltage is applied to the unselected BL terminals 74, zero or positive voltage is applied to the unselected BW terminals, and zero voltage is applied to the unselected substrate terminals 78.
The positive voltage applied to the gate 60 of the memory cell 250 has to be sufficiently high to create an inversion layer underneath the gate 60, regardless of the state of the memory cell 250. This will result in an inversion layer at the same potential level as the voltage that is applied to the bit line region 18. Hence, a strong electric field will be developed across the gap region 25, which results in hole injection into the floating body region 24 as a result of impact ionization.
In one particular non-limiting embodiment, about +1.0 volts is applied to the selected WL terminal 70a, about +1.8 volts is applied to the selected BL terminal 74a, about +0.4 volts is applied to the selected SL terminal 72a, about +1.0 volts is applied to the selected BW terminal 76a, and about 0.0 volts is applied to the selected substrate terminal 78a. However, these voltage levels may vary while maintaining the relative relationships between voltage levels as generally described above.
FIGS. 23 and 24 illustrate a write logic-1 operation using a band-to-band tunneling mechanism performed on the memory array 280 and the bias conditions applied to the terminals of the selected memory cell 250a, respectively. The following bias conditions are applied to the selected terminals to perform a write logic-1 operation: a negative voltage is applied to the selected WL terminal 70a, a slight positive voltage is applied to the SL terminal 72a, a positive voltage is applied to the selected BL terminal 74a, a positive voltage is applied to the selected BW terminal 76a, and zero voltage is applied to the substrate terminal 78a. The following bias conditions are applied to the unselected terminals: zero voltage is applied to the unselected WL terminals 70, a slight positive voltage is applied to the unselected SL terminals 72, zero voltage is applied to the unselected BL terminals 74, zero or positive voltage is applied to the unselected BW terminals, and zero voltage is applied to the unselected substrate terminals 78.
The negative-voltage on the gate 60 (connected to WL terminal 70) and the positive voltage on bit line region 18 (connected to BL terminal 74) create a strong electric field (for example, about 106 V/cm in silicon, as described in Sze, p. 104) between the bit line region 18 and the floating body region 24 in the proximity of gate 60. This bends the energy band sharply upward near the gate and bit line junction overlap region, causing electrons to tunnel from the valence band to the conduction band, leaving holes in the valence band. The electrons which tunnel across the energy band become the drain leakage current, while the holes are injected into floating body region 24 and become the hole charge that creates the logic-1 state.
In one particular non-limiting embodiment, about −1.0 volts is applied to the selected WL terminal 70a, about +1.0 volts is applied to the selected BL terminal 74a, about +0.4 volts is applied to the selected SL terminal 72a, about +1.0 volts is applied to the selected BW terminal 76a, and about 0.0 volts is applied to the selected substrate terminal 78a. However, these voltage levels may vary while maintaining the relative relationships between voltage levels as generally described above.
FIG. 25 illustrates a method to further lower the voltage applied to the bit line region 18 during the write logic-1 operation, which can be applied to memory cells 50, 150, and 250, and to the bi-stable memory cells described for example in Widjaja-1 and Widjaja-2. FIG. 25 illustrates the voltage applied to the BL terminal 74 and the BW terminal 76. To further lower the voltage applied to the BL terminal 74, the voltage applied to the BW terminal 76 may be increased during the write logic-1 operation. Increasing the BW terminal 76 lowers the barrier for the electron flow during the write logic-1 operation, and thus increasing the holes generated under the impact ionization mechanism.
A schematic cross-sectional view of a memory device 350 according to another embodiment of the present invention is shown in FIG. 26. Memory cell 350 includes a substrate 12 of a first conductivity type such as p-type, for example. Substrate 12 is typically made of silicon, but may alternatively or additionally comprise, for example, germanium, silicon germanium, gallium arsenide, carbon nanotubes, and/or other semiconductor materials. In some embodiments of the invention, substrate 12 can be the bulk material of the semiconductor wafer. In other embodiments, substrate 12 can be a well of the first conductivity type embedded in either a well of the second conductivity type or, alternatively, in the bulk of the semiconductor wafer of the second conductivity type, such as n-type, for example (not shown in the figures). To simplify the description, the substrate 12 will usually be drawn as the semiconductor bulk material as it is in FIG. 26.
Memory cell 350 also comprises a buried layer region 22 of a second conductivity type, such as n-type, for example; a floating body region 24 of the first conductivity type, such as p-type, for example; and source/drain regions 16 and 18 of the second conductivity type, such as n-type, for example.
Buried layer 22 may be formed by an ion implantation process on the material of substrate 12. Alternatively, buried layer 22 can also be grown epitaxially on top of substrate 12.
The floating body region 24 of the first conductivity type is bounded on top by surface 14, source line region 16, drain region 18, and insulating layer 62. Floating body 24 may be the portion of the original substrate 12 above buried layer 22 if buried layer 22 is implanted. Alternatively, floating body 24 may be epitaxially grown. Depending on how buried layer 22 and floating body 24 are formed, floating body 24 may have the same doping as substrate 12 in some embodiments or a different doping, if desired in other embodiments.
A source line region 16 having a second conductivity type, such as n-type, for example, is provided in floating body region 24, so as to bound a portion of the top of the floating body region in a manner discussed above, and is exposed at surface 14. Source line region 16 may be formed by an implantation process on the material making up substrate 12, according to any implantation process known and typically used in the art. Alternatively, a solid state diffusion or a selective epitaxial growth process could be used to form source line region 16.
A bit line region 18 having a second conductivity type, such as n-type, for example, is also provided in floating body region 24, so as to bound a portion of the top of the floating body region in a manner discussed above, and is exposed at cell surface 14. Bit line region 18 may be formed by an implantation process on the material making up substrate 12, according to any implantation process known and typically used in the art. Alternatively, a solid state diffusion or a selective epitaxial growth process could be used to form bit line region 18.
A gate 60 is positioned in between the source line region 16 and the drain region 18, above the floating body region 24. The gate 60 is insulated from the floating body region 24 by an insulating layer 62. Insulating layer 62 may be made of silicon oxide and/or other dielectric materials, including high-K dielectric materials, such as, but not limited to, tantalum peroxide, titanium oxide, zirconium oxide, hafnium oxide, and/or aluminum oxide. The gate 60 may be made of, for example, polysilicon material or metal gate electrode, such as tungsten, tantalum, titanium and/or their nitrides.
Insulating layers 26 (like, for example, shallow trench isolation (STI)), may be made of silicon oxide, for example, though other insulating materials may be used. The bottom of insulating layer 26 resides above the buried layer region 22. As can be seen in FIG. 26, there is no physical separation between adjacent memory cells 350 since the floating body region 24 of one memory cell will be connected to adjacent memory cells 350 when a plurality of memory cells 350 are connected to form a memory array 380. The isolation of floating body region 24 of adjacent memory cells 350 is accomplished electrically, through an application of a potential to the buried layer region 22, thus isolating floating body regions 24 of adjacent memory cells 350.
Cell 350 includes several terminals: word line (WL) terminal 70 electrically connected to gate 60, bit line (BL) terminal 74 electrically connected to bit line region 18, source line (SL) terminal 72 electrically connected to source line region 16, buried well (BW) terminal 76 electrically connected to buried layer 22, and substrate terminal 78 electrically connected to the substrate 12.
FIG. 27 illustrates a cross-sectional view of a plurality of memory cells 350 connected to form a memory array 380. FIG. 27 also illustrates a contact made to the buried layer region 22, which is made through regions 20 having second conductivity type. FIG. 27 illustrates a memory array 380 having region 20 on both sides of the plurality of the memory cells 350. FIG. 28 illustrates a memory array 380 according to another embodiment of the present invention, where the region 20 (which connects BW terminal 76 to the buried layer region 22), is only provided on one side of the plurality of the memory cells 350. As a result, the floating body region 24 is connected to the substrate region 12.
FIG. 29 illustrates a depletion region 23 (enclosed in dashed lines) formed in the junction of floating body region 24 and the buried layer 22 as a result of an application of a back bias to the BW terminal 76. If positive voltage is applied to the buried layer 22 while the source line region 16 or the bit line region 18 is grounded or negatively biased, depletion region 23 extends into the floating body region 24 because of reverse biased p-n junction, and the top portion of the depletion region 23 resides above the bottom of the insulating layer 26. Therefore, the floating body region 24 of one memory cell 350 is now isolated from that of adjacent memory cells 350 and forms a potential well for excess charge storage. A depletion region extends further into the region with a lower doping concentration. FIG. 29 illustrates an example where the depletion region 23 extends deeper into the floating body region 24, assuming that the doping concentration of the floating body region 24 is lighter than the doping concentration of the buried well region 22. However, this is only provided as an illustration and is not limiting.
In one embodiment, the bias conditions applied to the memory cell 350 (e.g., to establish the depletion region, as well as to establish a holding condition as described in the Widjaja references incorporated herein) are: about 0V is applied to the WL terminal 70, SL terminal 72, BL terminal 74, substrate terminal 78, and about +3.0V is applied to the BW terminal 76. These example bias conditions are provided for an n-type memory cell 350 (where the source region 16 and drain region 18 are of n-type conductivity, the floating body region 24 is of p-type conductivity, the buried layer region 22 is of n-type conductivity, and the substrate region 12 is of p-type conductivity). In other embodiments, different voltages may be applied to the various terminals of memory cell 350 and the exemplary voltages described are not limiting.
The operation of the memory cell 350 as a capacitorless DRAM has been described in Okhonin, Ohsawa, Villaret, Ranica, and Pulicani, while a holding operation to form a bi-stable memory cell, analogous to an SRAM cell, has been described in Widjaja-1 and Widjaja-2. A write logic-0 operation according to another embodiment of the present invention may be performed by removing the potential applied to the buried layer region 22. The removal of the back bias to the buried layer region 22 will in turn result in the removal of the depletion region 23, causing any charges stored in the floating region 24 to be removed, either through charge sharing to adjacent memory cells 350, or to the substrate 12. This write logic-0 operation is performed in parallel to all the cells connected to the same BW terminal 76. A parallel write logic-0 operation may also be used to accelerate the testing of the memory device. In order to write arbitrary binary data to different memory cells 350, a write logic-0 operation is first performed on all the memory cells to be written, followed by one or more write logic-1 operations on the memory cells that must be written to logic-1.
FIG. 30 illustrates memory cell 450 according to another embodiment of the present invention. Memory cell 450 consists of a memory device 450M and access device 450A, that are connected in series, as described for example in PCT/US13/26466 to Widjaja et al., titled “Memory Cell Comprising First and Second Transistors and Methods of Operating”, which is hereby incorporated herein, in its entirety, by reference thereto.
Memory cell 450 comprises a memory device 450M having a buried layer region 22, located below the insulating region 26, similar to the memory cell 350 illustrated in FIG. 26. Access transistor 450A consists of a well region 24 (which is physically connected to the floating body region 24 of the memory device 450M, but will be electrically separated through an application of a potential to the buried well region 22), source region 19 and drain region 20, and gate region 64 which is isolated from the well region 24 by an insulating layer 66. Insulating layer 66 may be made of silicon oxide and/or other dielectric materials, including high-K dielectric materials, such as, but not limited to, tantalum peroxide, titanium oxide, zirconium oxide, hafnium oxide, and/or aluminum oxide. The gate 64 may be made of, for example, polysilicon material or metal gate electrode, such as tungsten, tantalum, titanium and their nitrides.
The drain region 18 of the memory device 450M is connected to the source region 19 of the access device 450A through a conductive element 94. The conductive element 94 may be formed of, but not limited to, tungsten or silicided silicon.
In addition to the SL terminal 72 and BL terminal 74, memory cell 450 also includes word line 1 (WL1) terminal 70, which is electrically connected to the gate 60 of the memory device 450M, word line 2 (WL2) terminal 71, which is electrically connected to the gate 64 of the access transistor 450A, buried well (BW) terminal 76, which is electrically connected to the buried well region 22 of the memory device 450M, and substrate (SUB) terminal 78, which is connected to the substrate region 12.
As illustrated in FIG. 31, application of a potential to the buried well region 22 of the memory device 450M will electrically isolate the floating body region 24 from the substrate region 12 as well as from adjacent floating body region 24 of adjacent memory cells 450. In one embodiment, the bias conditions applied to the memory cell 450 (e.g., to electrically isolate the floating body region as well as to establish the holding condition, as described in the Widjaja references incorporated herein) are: about 0V is applied to the WL1 terminal 70, WL2 terminal 71, SL terminal 72, BL terminal 74, substrate terminal 78, and about +3.0V is applied to the BW terminal 76. These example bias conditions are provided for an n-type memory device 450M (where the source region 16 and drain region 18 are of n-type conductivity, the floating body region 24 is of p-type conductivity, the buried layer region 22 is of n-type conductivity, and the substrate region 12 is of p-type conductivity). In other embodiments, different voltages may be applied to the various terminals of memory cell 450 and the exemplary voltages described are not limiting.
A write logic-0 operation according to another embodiment of the present invention may be performed by removing the potential applied to the buried layer region 22. The removal of the back bias to the buried layer region 22 will in turn result in the removal of the depletion region 23, causing any charges stored in the floating region 24 to be removed, either through charge sharing to adjacent memory cells 450, or to the substrate 12. This write logic-0 operation is performed in parallel to all the cells connected to the same BW terminal 76. A parallel write logic-0 operation may also be used to accelerate the testing of the memory device. In order to write arbitrary binary data to different memory cells 450, a write logic-0 operation is first performed on all the memory cells to be written, followed by one or more write logic-1 operations on the memory cells that must be written to logic-1.
FIG. 32 illustrates the bias conditions for the write logic-0 operation performed on a memory array 480 comprising a plurality of memory cells 450 arranged in rows and columns.

## Claims

1. A semiconductor memory cell comprising:
a floating body region configured to be charged to a level indicative of a state of the memory cell selected from at least first and second states;
a first region in electrical contact with said floating body region;
a second region in electrical contact with said floating body region and spaced apart from said first region;
a gate positioned between said first and second regions;
a first insulating region located above said floating body region;
second insulating regions adjacent to said floating body region;
a buried layer region located below said floating body region and said second insulating regions and spaced from said second insulating regions so as not to contact said second insulating regions, wherein:
said floating body region is bounded by said first insulating region above said floating body region, said second insulating regions adjacent to said floating body region, and a depletion region formed as a result of an application of a back bias to said buried layer region.

2. The semiconductor memory cell of claim 1, further comprising a substrate region, wherein said buried layer region is positioned between said substrate region and said floating body region.

3. The semiconductor memory cell of claim 1, wherein said first region has a first conductivity type selected from a p-type conductivity type and an n-type conductivity type;
said second region has a first conductivity type selected from a p-type conductivity type and an n-type conductivity type;
said floating body region has a second conductivity type selected from a p-type conductivity type and an n-type conductivity type and different from said first conductivity type; and
said buried layer region has said first conductivity type.

4. The semiconductor memory cell of claim 1, wherein said back bias is applied as a constant positive voltage bias.

5. The semiconductor memory cell of claim 1, wherein said buried layer region is configured to inject charge into or extract charge out of said floating body region to maintain said state of the memory cell.

6. The semiconductor memory cell of claim 1, wherein said floating body region, said first region, said second region, said gate, said first insulating region, said second insulating regions and said buried layer form a memory device; and
wherein said semiconductor memory cell further comprises an access transistor connected in series with said memory device.

7. A semiconductor memory cell comprising:
a floating body region configured to be charged to a level indicative of a state of the memory cell selected from at least first and second states;
a first region in electrical contact with said floating body region;
a second region in electrical contact with said floating body region and spaced apart from said first region;
a gate positioned between said first and second regions;
a first insulating region located above said floating body region;
second insulating regions adjacent to said floating body region;
a buried layer region located below said floating body region and said second insulating regions and spaced from said second insulating regions so as not to contact said second insulating regions,
wherein said floating body region is bounded by said first insulating region above said floating body region, said second insulating regions adjacent to said floating body region, and a depletion region formed as a result of an application of a back bias to said buried layer region, and
wherein application of said back bias results in at least two stable floating body charge levels.

8. The semiconductor memory cell of claim 7, further comprising a substrate region, wherein said buried layer region is positioned between said substrate region and said floating body region.

9. The semiconductor memory cell of claim 7, wherein said first region has a first conductivity type selected from a p-type conductivity type and an n-type conductivity type;
said second region has said first conductivity type;
said floating body region has a second conductivity type selected from a p-type conductivity type and an n-type conductivity type and different from said first conductivity type; and
said buried layer region has said first conductivity type.

10. The semiconductor memory cell of claim 7, wherein said back bias is applied as a constant positive voltage bias.

11. The semiconductor memory cell of claim 7, wherein said buried layer region is configured to inject charge into or extract charge out of said floating body region to maintain said state of the memory cell.

12. The semiconductor memory cell of claim 7, wherein said floating body region, said first region, said second region, said gate, said first insulating region, said second insulating regions and said buried layer form a memory device; and
wherein said semiconductor memory cell further comprises an access transistor connected in series with said memory device.