Patent Grant
10141046
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.
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.
A DRAM cell without a capacitor has been investigated previously. 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. Chatterjee et al. have proposed a Taper Isolated DRAM cell concept in “Taper Isolated Dynamic Gain RAM Cell”, P. K. Chatterjee et al., pp. 698-699, International Electron Devices Meeting, 1978 (“Chatterjee-1”), “Circuit Optimization of the Taper Isolated Dynamic Gain RAM Cell for VLSI Memories”, P. K. Chatterjee et al., pp. 22-23, IEEE International Solid-State Circuits Conference, February 1979 (“Chatterjee-2”), and “dRAM Design Using the Taper-Isolated Dynamic RAM Cell”, J. E. Leiss et al., pp. 337-344, IEEE Journal of Solid-State Circuits, vol. SC-17, no. 2, April 1982 (“Leiss”), all of which are hereby incorporated herein, in their entireties, by reference thereto. The holes are stored in a local potential minimum, which looks like a bowling alley, where a potential barrier for stored holes is provided. The channel region of the Taper Isolated DRAM cell contains a deep n-type implant and a shallow p-type implant. As shown in “A Survey of High-Density Dynamic RAM Cell Concepts”, P. K. Chatterjee et al., pp. 827-839, IEEE Transactions on Electron Devices, vol. ED-26, no. 6, June 1979 (“Chatterjee-3”), which is hereby incorporated herein, in its entirety, by reference thereto, the deep n-type implant isolates the shallow p-type implant and connects the n-type source and drain regions.
Terada et al. have proposed a Capacitance Coupling (CC) cell in “A New VLSI Memory Cell Using Capacitance Coupling (CC) Cell”, K. Terada et al., pp. 1319-1324, IEEE Transactions on Electron Devices, vol. ED-31, no. 9, September 1984 (“Terada”), while Erb has proposed Stratified Charge Memory in “Stratified Charge Memory”, D. M. Erb, pp. 24-25, IEEE International Solid-State Circuits Conference, February 1978 (“Erb”), both of which are hereby incorporated herein, in their entireties, by reference thereto.
DRAM based on the electrically floating body effect has been proposed both in silicon-on-insulator (SOI) substrate (see for example “The Multistable Charge-Controlled Memory Effect in SOI Transistors at Low Temperatures”, Tack et al., pp. 1373-1382, IEEE Transactions on Electron Devices, vol. 37, May 1990 (“Tack”), “A Capacitor-less 1T-DRAM Cell”, S. Okhonin et al., pp. 85-87, IEEE Electron Device Letters, vol. 23, no. 2, February 2002 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, all of which are hereby incorporated herein, in their entireties, by reference thereto) and in bulk silicon (see for example “A one transistor cell on bulk substrate (1T-Bulk) for low-cost and high density eDRAM”, R. Ranica et al., pp. 128-129, Digest of Technical Papers, 2004 Symposium on VLSI Technology, June 2004 (“Ranica-1”), “Scaled 1T-Bulk Devices Built with CMOS 90 nm Technology for Low-Cost eDRAM Applications”, R. Ranica et al., 2005 Symposium on VLSI Technology, Digest of Technical Papers (“Ranica-2”), “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”), “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 (ICECS) (“Pulicani”), which are hereby incorporated herein, in their entireties, by reference thereto).
Widjaja and Or-Bach describe a bi-stable SRAM cell incorporating a floating body transistor, where more than one stable state exists for each memory cell (for example as described in U.S. Pat. No. 8,130,548 to Widjaja et al., titled “Semiconductor Memory Having Floating Body Transistor and Method of Operating” (“Widjaja-1”), U.S. Pat. No. 8,077,536, “Method of Operating Semiconductor Memory Device with Floating Body Transistor Using Silicon Controlled Rectifier Principle” (“Widjaja-2”), U.S. Patent Application Publication No. 2013/0264656 A113/746,523, “Memory Device Having Electrically Floating Body Transistor” (“Widjaja-3”), all of which are hereby incorporated herein, in their entireties, by reference thereto). This is bi-stability is achieved due to the applied back bias which causes impact ionization and generates holes to compensate for the charge leakage current and recombination.
According to an aspect of the present invention, a bi-stable SRAM cell is provided that incorporates a floating body transistor in SOI substrate.
In another aspect of the present invention, a memory cell includes: a silicon-on-insulator (SOI) substrate; an electrically floating body transistor fabricated on the silicon-on-insulator (SOI) substrate; and a charge injector region;
wherein the floating body transistor is configured to have more than one stable state through an application of a bias on the charge injector region.
In at least one embodiment, the floating body region comprises a first conductivity type selected from p-type conductivity type and n-type conductivity type, and the memory cell further includes: a source line region comprising a second conductivity type selected from the p-type conductivity type and the n-type conductivity type and being different from the first conductivity type, the source line region in physical contact with the floating body region; a drain region comprising the second conductivity type in physical contact with the floating body region and spaced apart from the source line region; and wherein the charge injection region comprises the second conductivity type and is in physical contact with the floating body region and spaced apart from the source line region and the drain line region.
In at least one embodiment, a memory array is provided that includes a plurality of the above-described memory cells arranged in at least one row and at least one column.
In at least one embodiment, the memory cell further includes a gate positioned in between the source line region and the drain region, above the floating body region.
In at least one embodiment, the memory cell is configured such that a holding operation on the memory cell does not require any interruption to access to the memory cell.
In at least one embodiment, the above-described array includes a charge injector terminal connected to all of the memory cells in a row or column of the array; wherein the charge injector terminal is configured to perform a holding operation on all of the memory cells connected to the charge injector terminal in a batch operation, so that no individual selection of one of the memory cells is required to perform the holding operation.
In at least one embodiment, application of a charge to the charge injector region in performance of a holding operation also increases a size of a memory window of the floating body region.
In at least one embodiment, the memory cell further includes a second charge injection region in contact with the floating body region and spaced apart from the first charge injection region.
In at least one embodiment, the charge injector region comprises a first charge injector region, the memory cell further comprising a second charge injector region, wherein the first and second charge injector regions are configured to maintain a state of the memory cell.
In at least one embodiment, a depth of at least one of the drain region, the source line region and the charge injector region is less than a depth of at least one other of the drain region, the source line region and the charge injector region.
In at least one embodiment, a depth of at least one of the drain region, the source line region and the charge injector region is equal to a depth of the floating body region, and a depth of at least one other of the drain region, the source line region and the charge injector region is less than the depth of the floating body region.
In at least one embodiment, the memory cell further includes a second gate region.
In at least one embodiment, the second gate region is positioned in between the charge injection region and one of the source line region and the drain region.
In at least one embodiment, a memory device is provided that includes a memory cell as described above connected in series to an access device.
In at least one embodiment, a memory device is provided that includes a memory cell as described above, connected in series to an access device; wherein the access device includes: a body region comprising the first conductivity type; a second source line region comprising the second conductivity type in contact with the body region; a second drain region comprising the second conductivity type in contact with the body region and spaced apart from the second source line region; and a body tap region comprising the first conductivity type in contact with the body region.
In at least one embodiment, the body tap region is configured to apply a bias on the body region.
In at least one embodiment, the memory cell and the access device comprise two transistors each having a same conductivity type.
In at least one embodiment, the memory cell and the access device comprise two transistors each having a different conductivity type.
In at least one embodiment, a memory device includes a memory cell as described above connected in series to an access device, wherein the access device includes: a body region comprising the first conductivity type; a second source line region comprising the second conductivity type in contact with the body region; and a second drain region comprising the second conductivity type in contact with the body region and spaced apart from the second source line region.
In another aspect of the present invention, a content addressable memory cell includes: a memory cell as described above, wherein the memory cell includes a first floating body transistor and a second floating body transistor; wherein the first floating body transistor and the second floating body transistor are electrically connected in series through a common node; and wherein the first floating body transistor and the second floating body transistor store complementary data.
In at least one embodiment, the content addressable memory cell is configured as a ternary content addressable memory cell.
In another aspect of the present invention, a memory cell includes: a memory device; and an access device connected in series to the memory device; wherein the memory device is configured to store a state of the memory cell, and the access device is configured to access the memory device.
In at least one embodiment, the memory device includes: a floating body region comprising a first conductivity type selected from p-type conductivity type and n-type conductivity type: a first source line region comprising a second conductivity type selected from the p-type conductivity type and the n-type conductivity type and being different from the first conductivity type, the first source line region in contact with the floating body region; a first drain region comprising the second conductivity type in physical contact with the floating body region and spaced apart from the first source line region; and a charge injection region comprising the second conductivity type, contacting the floating body region and spaced apart from the first source line region and the first drain line region; and the access device includes: a body region comprising the first conductivity type; a second source line region comprising the second conductivity type in contact with the body region; and a second drain region comprising the second conductivity type in contact with the body region and spaced apart from the second source line region.
In at least one embodiment, the memory cell further includes a conductive element connecting the first source line region to the second source line region.
In at least one embodiment, the access device further includes a body tap region comprising the first conductivity type in contact with the body region.
In another aspect of the present invention, a method of performing an operation on a memory cell is provided, including: providing the memory cell including: a substrate; an electrically floating body transistor; a source line region; a drain region; a charge injector region; and a gate positioned in between the source line region and the drain region, above the floating body region; and imposing bias conditions on the substrate, source line region, drain region, charge injector region and gate to perform the operation.
In at least one embodiment, the operation includes a holding operation, and the bias conditions imposed include: applying a positive bias to the charge injector region; applying zero or low negative bias to the gate; applying zero bias to the source line region; applying zero bias to the drain region; and applying zero bias to the substrate.
In at least one embodiment, the operation includes a read operation, and the bias conditions imposed include: applying zero bias or a positive bias to the charge injector region; applying a positive bias to the gate; applying zero bias to the source line region; applying a positive bias to the drain region; and applying zero bias to the substrate.
In at least one embodiment, the positive bias applied to the drain region is less than the positive bias applied to the gate.
In at least one embodiment, the positive bias applied to the drain region is greater than or equal to the positive bias applied to the gate.
In at least one embodiment, the operation includes a write logic-1 operation, and the bias conditions imposed include: applying a positive bias to the charge injector region; applying a positive bias to the gate; applying zero bias to the source line region; applying a positive bias to the drain region; and applying zero bias to the substrate.
In at least one embodiment, the positive bias applied to the drain region is greater than or equal to the positive bias applied to the gate and generates a sufficiently high electric field to trigger an impact ionization mechanism.
In at least one embodiment, the operation includes a write logic-1 operation, and the bias conditions imposed include: applying a positive bias to the charge injector region; applying a negative bias to the gate; applying zero bias to the source line region; applying a positive bias to the drain region; and applying zero bias to the substrate.
In at least one embodiment, the operation includes a write logic-0 operation, and the bias conditions imposed include: applying zero bias or a positive bias to the charge injector region; applying zero bias to the gate; applying a negative bias to the source line region; applying zero bias to the drain region; and applying zero bias to the substrate.
In at least one embodiment, the operation includes a bit selective, write logic-0 operation, and the bias conditions imposed include: applying zero bias or a positive bias to the charge injector region; applying a positive bias to the gate; applying zero bias to the source line region; applying a negative bias to the drain region; and applying zero bias to the substrate.
In at least one embodiment, the positive bias applied to the gate is configured to increase a potential of the floating body by half of a potential corresponding to state logic-1, and the negative bias applied to the drain region comprises a negative voltage of half of the potential corresponding to state logic-1.
These and other features of the present invention will become apparent to those persons skilled in the art upon reading the details of the memory cells, memory devices, arrays and methods as more fully described below.
Before the present memory cells, arrays 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 array” includes reference to one or more arrays 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.
Memory cell 50 is fabricated on silicon-on-insulator (SOI) or germanium-on-insulator (GOI) or silicon-germanium-on-insulator (SiGeOI) or strained silicon-on-insulator (sSOI), strained germanium-on-insulator (sGOI), or strained silicon-germanium-on-insulator (sSiGeOI) substrate, which comprises buried insulator layer 22, which may be buried oxide (BOX). Substrate 12 is typically made of silicon, but may also comprise, for example, amorphous or polycrystalline silicon, strained silicon, germanium, silicon germanium, gallium arsenide, carbon nanotubes, or other semiconductor materials.
Memory cell 50 further comprises a floating body region 24 of a first conductivity type, such as p-type conductivity for example (but alternatively may be n-type conductivity), bounded on top by surface 14 and insulating layer 62, on the sides by source line region 16 of a second conductivity type, such as n-type conductivity for example (but, alternatively, may be p-type conductivity when floating body region 24 is n-type conductivity), drain region 18 of the second conductivity type (but, like source line region 16, may be p-type conductivity when floating body region is n-type conductivity), and at the bottom by buried insulator layer 22. Memory cell 50 may store charge in the floating body region 24, which indicates the states of the memory cell 50.
Source line region 16 and drain 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 source line region 16 and drain 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 their nitrides.
Memory cell 50 also comprises charge injector region 20 (see
Cell 50 includes several terminals: word line (WL) terminal 70 electrically connected to gate 60, bit line (BL) terminal 74 electrically connected to drain region 18, source line (SL) terminal 72 electrically connected to source line region 16, charge injector (CI) terminal 76 electrically connected to charge injector region 20, and substrate (SUB) terminal 78 electrically connected to the substrate 12. Alternatively, the SL terminal 72 may be electrically connected to drain region 18 and BL terminal 74 may be electrically connected to source line region 16.
As illustrated in
During the operation of memory cell 50, at least one of the MOS transistors 20b or 20c is used to maintain the state stored in memory cell 50, while the other of MOS transistors 20a used for other operations, such as read and write operations. At least one of the bipolar transistors 30b or 30c may also be used to maintain the charge stored in the floating body region 24, while the other bipolar transistor 30a is used for other memory cell operations, such as read and write operations.
The BL terminal 74 may be connected to a sensing circuit, which senses the state of a selected memory cell 50, as shown in
When no reading operation is performed, the precharge signal is high, which will turn off transistors 1202 and 1204 and turn on transistors 1214 and 1216, bringing the BL terminals 74a and 74R to GND. The latch 1200 is also turned off when no reading operation is performed by turning off transistors 1206 and 1208. During a read operation, the precharge signal switches to low, enabling transistors 1202 and 1204, while turning off the transistors 1214 and 1216. Subsequently, the selected memory cell BL and reference memory cell BL (through column select CS transistors 1210 and 1212) are enabled. If memory cell 50a is in logic-1 state, it will pull node 1250 to a lower voltage than node 1252 and vice versa. The latch signal is subsequently turned on, amplifying the difference between nodes 1250 and 1252 to Vdd and GND through latch 1200.
Several operations can be performed to memory cell 50, such as: holding, read, write logic-1 and write logic-0 operations.
In one embodiment the bias conditions for the holding operation on memory cell 50 are: 0 volts is applied to WL terminal 70, 0 volts is applied to BL terminal 74, 0 volts is applied to SL terminal 72, a positive voltage, for example, +1.2 volts is applied to CI terminal 76, and 0 volts is applied to the substrate terminal 78. In other embodiments, different voltages may be applied to the various terminals of memory cell 50 and the exemplary voltages described are not limiting.
The region where the product β×(M−1) approaches 1 and is characterized by hole current moving into the base region of a bipolar transistor is sometimes referred to as the reverse base current region and 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”), and “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. In memory cell 50, the state of the memory cell is maintained due to the bipolar transistors 30b and 30c, while the remaining cell operations (i.e. read and write operations) are governed by the lateral bipolar transistor 30a and MOS transistor 20a. Hence, the holding operation does not require any interruptions to the memory cell 50 access.
If floating body 24 is neutrally charged (the voltage on floating body 24 being equal to the voltage on grounded bit line region 18), a state corresponding to logic-0, no (or low) current will flow through the n-p-n bipolar devices 30b and 30c. The bipolar devices 30b and 30c will remain off and no impact ionization occurs. Consequently memory cells in the logic-0 state will remain in the logic-0 state.
An autonomous refresh for a floating body memory, without requiring to first read the memory cell state, has been described for example in “Autonomous Refresh of Floating Body Cell (FBC)”, Ohsawa et al., pp. 801-804, International Electron Device Meeting, 2008 (“Ohsawa”), U.S. Pat. No. 7,170,807 and “Data Storage Device and Refreshing Method for Use with Such Device”, Fazan et al. (“Fazan”), which are hereby incorporated herein, in their entireties, by reference thereto. Ohsawa and Fazan teach an autonomous refresh method by applying periodic gate and drain voltage pulses, which interrupt access to the memory cells being refreshed. In memory cell 50, more than one stable state is achieved because of the bipolar transistors 30b and 30c. The read and write operations of the memory cell 50 are governed by the lateral bipolar transistor 30a and MOS transistor 20a. Hence, the holding operation does not require any interruptions to the memory cell 50 access.
In the holding operation described with regard to
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.
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 CI terminal 76, 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 CI terminal 76 as shown in
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 that 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.
In one embodiment the bias conditions for the read operation for memory cell 50 are: +1.2 volts is applied to WL terminal 70, +0.4 volts is applied to BL terminal 74, 0.0 volts is applied to SL terminal 72, +1.2 volts is applied to CI terminal 76, and 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 50 and the exemplary voltages described are not limiting. The positive voltage applied to BL terminal 74 may be less than the positive voltage applied to WL terminal 70, in which the difference in the threshold voltage of the memory cell 50 is employed to represent the state of the memory cell 50. The positive voltage applied to BL terminal 74 may also be greater than or equal to the positive voltage applied to WL terminal 70 and may generate sufficiently high electric field to trigger the bipolar read mechanism.
A sensing circuit typically connected to BL terminal 74 can be used to determine the data state of the memory cell 50. Any sensing scheme known in the art can be used in conjunction with memory cell 50. For example, the sensing schemes disclosed in Ohsawa-1 and Ohsawa-2 are incorporated by reference herein in there entireties, and can be used to determine the state of the memory cell 50.
In one particular non-limiting embodiment, about +1.2 volts is applied to the selected WL terminal 70, about +1.2 volts is applied to the selected BL terminal 74, about 0.0 volts is applied to SL terminal 72, about 0.0 volts or +1.2 volts is applied to CI terminal 76, and about 0.0 volts is applied to substrate terminal 78; while about 0.0 volts is applied to the unselected WL terminals 70, unselected BL terminals 74, unselected SL terminals, and substrate terminal 78, and 0.0 volts or +1.2 volts is applied to unselected CI terminal 76. These voltage levels are exemplary only and may vary from embodiment to embodiment. Thus the exemplary embodiments, features, bias levels, etc., described are not limiting.
In one particular non-limiting embodiment, about −1.2 volts is applied to the selected WL terminal 70, about +1.2 volts is applied to the selected BL terminal 74, about 0.0 volts is applied to SL terminal 72, about +1.2 volts is applied to CI terminal 76, and about 0.0 volts is applied to substrate terminal 78; while about 0.0 volts is applied to the unselected WL terminals 70, unselected BL terminals 74, unselected SL terminals, and substrate terminal 78, and 0.0 volts or +1.2 volts is applied to unselected CI terminal 76. These voltage levels are exemplary only may vary from embodiment to embodiment. Thus the exemplary embodiments, features, bias levels, etc., described are not limiting.
The negative bias 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 of the floating body region 24 to the conduction band of the bit line region 18, 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 −1.2 volts is applied to selected SL terminal 72, about 0.0 volts is applied to selected WL terminal 70, about 0.0 volts is applied to selected BL terminal 74, about 0.0 volts or +1.2 volts is applied to selected CI terminal 76, and about 0.0 volts is applied to substrate terminal 78, while zero voltage is applied to the unselected SL terminals 72, zero voltage bias applied to the unselected WL terminals 70, zero or positive bias applied to the unselected CI terminal 76, and zero voltage bias applied to the substrate 78. These voltage levels are exemplary only may vary from embodiment to embodiment. Thus the exemplary embodiments, features, bias levels, etc., described are not limiting.
To reduce undesired write logic-0 disturb to other memory cells 50 in a memory array, the applied potential can be optimized as follows: if the floating body 24 potential of state logic-1 is referred to as VFB1, then the voltage applied to the WL terminal 70 is configured to increase the floating body 24 potential by VFB1/2 while −VFB1/2 is applied to BL terminal 74. Additionally, either ground or a slightly positive voltage may also be applied to the BL terminals 74 of unselected memory cells 50 that do not share the same BL terminal 74 as the selected memory cell 50a, while a negative voltage may also be applied to the WL terminals 70 of unselected memory cells 50 that do not share the same WL terminal 70 as the selected memory cell 50.
As illustrated in
In order to form the charge injector regions 20 in memory cell 250, an additional masking step may be needed. An exemplary process step to form charge injector regions 20 is shown in
The operation of memory cell 350 is similar to that of memory cells 50, 150, and 250, where the charge injection region 20 maintains the state of the memory cell 350, and the read and write operations are performed by the MOS or bipolar transistors formed by the source region 16, the bit line region 18, and the gate 60. The gate region 64 (which may be connected to a word line 2 (WL2) terminal 77) may enhance the efficiency of the holding operation by enhancing the impact ionization efficiency.
Memory device 740 may be constructed from any of the memory cells 50, 150, 250, 350, 450, 550, or 650 described above. The state of the memory cell is stored in the floating body region 24 (e.g., having the first conductivity type). The charge injector region 20 (e.g., of the second conductivity type) is configured to maintain the state of the memory cell 750, while the read and write operations are performed by the MOS or bipolar transistors formed by the source region 16, the bit line region 18, and the gate 60.
Access device 742 comprises regions 32 and 34 (e.g., of second conductivity type), a body region 38 (e.g., of a first conductivity type), and a body tap region 36 (e.g., of a first conductivity type). The body region 38 of the access device 742 is not floating as the body tap region 36 may be used to apply a bias on the body region 38.
The source region 16 of the memory device 740 is connected to the source region 32 of the access device 742 through a conductive element 94.
Memory cell 750 comprises several terminals: word line #1 (WL1) terminal 770 (see
In one embodiment the bias conditions for the holding operation for memory cell 750 are: 0.0 volts is applied to WL1 terminal 770, WL2 terminal 772, SL terminal 774, BL terminal 776, and TAP terminal 780, and a positive voltage like, for example, +1.2 volts is applied to CI terminal 778. In other embodiments, different voltages may be applied to the various terminals of memory cell 750 as a matter of design choice and the exemplary voltages described are not limiting.
A read operation of the memory cell 750, according to an embodiment of the present invention, will be described in conjunction with
A read operation for example can be performed on memory cell 750 by applying the following bias conditions. A positive voltage is applied to the WL2 terminal 772, which turns on the access transistor 742, a positive voltage is applied to the BL terminal 776, zero voltage is applied to the SL terminal 774, positive voltage is applied to the CI terminal 778, and zero voltage is applied to the TAP terminal 780. Positive voltage may also be applied to the WL1 terminal 770 to further enhance the current flowing through the memory cell 750, from the BL terminal 776 to the SL terminal 774. If memory cell 750 is in a logic-1 state having holes in the floating body region 24, then a higher current will flow from the BL terminal 776 to the SL terminal 774 of the selected memory cell 750, compared to if memory cell 750 is in a logic-0 state having no holes in the floating body region 24. In one particular embodiment, +1.2 volts is applied to the WL1 terminal 770, WL2 terminal 772, BL terminal 776, CI terminal 778, and 0.0 volts is applied to the SL terminal 774 and TAP terminal 780. In other embodiments, different voltages may be applied to the various terminals of memory cell 750 as a matter of design choice and the exemplary voltages described are not limiting in any way.
The access transistor 742 is used to assist the selection of the memory cell 750 during a read operation. Because the access transistor 742 of the unselected memory cells in different rows are turned off, it will not pass the positive voltage applied to the BL terminal 776 to the region 16 of the floating body transistor 740. As a result, no current will flow through the floating body transistor 740 of the unselected memory cells in different rows.
In one particular non-limiting embodiment, about +1.2 volts is applied to the selected WL2 terminal 772, about −1.2 volts is applied to the selected WL1 terminal 770, about +1.2 volts is applied to the selected BL terminal 776, about +1.2 volts is applied to the selected CI terminal 778, about 0.0 volts is applied to SL terminal 774 and about 0.0 volts is applied to the TAP terminal 780.
The positive bias applied to the WL2 terminal 772 will turn on the access transistor 742, which will pass the positive bias applied to the BL terminal 776 to the region 16 of the floating body transistor 740. The positive bias now present on the region 16 of the floating body transistor 740, along with the negative voltage applied to the WL1 terminal 770 (connected to the gate 60), will create a strong electric field around the junction area of the region 16 in the proximity of the gate 60. The strong electric field bends the energy band sharply upward near the gate 60 and region 16 junction overlap region, causing electrons to tunnel from the valence band of the floating body region 24 to the conduction band of the region 16, leaving holes in the valence band of the floating body region 24. The electrons which tunnel across the energy band become the region 16 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 +1.2 volts is applied to the selected WL2 terminal 772, about +0.5 volts is applied to the selected WL1 terminal 770, about +1.2 volts is applied to the selected BL terminal 776, about +1.2 volts is applied to the selected CI terminal 778, about 0.0 volts is applied to SL terminal 774 and about 0.0 volts is applied to the TAP terminal 780. These voltage levels are exemplary only and may vary from embodiment to embodiment. Thus the exemplary embodiments, features, bias levels, etc., described are not limiting.
In one particular non-limiting embodiment, about 0.0 volts is applied to the WL2 terminal 772, the voltage applied to the WL1 terminal 770 is increased from 0.0 volts to about +1.2, about +1.2 volts is applied to the SL terminal 774, about +1.2 volts is applied to the BL terminal 776, about +1.2 volts is applied to the CI terminal 778, and about 0.0 volts is applied to the TAP terminal 780. These voltage levels are exemplary only and may vary from embodiment to embodiment. Thus the exemplary embodiments, features, bias levels, etc., described are not limiting.
The ramp rate of the positive bias applied to the WL1 terminal 770 (connected to gate electrode 60) may be optimized to increase the coupling ratio from the gate 60 to the floating body region 24. As described for example in “Substrate Response of a Floating Gate n-channel MOS Memory Cell Subject to a Positive Linear Ramp Voltage”, H.-S. Lee and D. S. Lowrie, Solid-State Electronics 24, no. 3, pp. 267-273, 1981, which is hereby incorporated herein, in its entirety, by reference thereto, a higher coupling from the gate 60 to the floating body region 24 can be achieved with a higher ramp rate. The ramp rate applied to the gate 60 may also be higher in the write logic-1 operation than in other operations, such as read operation, to further improve the write logic-1 operation time. For example, during write logic-1 operation, the voltage applied to the gate 60 may be ramped from about 0.0 volts to about +1.2 volts in about 50 picoseconds (ps) or less. This ramp rate is exemplary only and may vary from embodiment to embodiment and is not limiting.
In one particular non-limiting embodiment, about −1.2 volts is applied to the selected SL terminal 774, about 0.0 volts is applied to the WL1 terminal 770, WL2 terminal 772, BL terminal 776, and TAP terminal 780, and about +1.2 volts is applied to the CI terminal 778. These voltage levels are exemplary only and may vary from embodiment to embodiment. Thus the exemplary embodiments, features, bias levels, etc., described are not limiting.
In one particular non-limiting embodiment, about +1.2 volts is applied to the WL2 terminal 772, about +1.2 volts is applied to the WL1 terminal 770, about 0.0 volts is applied to the SL terminal 774, about −0.2 volts is applied to the BL terminal 776, about +1.2 volts is applied to the CI terminal 778, and about 0.0 volts is applied to the TAP terminal 780. These voltage levels are exemplary only and may vary from embodiment to embodiment. Thus the exemplary embodiments, features, bias levels, etc., described are not limiting.
Memory cell 750 illustrated in
The memory cells that have been described herein (memory cells 50, 150, 250, 350, 450, 550, 650, 750, and 850) are all fabricated on planar SOI substrate. Memory cell 950 illustrated in
The memory cell described in this disclosure may also be used to form a content addressable memory (CAM) cell or ternary content addressable memory (TCAM) cell as described in U.S. patent application Ser. No. 14/154,138 by Widjaja et al., “Content Addressable Memory Device Having Electrically Floating Body Transistor”, which is hereby incorporated herein, in their entirety, by reference thereto.
From the foregoing it can be seen that a memory cell comprising an electrically floating body transistor fabricated on a silicon-on-insulator (SOI) substrate having more than one stable state through an application of a bias on a charge injector region has been described. While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The invention should therefore not be limited by the above described embodiment, method, and examples, but by all embodiments and methods within the scope and spirit of the invention as claimed.
This application is a continuation of U.S. application Ser. No. 15/375,236, filed Dec. 12, 2016, which is a continuation of U.S. application Ser. No. 14/597,444, filed Jan. 15, 2015, now U.S. Pat. No. 9,548,119, which claims the benefit of U.S. Provisional Application No. 61/927,484, filed on Jan. 15, 2014, which applications and patent are hereby incorporated herein, in their entireties, by reference thereto.
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| Number | Date | Country | |
|---|---|---|---|
| 20180122457 A1 | May 2018 | US |
| Number | Date | Country | |
|---|---|---|---|
| 61927484 | Jan 2014 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15375236 | Dec 2016 | US |
| Child | 15846566 | US | |
| Parent | 14597444 | Jan 2015 | US |
| Child | 15375236 | US |
