SELF-BIASING TRANSISTOR STRUCTURE AND AN SRAM CELL HAVING LESS THAN SIX TRANSISTORS
By providing a self-biasing semiconductor switch, an SRAM cell having a reduced number of individual active components may be realized. In particular embodiments, the self-biasing semiconductor device may be provided in the form of a double channel field effect transistor that allows the formation of an SRAM cell with less than six transistor elements and, in preferred embodiments, with as few as two individual transistor elements.
1. Field of the Invention
The present invention generally relates to the fabrication of integrated circuits, and, more particularly, to transistor architectures that enable an extended functionality of transistor devices, thereby providing the potential for simplifying the configuration of circuit elements, such as registers, static RAM cells, and the like.
2. Description of the Related Art
In modern integrated circuits, such as microprocessors, storage devices, and the like, a huge number of circuit elements, especially transistors, are provided and operated on a restricted chip area. Although immense progress has been made over the recent decades with respect to increased performance and reduced feature sizes of the circuit elements, the ongoing demand for enhanced functionality of electronic devices forces semiconductor manufacturers to steadily reduce the dimensions of the circuit elements and to increase the operating speed thereof. However, the continuing scaling of feature sizes involves great efforts in redesigning process techniques and developing new process strategies and tools to comply with new design rules. Generally, in complex circuitry including complex logic portions, the MOS technology is presently a preferred manufacturing technique in view of device performance and/or power consumption. In integrated circuits including logic portions formed by the MOS technology, a large number of field effect transistors (FETs) are provided that are typically operated in a switched mode, that is, these devices exhibit a highly conductive state (on-state) and a high impedance state (off-state). The state of the field effect transistor is controlled by a gate electrode, which may influence, upon application of an appropriate control voltage, the conductivity of a channel region formed between a drain terminal and a source terminal.
As previously mentioned, an appropriate manufacturing process involves a plurality of highly complex process techniques, which depend on the specified design rules that prescribe the critical dimensions of the transistor element 100 and respective process margins. For example, one essential dimension of the transistor 100 is the channel length, i.e., in
It should be noted that a similar behavior is obtained for P-channel enhancement and depletion transistors, wherein, however, the channel conductivity is high for negative gate voltages and abruptly decreases at the respective threshold voltages with a further increasing gate voltage.
On the basis of field effect transistors, such as the transistor element 100, more complex circuit components may be created. For instance, storage elements in the form of registers, static RAM (random access memory), and dynamic RAM represent an important component of complex logic circuitries. For example, during the operation of complex CPU cores, a large amount of data has to be temporarily stored and retrieved, wherein the operating speed and the capacity of the storage elements significantly influence the overall performance of the CPU. Depending on the memory hierarchy used in a complex integrated circuit, different types of memory elements are used. For instance, registers and static RAM cells are typically used in the CPU core due to their superior access time, while dynamic RAM elements are preferably used as working memory due to the increased bit density compared to registers or static RAM cells. Typically, a dynamic cell comprises a storage capacitor and a single transistor, wherein, however, a complex memory management system is required to periodically refresh the charge stored in the storage capacitors, which may otherwise be lost due to unavoidable leakage currents. Although the bit density of DRAM devices may be extremely high, a charge has to be transferred from and to storage capacitors in combination with periodic refresh pulses, thereby rendering these devices less efficient in terms of speed and power consumption when compared to static RAM cells. On the other hand, static RAM cells require a plurality of transistor elements to allow the storage of an information bit.
During operation of the RAM cell 150, the bit cell 110 may be “programmed” by pre-charging the bit lines 112, 113, for example with logic high and logic zero, respectively, and by activating the select line 116, thereby connecting the bit cell 110 with the bit lines 112, 113. After deactivating the select line 116, the state of the bit cell 110 is maintained as long as the supply voltage is connected to the cell 150 or as long as a new write cycle is performed. The state of the bit cell 110 may be retrieved by, for example, bringing the bit lines 112, 113 in a high impedance state and activating the select line 116.
As is evident from
In view of the problems identified above, a need exists for an improved device architecture that enables the formation of storage elements in a more space efficient manner.
SUMMARY OF THE INVENTIONThe following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an exhaustive overview of the invention. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later.
Generally, the present invention is directed to a technique that enables the formation of circuit components including transistor elements in a more space-efficient manner, especially in static memory devices, in that the functionality of a transistor element is extended so that a self-biasing conductive state may be obtained.
According to one illustrative embodiment of the present invention, a semiconductor device comprises a drain region formed in a substantially crystalline semiconductor material and doped with a first type of dopant material to provide a first conductivity type. The device further comprises a source region formed in the substantially crystalline semiconductor material, which is doped with the first type of dopant material to provide the first conductivity type. A first channel region is located between the drain region and the source region and is doped with the first type of dopant material to provide the first conductivity type. Furthermore, a second channel region is located between the drain region and the source region and adjacent to the first channel region and is doped with a second type of dopant material to provide a second conductivity type that differs from the first conductivity type. Finally, a gate electrode is located to enable control of the first and second channel regions.
In accordance with another illustrative embodiment of the present invention, a transistor element comprises a drain region, a source region, and a channel region, which is formed between the drain region and the source region and which is configured to define at least a first threshold of a first abrupt conductivity change and a second threshold of a second abrupt condutdivity change of the channel region. The transistor element further comprises a gate electrode that is located to enable control of the channel region by capacitive coupling.
According to yet another illustrative embodiment of the present invention, a static RAM cell comprises a select transistor and an information storage element coupled to the select transistor, wherein the information storage element includes less than four transistor elements.
According to still another illustrative embodiment of the present invention, a static RAM cell comprises a transistor element having a gate electrode, a drain region, a source region, and a channel region that is electrically connected with the gate electrode. Moreover, the transistor element is configured to self-bias the gate electrode to maintain the channel region in a stationary conductive state.
According to yet another illustrative embodiment of the present invention, a static RAM cell comprises two or less transistor elements.
The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which:
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
DETAILED DESCRIPTION OF THE INVENTIONIllustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as comphance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.
The present invention will now be described with reference to the attached figures. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the present invention with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present invention. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase.
Generally, the present invention is based on the concept that the circuit architecture of a plurality of logic circuit portions, especially of registers, static memory cells, and the like, may be significantly simplified in that one or more characteristics of a semiconductor switch element may be modified to obtain extended functionality. In particular, the inventors contemplated to provide a self-biasing semiconductor switch, which may be based in particular embodiments of the present invention on a field effect transistor design with a modified channel region, wherein a conductive state, once initiated, is maintained as long as the supply voltage is applied, unless a change of conductivity state is externally initiated. In this way, particularly the number count of individual switch elements in a static RAM cell may be drastically reduced compared to conventional RAM cell designs and may be less than six, thereby enabling the fabrication of fast storage devices with a bit density that is comparable with that of dynamic RAM devices.
Again referring to
During reading of the bit cell 210, the bit line 212 may be in a high impedance state and the select transistor 214 may be switched into its on-state by activating the select line 216. Due to the self-biased high conductivity state of the bit cell 210, charge may be supplied from the supply voltage source VDD to the bit line 212 to establish the voltage VDD 5 at the bit line 212, which may be sensed by a corresponding sense amplifier (not shown). Thus, a logic state corresponding to the self-biased state of the bit cell 210 may be identified and read out. Similarly, a high impedance state may be written into the bit cell 210 by, for instance, pre-charging the bit line 212 with ground potential and activating the select line 216. In this case, the ground potential is supplied to the gate electrode 205 via the feedback section 208—the inherent resistance of the bit line 212 is assumed to be significantly lower than the resistance of the channel region 203 in its high conductivity state—and hence the channel region 203 is brought into its high impedance state, which is maintained even if the bit line 212 is decoupled from the output 204s by deactivating the select line 216.
As a result, by means of the semiconductor bit cell 210, a significantly simplified architecture for a static RAM cell is obtained, wherein particularly the number of individual semiconductor elements may be less than in the conventional RAM cell described with reference to
In one particular embodiment, the channel region 303 may comprise a first channel sub-region 303a that is inversely doped with respect to the drain and source regions 304. Thus, the first channel sub-region 303a may be considered as a “conventional” channel region of a conventional enhancement transistor, such as, for instance, the transistor 100 in
A typical process flow for forming the semiconductor device 300 as shown in
The basic operational behavior of the transistor element 300 will now be explained with reference to the N-type transistor of
The operation of the cell 450 is substantially the same as is previously described with reference to
During the operation of the cell 450, the write and read cycles may be performed as previously described, wherein, when operated at a higher VDD, the transistor element 400b is operated in the self-biasing mode and thus maintains its gate voltage and the gate voltage of the transistor element 400a at the high threshold voltage VT2b when remaining in the high conductivity state. Likewise, when being operated with a low VDD that may range between the threshold VT2b and VT2a of the transistor 400b and the transistor 400a, the device 400a remains in the high conductivity state and thus keeps the gate voltages of the devices 400a and 400b at the lower threshold voltage VT2a.
It should also be appreciated that more than two devices with different threshold voltages VT2 may be provided in the cell 450, thereby providing the potential for an enhanced functionality. For example, the device 450 may be used to store three different states, one state representing a high impedance state, one state representing a high conductivity state with a gate voltage at the lower threshold voltage VT2a, and one state representing a high conductivity state at the higher threshold voltage VT2b of the device 400b. When writing corresponding states into the cell 450, the bit line has to be pre-charged with respective voltages. Likewise, when more than two transistor elements with different threshold voltages VT2 are provided, a corresponding number of different states may be stored in the cell 450, wherein a single select line 416 and a single bit line 412 is sufficient to address the cell 450 having stored therein a plurality of different states. In other applications, the lower threshold VT2a may be considered as a stand-by threshold, to ensure data integrity when the supply voltage VD1) decreases below the normal operating voltage due to a sleep mode, during which the supply voltage may be delivered by a storage capacitor or the like.
The transistor element 500 may be manufactured in accordance with conventional process techniques, wherein the channel regions 503a, 503b may be formed by ion implantation and/or epitaxial growth techniques, as is previously described with reference to FIGS. 3a and 3b. The SOI device 500 may be advantageously incorporated into complex microprocessors, which are increasingly fabricated as SOI devices.
In other embodiments, a specific internal strain in the channel region 603a and/or 603b may be created by applying external stress, for instance by means of a specifically stress-containing capping layer enclosing the transistor element 600. In other embodiments, stress may be created additionally or alternatively by a corresponding implantation of specific ion species, such as hydrogen, helium, oxygen, and the like, in or in the vicinity of the first and second channel regions 603a, 603b, thereby specifically adjusting the respective threshold voltages. The adjustment of threshold voltages by stress created by ion implantation is advantageous when a plurality of different threshold voltages have to be created at different die locations or different substrate locations, since respective implantations may readily be performed with different mask schemes in conformity with device requirements.
As a result, the present invention provides a self-biasing semiconductor device that may mostly be advantageously used in combination with static storage cells, such as RAM cells, to significantly reduce the number of transistor elements required. Since already well-established process techniques may be used in forming a corresponding self-biasing transistor element, for instance in the form of a double channel transistor, a significant improvement in bit density and/or performance may be achieved for a given technology node. Moreover, since SWAM devices may now be fabricated in a highly efficient manner with a bit density comparable to dynamic RAM devices, the dynamic devices, usually employed as external operating memory for CPUs, may be readily replaced, thereby providing immense cost and performance advantages. Moreover, the simplified SRAM design of the present invention in combination with a low-cost power supply enables a cost-effective utilization of SRAM devices in a wide variety of applications, which may currently employ magnetic storage devices or EEPROMs.
The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. For example, the process steps set forth above may be performed in a different order. Furthermore, no limitations are intended to the details of construction or design herein shown, other tan as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.
Claims
1.-11. (canceled)
12. A transistor element, comprising:
- a drain region;
- a source region;
- a channel region formed between said drain region and said source region and being configured to define a first threshold voltage of the semiconductor device for transitioning a total conductivity of the channel region into a low impedance state and a second threshold voltage of the semiconductor device that results in a first abrupt conductivity change of the total conductivity of said channel region when said total conductivity is in the low impedance state; and
- a gate electrode located to control said channel region by capacitive coupling.
13. The transistor element of claim 12, wherein said first abrupt conductivity change is defined for an absolute amount of gate voltage that maintains said channel region in a low impedance state to define a local maximum of the conductivity with respect to said absolute amount.
14. The transistor element of claim 12, wherein said channel region comprises at least a first channel sub-region having a first conductivity type and a second channel sub-region having a second conductivity type that differs from the first conductivity type.
15. The transistor element of claim 14, wherein said first conductivity type of said first channel sub-region differs from a conductivity type of said drain and source regions, said first channel sub-region being located more closely to said gate electrode than said second channel sub-region.
16. The transistor element of claim 12, further comprising a doped semiconductor region having a conductivity type and being located adjacent to said drain region, source region and said channel region, said conductivity type differing from a conductivity type of said drain region and source region.
17. The transistor element of claim 12, further comprising an insulation layer formed adjacent to said drain and source regions and said channel region, said insulation layer isolating said transistor element from a substrate.
18. The transistor element of claim 14, wherein said first and second channel sub-regions differ in at least one of material composition and internal strain.
19. A static RAM cell, comprising:
- a select transistor; and
- an information storage element coupled to said select transistor, said information storage element including less than four transistor elements, wherein one of the transistor elements comprises a first controllable semiconductor device including at least: a drain region; a source region; a channel region formed between said drain region and said source region and being configured to define a first threshold voltage of the semiconductor device for transitioning a total conductivity of the channel region into a low impedance state and a second threshold voltage of the semiconductor device that results in a first abrupt conductivity change of the total conductivity of said channel region when said total conductivity is in the low impedance state; and a gate electrode located to control said channel region by capacitive coupling.
20. The static RAM cell of claim 19, wherein said controllable semiconductor device has at least one stationary conductive state and wherein said channel region is connected to said gate electrode and configured to self-bias said gate electrode when said semiconductor device is in said at least one stationary conductive state.
21. The static RAM cell of claim 20, wherein
- said channel region is configured to define at least a first threshold for an absolute amount of voltage applied to the gate electrode, said first threshold being a lower limit for a gate voltage to transit into said self-biased at least one stationary conductive state.
22. The static RAM cell of claim 21, wherein said channel region comprises at least a first channel sub-region having a first conductivity type and a second channel sub-region having a second conductivity type that differs from the first conductivity type.
23. The static RAM cell of claim 22, wherein said first conductivity type of said first channel sub-region differs from a conductivity type of said drain and source regions, said first channel sub-region being located more closely to said gate electrode than said second channel sub-region.
24. The static RAM cell of claim 21, wherein said controllable semiconductor device further comprises a doped semiconductor region having a conductivity type and being located adjacent to said drain region, source region and said channel region, said conductivity type differing from a conductivity type of said drain region and source region.
25. The static RAM cell of claim 21, wherein said controllable semiconductor device further comprises an insulation layer formed adjacent to said drain and source regions and said channel region, said insulation layer isolating said semiconductor device from a substrate.
26. The static RAM cell of claim 23, wherein said first and second channel sub-regions differ in at least one of material composition and internal strain.
27. The static RAM cell of claim 19, wherein said information storage element comprises a second controllable semiconductor device having at least one stationary conductive state and having a second channel region and a second gate electrode configured to control a conductivity of said second channel region, said second channel region connected to said second gate electrode and being configured to self-bias said second gate electrode when said semiconductor device is in said at least one stationary conductive state.
28. The static RAM cell of claim 27, wherein said second controllable semiconductor device is operable in a self-biased state with a second control voltage that differs from a first control voltage required to operate said controllable semiconductor device in a self-biased state.
29. A static RAM cell, comprising:
- a transistor element having a gate electrode, a drain region, a source region, a channel region formed between said drain region and said source region and electrically connected with said gate electrode and controllable by said gate electrode, said transistor element being configured to define a first threshold voltage of the semiconductor device for transitioning a total conductivity of the channel region into a low impedance state and a second threshold voltage of the semiconductor device that results in a first abrupt conductivity change of the total conductivity of said channel region when said total conductivity is in the low impedance state and self-bias said gate electrode to maintain said channel region in a stationary conductive state.
30. The static RAM cell of claim 29, further comprising a select transistor element coupled to said transistor element.
31. The static RAM cell of claim 30, wherein a total number of transistor elements is less than six.
32. The static RAM cell of claim 29, wherein said channel region is formed between said drain region and said source region and is configured to define at least a first threshold for an absolute amount of voltage applied to the gate electrode, said first threshold being a lower limit for a gate voltage to transit into said self-biased stationary conductive state.
33. The static RAM cell of claim 32, wherein said channel region comprises at least a first channel sub-region having a first conductivity type and a second channel sub-region having a second conductivity type that differs from the first conductivity type.
34. The static RAM cell of claim 33, wherein said first conductivity type of said first channel sub-region differs from a conductivity type of said drain and source regions, said first channel sub-region being located more closely to said gate electrode than said second channel sub-region.
35. The static RAM cell of claim 29, wherein said transistor element further comprises a doped semiconductor region having a conductivity type and being located adjacent to said drain region, source region and said channel region, said conductivity type differing from a conductivity type of said drain region and source region.
36. The static RAM cell of claim 29, wherein said transistor element further comprises an insulation layer formed adjacent to said drain and source regions and said channel region, said insulation layer isolating said transistor element from a substrate.
37. The static RAM cell of claim 33, wherein said first and second channel sub-regions differ in at least one of material composition and internal strain.
38. The static RAM cell of claim 29, further comprising a second transistor element having at least one self-biased stationary conductive state and having a second channel region and a second gate electrode configured to control a conductivity of said second channel region, said second channel region connected to said second gate electrode and being configured to self-bias said second gate electrode when said transistor element is in said at least one self-biased stationary conductive state.
39. A static RAM cell comprising two or less transistor elements, wherein at least one of the two or less transistor elements comprises:
- a drain region;
- a source region;
- a channel region formed between said drain region and said source region and being configured to define a first threshold voltage of the semiconductor device for transitioning a total conductivity of the channel region into a low impedance state and a second threshold voltage of the semiconductor device that results in a first abrupt conductivity change of the total conductivity of said channel region when said total conductivity is in the low impedance state; and
- a gate electrode located to control said channel region by capacitive coupling.
40. The static RAM cell of claim 39, wherein at least one of the two or less transistor elements is a double channel transistor element.
41. The static RAM cell of claim 40, wherein said double channel transistor element comprises:
- a drain region formed in a substantially crystalline semiconductor material and doped to provide a first conductivity type;
- a source region formed in said substantially crystalline semiconductor material and doped to provide the first conductivity type;
- a first channel region located between said drain region and said source region and doped to provide the first conductivity type; and
- a second channel region located between said drain region and said source region and adjacent to the first channel region and being doped to provide a second conductivity type differing from said first conductivity type.
Type: Application
Filed: Sep 29, 2008
Publication Date: Jan 29, 2009
Inventors: Frank Wirbeleit (Freiberg), Manfred Horstmann (Duerroehrsdorf-Dittersbach), Christian Hobert (Pima)
Application Number: 12/240,312
International Classification: H01L 29/00 (20060101);