# Patent

## Title
Digital circuits having improved transistors, and methods therefor

## Abstract

Digital circuits are disclosed that may include multiple transistors having controllable current paths coupled between first and second logic nodes. One or more of the transistors may have a deeply depleted channel formed below its gate that includes a substantially undoped channel region formed over a relatively highly doped screen layer formed over a doped body region. Resulting reductions in threshold voltage variation may improve digital circuit performance. Logic circuit, static random access memory (SRAM) cell, and passgate embodiments are disclosed.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a divisional application of U.S. application Ser. No. 13/891,929 filed May 10, 2013 which is a continuation application of U.S. application Ser. No. 13/030,939 filed Feb. 18, 2013 which is hereby incorporated herein by reference.


TECHNICAL FIELD
The present invention relates generally to electronic circuits, and more particularly to digital circuits for processing and/or storing digital values.
BACKGROUND
Electronic circuits, typically incorporated within integrated circuit (IC) devices, determine the function of various electronic systems, ranging from large systems (such as computer server “farms” that enable the wide variety of Internet services and businesses, and can include hundreds or even thousands of server computers), to the small portable electronic devices such as cellular telephones.
Electronic circuits typically include transistors interconnected to one another to a same integrated circuit substrate and/or package. An integrated circuit (IC) substrate may be a single semiconductor substrate (e.g., die created by dividing a fabricated “wafer”) that includes circuit elements of the electronic circuit. An integrated circuit package may present a set of external connections, but include one or more ICs substrates and circuit components having conductive interconnections to one another.
Digital electronic circuits (hereinafter digital circuits) may form all or a portion of a large majority of circuits included within ICs. Digital circuits may receive and output digital values, typically binary values that vary between low and high logic levels.
Continuing goals for circuits (including digital circuits) include reductions in power consumption, improvements in performance, and reductions in area occupied by the circuit. Because ICs may employ vast numbers (up to millions) of digital circuits, even incremental reductions in power consumption may translate into significant power savings of devices or systems employing such circuits. In the case of large systems, reductions in power consumption can reduce power costs of an enterprise. In the case of portable electronic devices, reductions in power consumption can advantageously lead to longer battery life and/or the ability to provide additional functions for a given amount of charge.
Performance may include various aspects of circuit operation, including but not limited to: the speed at which data values transmitted and/or accessed by digital circuits. Improvements in signal propagation time (e.g., speed) may enable a device to increase the speed at which data is transmitted between locations of a device, thus reducing the time for the device to execute operations. In devices where data is stored, the speed at which data is written and/or read from storage locations may likewise improve device performance. Performance may also include circuit stability. Stability may be the ability of a circuit to provide a sufficient response under particular operating conditions.
Reductions in circuit size may directly translate into cost savings. In the case of ICs, reductions in size may allow more devices to fit on a fabrication substrate: As understood from above, digital circuits may occupy substantially all of the substrate area for some devices, and significant amount of are for others.
As device fabrication technologies approach limits to scaling (i.e., the ability to reduce circuit element sizes) the ability to further advance any of the goals noted above has grown increasingly costly or technically challenging.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an integrated circuit device according to an embodiment.
FIG. 2A shows a deeply depleted channel (DOC) transistor that may be included in embodiments.
FIG. 2B shows a conventional transistor.
FIGS. 3A and 3B show inverter circuits according to embodiments.
FIGS. 4A and 4B show NAND and NOR logic circuits according to embodiments.
FIG. 4C is a graph illustrating simulation results of a threshold voltage (Vt) of a DOC device at various bias voltages.
FIG. 5 shows a digital circuit according to an embodiment that includes serially connected digital stages.
FIG. 6 is a table showing signal propagation simulation results for a clock tree according to an embodiment and according to a conventional approach.
FIG. 7 shows a passgate circuit according to an embodiment.
FIG. 8 shows a flip-flop circuit according to an embodiment.
FIG. 9 shows a dynamic logic circuit according to an embodiment.
FIGS. 10A and 10B show one of many logic sections that can be included in the embodiment of FIG. 9.
FIG. 11 shows a current steering logic circuit according to an embodiment.
FIGS. 12A and 12B show one of many logic sections that can be included in the embodiment of FIG. 11.
FIG. 13 shows a latch circuit according to an embodiment.
FIG. 14 shows a six transistor (6-T) static random access memory (SRAM) cell according to an embodiment.
FIGS. 15A and 15B show simulation conditions and results for a 6-T SRAM cell according to one embodiment.
FIGS. 16A and 16B show simulation conditions and results for a conventional 6-T SRAM cell.
FIG. 17 shows a DOC transistor that may be included in embodiments.
FIG. 18 shows another DOC transistor that may be included in embodiments.
FIG. 19 is a top plan view of an IC device according to an embodiment having both DOC transistors and non-DOC transistors.



DETAILED DESCRIPTION
Various embodiments of the present invention will now be described in detail with reference to a number of drawings. The embodiments show digital circuits and related methods that may be included in integrated circuit devices to provide improved performance over conventional digital circuit approaches.
In the various embodiments below, like items are referred to by the same reference character but the leading digits corresponding to the figure number.
Referring now to FIG. 1 an integrated circuit (IC) device according to one embodiment is show in a top plan view, and designated by the general reference character 100. An IC device 100 may be formed as a “die” having substrate 101 containing the various circuits therein.
An IC device 100 may include one or more circuit sections, and FIG. 1 identifies four circuit sections as 102-0 to 102-3. Any or all of circuit sections (102-0 to 102-3) may include digital circuit blocks that perform digital functions for the IC device 100.
In the embodiment shown, circuit section 102-2 may be a digital circuit block that includes one or more digital circuits, one shown as 104. A digital circuit 104 may generate output signals on one or more output nodes (e.g., 108) in response to input signals received on one or more input nodes (e.g., 110). It is noted that in some embodiments, an output node and input node may be the same node.
Referring still to FIG. 1, a digital circuit 104 may include one or more “deeply depleted channel” (DDC) transistors. A DDC transistor includes both a highly doped “screening” layer below a gate that defines the extent of the depletion region below the gate in operation, and an undoped channel extending between source and drain of a transistor. Typically, to prevent contamination of the undoped channel, transistors are manufactured without halo or “pocket” implants, and anneal conditions are tightly controlled to prevent unwanted diffusion of dopants into the undoped channel. While conventional threshold voltage (Vt) implants are also avoided to prevent channel contamination, Vt set layers that are grown as blanket or selective epitaxial layers on the screening layer can be used to finely adjust or tune the threshold voltage of individual or blocks of transistors. Further examples of DDC transistor structure and manufacture are disclosed in U.S. patent application Ser. No. 12/708,497, filed on Feb. 18, 2010, titled ELECTRONIC DEVICES AND SYSTEMS, AND METHODS FOR MAKING AND USING THE SAME, by Scott E. Thompson et al., as well as U.S. patent application Ser. No. 12/971,884, filed on Dec. 17, 2010 titled LOW POWER SEMICONDUCTOR TRANSISTOR STRUCTURE AND METHOD OF FABRICATION THEREOF and U.S. patent application Ser. No. 12/971,955 filed on Dec. 17, 2010 titled TRANSISTOR WITH THRESHOLD VOLTAGE SET NOTCH AND METHOD OF FABRICATION THEREOF the respective contents of which are incorporated by reference herein.
DDC transistors included within a digital circuit may include n-channel transistors, p-channel transistors or both. N-channel DDC transistors will be represented in this disclosure by the symbol shown as 106-0 in FIG. 1. P-channel DDC transistors will be represented in this disclosure by the symbol shown as 106-1 in FIG. 1. DDC transistors may advantageously include a substantially undoped channel region formed over a relatively highly doped screening layer.
Specifically referring now to FIG. 2A, one exemplary representation of a DDC transistor is shown in a side cross sectional view, and designated by the general reference character 206. DDC transistor 206 may include a gate 212 separated from a substrate 224 by a gate insulator 222. A substantially undoped channel region 214 may be formed below gate 212. A doped screening layer 216 may be formed below channel region 214. It is understood that there may be other layers between channel region 214 and screening layer 216. A substrate 224 may be formed of more than one semiconductor layer. As but one example, a substrate may include one or more “epitaxial” layers formed on a bulk semiconductor substrate.
A screening layer 216 may be doped to an opposite conductivity type of the transistor channel type (e.g., an n-channel DDC transistor will have a p-doped screening layer). A screening layer 216 doping concentration may be greater than a concentration of a body region 218. FIG. 2A also shows source and drain regions 220 on opposing lateral sides of channel region 214. Source and drain regions 220 may include a source and drain diffusion. More particular types of DDC source and drain structures, relative to substantially undoped channel region will be described in more detail below.
Referring to FIG. 2B, one representation of a conventional transistor is shown for comparison to that shown in FIG. 2A. Conventional transistor 205 may include a gate 213 separated from a substrate 225 by a gate insulator 223. A channel region 215 may be formed below a gate 213 between source/drain diffusions 221. A channel region 215 may be doped to a conductivity type opposite to that of source/drain diffusions 221, and the same as that of a transistor body region 219.
In this way, a digital circuit may be formed with one or more DDC transistors.
Referring now to FIG. 3A one example of a digital circuit according to an embodiment is shown in a schematic diagram, and designated by the general reference character 304. Digital circuit 304 may be an inverter that inverts an input signal received on an input node 310 to generate an output signal OUT on an output node 308. Digital circuit 304 may include a pull-down transistor 326-0 and a pull-up transistor 326-1 that 30 drive output node 308 between a high logic level (e.g., VHI) and a low logic level (e.g., VLO). In the embodiment shown, pull-up transistor 326-1 may be an re-channel DDC transistor, while pull-up transistor 326-1 may optionally be a p-channel DDC transistor (as indicated by hashing). However, in alternate embodiments all or anyone of the transistors may be a DDC transistor. It is noted that FIG. 3A shows a “static” logic embodiment where a signal generated on output node 308 may have a value, a timing established by input signal IN.
In the embodiment of FIG. 3A, transistors 326-0/1 have source tied “bodies”. Thus, any screening region of a transistor may be driven with logic supply voltage (Le., a logic high or low voltage shown as VHI and VLO).
FIG. 3B shows a digital circuit 304′ having a configuration with similar connections to those of FIG. 3A. However, unlike FIG. 3A, transistors (326-2/3) of FIG. 3B may have body bias voltages different than corresponding drain voltages. Thus, a screening region of any such independent body transistor may be driven with a body bias voltage (shown as Pbias, Nbias) that may be different than drain voltages. A body bias voltage may be static or dynamic. In embodiments having DDC transistors of both conductivity types, one or both types of transistors may receive a body bias voltage. In embodiments having a DDC transistor and non-DDC transistor, one or both types of transistors may receive a body bias voltage.
In this way, an inverter may include one or more DDC transistors with or without separately biased bodies.
Referring now to FIGS. 4A and 4B, additional digital circuits according to embodiments are shown in block schematic diagrams, and designated by the general reference characters 404 and 404′, respectively. Digital circuit 404/404′ may include multiple transistors (426-0 to -3) interconnected to one another between a high logic node VHI and low logic node VLO. The transistor types and body connections may vary as noted for FIGS. 3A and 3B. In particular, any number (including all) transistors may be DDC transistors. Further, any number (including all) transistors (regardless of whether they DDC transistors or not), may have bodies connected to a logic high or low voltage, or may have bodies biased with a different voltage (such a body bias voltage being static or dynamic). FIG. 4A shows a static logic NAND gate, while FIG. 4B shows a static logic NOR gate. From these logic gates one skilled in the art could arrive at more complex logic circuits that may include DDC type transistors.
Digital circuits employing DDC transistors as described herein, and equivalents, may provide a wider range of performance modulation than conventional approaches. As noted above, in various logic circuits shown herein, bodies of DDC transistors may be biased with a voltage other than a logic high or logic low voltage. Such body biasing of DDC transistors may provide for greater variation in transistor threshold voltage per applied body bias, as compared to doped channel devices. One very particular example of such bias variation is shown in FIG. 4C.
FIG. 4C is a graph showing simulation results of a threshold voltage (Vt) of a DDC device at various bias voltages. Curve 400 represents results corresponding to a 500 nanometer (nm) gate length DDC transistor. Curve 402 shows a conventional 500 nm transistor response. It is understood that FIG. 4C is but one response, and such biasing response may vary according to device geometry, doping, screening channel position, any Vt adjustment layer, gate insulator thickness, gate material, to name but a few examples.
Digital circuits employing DDC transistor as described herein, may vary operations with body biasing. For example, higher body bias may be utilized to reduce power in standby states. Such body biasing may also vary with changes in logic levels, which may be a power supply voltage in some embodiments (e.g., VHI=VDD, VLO=VSS).
Referring now to FIG. 5, a digital circuit according to a further embodiment is shown in a block schematic diagram, and designated by the general reference character 504. A digital circuit 504 may include a series of stages 504-0 to -M, where each stage may be a logic circuit as described herein or an equivalent. One or more input signals (IN0 to INj) may be logically operated on, to propagate signals through stages (504-0 to -M) and generate one or more output stages OUT0 to OUTk. That is, each of stages (504-0 to -M) may drive nodes within between logic high (VHI) and logic low (VLO) levels.
Such digital circuits that include stages with DDC transistors may have various advantageous features over conventional digital circuits as would be understood by those skilled in the art in light of the various discussions herein. However, in particular embodiments, digital circuits that employ DDC devices may have less variation in response. Consequently, when such digital circuits include stages, as shown in FIG. 5, an average expected signal propagation time may exhibit less variation, and hence be faster.
FIG. 6 is a table showing simulation data comparing digital circuits according embodiments (DDC DEVICES) versus a corresponding conventional approach. The digital circuits of FIG. 6 may be signal distribution chains composed of a number of stages (60, 30, 10), where each stage includes an inverter. That is, the DDC DEVICES may be one implementation of that shown in FIG. 5, where each stage includes an inverter like that of FIG. 2A.
FIG. 6 shows results for high and low simulation temperatures (i.e., 0° C. and 85° C.) for signal chains of 60 stages, 30 stages and 10 stages. Results are given as 3σ distributions. As shown, distributions may be tighter in the DDC DEVICES cases. Such tighter distributions may allow for faster signal distribution and/or processing environments, improving a device or system performance.
In this way, digital circuits having series connected stages may include DDC devices to reduce signal propagation time.
While embodiments may include DDC transistors that drive nodes between high and/or a low logic levels, other embodiments may serve to pass signals from one node to another. One such embodiment is shown in FIG. 7.
Referring now to FIG. 7 one example of a passgate circuit according to an embodiment is shown in a schematic diagram, and designated by the general reference character 704. Passgate circuit 704 may receive an input signal (IN) on an input node 710 from a signal source 728 and pass such a signal to an output node 708. Such signals may vary between high and low logic levels (VHI and VLO). Passgate circuit 704 may include, first and second transistors 726-0 and 726-1 of different conductivity types having source-drain paths arranged in parallel to one another between an input node 710 and an output node 708. Transistors 726-0 and 726-1 may receive complementary enable signals EN and /EN at their gates, respectively.
It is understood that either (or both) transistors may be DDC transistors. Further, either or both transistors (regardless of whether they DDC transistors or not), may have bodies driven by high or low logic levels (VHI or VLO), or bodies static or dynamically biased to different voltages.
Embodiment may also include passgate and logic combinations. One such particular embodiment is shown in FIG. 8.
Referring now to FIG. 8, one example of a flip-flop circuit according to an embodiment is shown in a schematic diagram, and designated by the general reference character 804. Passgate circuit 804 may include two stages 804-0/1, each of which includes an input passgate 830-0 and clocked latch formed by a latch passgate 830-1 and cross coupled latch inverters 832-0/1. Stages 804-0/1 may be enabled on complementary clock duty cycles. That is, as data is input to stage 804-0, data may be latched in stage 804-1 (e.g., CLK low), and as data is latched in stage 804-0, it may be input to stage 804-1 (e.g., CLK high).
Any of passgates 830-0/1 may take the form of those passgate embodiments shown herein, or equivalents. Similarly, any of inverters 830-0/1 may take the form inverter embodiment shown herein, or equivalents. Accordingly, any or both of passgates 830-0/1 may include DDC transistors, or may have separately biased bodies. Further, any or all of inverters 830-0/1 may include one or more DDC transistors, with any such DDC transistor having a bodies tied to a logic level, or biased to some other voltage.
Embodiments like those of FIGS. 3A to 4B have shown “static” logic circuit in which an output value and timing may be established by input signals. Alternate embodiments may include “dynamic” logic approaches. In a dynamic logic embodiment, an output logic level may be determined according to input signals, however a timing of such an output signal may be established by one or more timing signals. Particular dynamic logic circuits embodiments are shown in FIGS. 9 to 10B.
Referring now to FIG. 9, one example of a dynamic logic circuit according to an embodiment is shown in a schematic diagram, and designated by the general reference character 904. A dynamic logic circuit 904 may include a precharge transistor 936, an evaluation transistor 938, and a logic section 934. A precharge transistor 936 may connect a precharge node 940 to a logic high level (VHI) in response to a timing signal (P/E) being low. An evaluation transistor 938 may connect a discharge node 942 to a low logic level (VLO) in response to the timing signal (P/E) being high. A logic section 934 may receive one or more input signals (IN0 to INj) and provide one or more output signals (OUT0 to OUTk). Input signals (IN0 to INj) may determine a state of any output signals (OUT0 to OUTk). However the timing of such a determination may be controlled according to timing signal (PIE).
While FIG. 9 shows precharge and evaluation transistors (936 and 938) being enabled/disabled in response to the same timing signal P/E, alternate embodiments may utilize separate signals to enable and disable such devices.
In some embodiments, a precharge transistor 936, an evaluation transistor 938, or both, may be DDC transistors. In addition or alternatively, a logic section 934 may include one or more DDC transistors. As in other embodiments above, DDC transistors may have logic level tied bodies, or bodies statically or dynamically biased to other levels.
Referring now to FIGS. 10A and 10B, logic sections according to particular embodiments that may included in a dynamic logic circuit, like that of FIG. 9, are shown in schematic diagrams.
Referring to FIG. 10A, a logic section 1034 may include a number of logic transistors 1040-0 to 1040-2 that can selectively connect an output node 1008 to a precharge node 940 or a discharge node 942 in response to input values (IN0 to IN2). Any or all of such transistors may be DDC transistors. In the embodiment of FIG. 10A, all logic transistors 1040-0 to -2 may have source-drain paths that are connected to a logic high or low level through a corresponding precharge or evaluation device (not shown in FIG. 10A). In the very particular embodiment shown, logic section 1034 may provide a logic function where an output signal OUT0=inverse [IN0+IN1+IN2].
Referring to FIG. 10B, a logic section 1034′ may include an evaluation section 1038 and an output inverter 1036. An evaluation section 1038 can selectively connect an input of inverter 1036 a precharge node 940 or a discharge node 942 in response to input values (IN0 to IN2), and according to a timing established by corresponding precharge or evaluation devices (not shown in FIG. 10B). Any or all the transistors within evaluation section 1038 may be DDC transistors. Output inverter 1036 may drive output node 1008′ according to an output of evaluation section 1038. In the very particular embodiment shown, logic section 1034′ may provide a logic function where an output signal OUT0=IN0*IN2+IN1.
One skilled in the art could arrive at various other logic functions according to teachings set forth.
In this way, dynamic logic circuits may include one or more DDC devices.
From the above examples, one skilled in the art would recognize that digital circuits according to the embodiments may include logic circuit conventions beyond static and dynamic approaches. As but one example, other embodiments may include “current steering” logic approaches. In a current steering embodiment, an output logic level may be determined by steering current from two current paths according to received input signals. Particular current steering logic circuit embodiments are shown in FIGS. 11 to 12B.
Referring now to FIG. 11, one example of a current steering circuit according to an embodiment is shown in a block schematic diagram, and designated by the general reference character 1104. A current steering logic circuit 1104 may include respective output nodes 1108-0 and 1108-1, a first current source 1144-0, a second current source 1144-1, a current sink 1146, and a logic section 1134. First and second current sources 1144-0/1 may be connected in parallel between a logic high node VHI and a logic section 1134. First current source 1144-0 may provide a current to logic section 1134 via a first input current path 1148-0, and second current source 1144-1 may provide a current to logic section 1134 via a second input current path 1148-1. According to input values (in this embodiment, IN0 to INj and their complements), a logic section 1134 may steer current from either input current path 1148-0/1 to current sink 1146, and thus generate complementary output signals on such current paths 1148-0/1. A current sink 1146 may provide a current path to a low voltage node VLO through sink current path 1150.
In the particular embodiment of FIG. 11, first and second current sources 1144-0/1 may be p-channel transistors, which in particular embodiments may be DDC transistors. Similarly, current sink 1146 may be an re-channel transistor, which may in particular embodiments, be a DDC transistor. As in other embodiments above, such DDC transistors may have bodies tied to logic levels, or have bodies driven by another voltage, either statically or dynamically.
Referring now to FIGS. 12A and 12B, logic sections according to particular embodiments that may be included in a current steering logic circuit, like that of FIG. 11, are shown in schematic diagrams.
Referring to FIG. 12A, a logic section 1234 may include logic transistors 1240-0 and 1240-1 that can selectively steer either of input current paths 1148-0/1 to current sink path 1150 according to an input values (IN0 and its complement). When current is steered down one path and not the other, complementary output values may be generated at input current paths 1148-0/1. Any or all of such transistors may be DDC transistors subject to the various body biasing configurations noted herein. In the embodiment of FIG. 12A, logic section 1234 may provide a logic function of a buffer or an inverter, depending upon which input values and output nodes are considered.
Referring to FIG. 12B, a logic section 1234′ may include a more complex arrangement of logic transistors 1240-2 to 1240-5. However, operations may occur in the same generation fashion as FIG. 12A, with such transistors (1240-2 to 1240-5) selectively steering current from one of steering current paths 1148-0/1 to current sink path 1150, to generate complementary output values at input current paths 1148-0/1. Any or all of such transistors may be DDC transistors subject to the various body biasing configurations noted herein. In the embodiment of FIG. 12B, logic section 1234′ may provide a NAND, NOR, AND or OR function depending upon which input values and output nodes are considered.
In this way, current steering logic circuits may include one or more DDC devices.
While the flip-flop embodiments shown above may store data values, embodiments may include more compact digital data storage circuits. In particular, embodiments may include latches, and in particular embodiments, latches and memory cells with symmetrical matching devices.
Referring now to FIG. 13, a latch according to an embodiment is shown in a schematic diagram and designated by the general reference character 1300. A latch 1300 may include driver transistors 1354-0/1 and load devices 1356-0/1. Driver transistors 1354-0/1 may be cross-coupled between complementary data nodes 1352-0/1, having source-drain paths connected to a first logic level (in this case VLO), with a gate of one transistor being connected to the drain of the other. Load devices 1356-0/1 may be passive or active devices, connected in parallel between data nodes 1352-0/1 and second logic level (in this case VHI).
Driver transistors 1354-0/1 may be DDC transistors, and in particular embodiments, matching DDC transistors. DDC driver transistors may have bodies driven to logic levels, or to some other bias voltage, dynamically and/or statically.
A latch 1300 may store a data value on complementary data nodes 1352-0/1, and may form part of various memory cell types, including but not limited to four transistor (4T), 6T, and 8T static random access memory (SRAM) cells, to name but a few. Further, while FIG. 13 shows a latch with n-channel driver transistors, alternate embodiments may include p-channel driver transistors.
In this way, a latch circuit may include DDC driver devices.
Referring now to FIG. 14, a 6-T SRAM cell according to one embodiment is shown in schematic diagram and designated by the general reference character 1400. An SRAM cell 1400 may include items like those shown in FIG. 13, and such like items are referred to by the same reference character.
FIG. 14 differs from FIG. 13 in that load devices may be p-channel transistors 1456-0/1 cross coupled to one another. Further, n-channel access transistors 1458-0 and 1458-1 may connect bit lines 1460-0 and 1460-1 to data nodes 1352-0 and 1352-1, respectively. In the embodiment of FIG. 14, all transistors may have bodies connected to a logic high or logic low level, according to conductivity type. Further all transistors may be DDC transistors. Still further, transistors may match one another in a symmetrical fashion. That is, transistors 1456-0, 1458-0, and 1354-0 may be the same size as, and fabricated in the same fashion as corresponding transistors 1456-1, 1458-1, and 1354-1 respectively.
As will be described in more detail below, DDC transistors, by employing a substantially undoped channel, may provide less threshold variation than conventional transistors, as such channels are less (or not) susceptible to random doping fluctuation (RDF). Consequently, a symmetrical latching structure may provide performance advantages over conventional latch circuits having doped channels subject to RDF.
Referring now to FIGS. 15A and 15B, a response of a 6-T SRAM cell according to an embodiment is shown. FIG. 15A is a schematic diagram showing a 6-T cell like that of FIG. 14 under static noise margin (SNM) simulation conditions. Transistors in such a 6-T SRAM cell may have gate lengths of 28 nm. In the particular conditions shown, bit lines may held at 0.7 volts, access devices may be driven to 1.0 V, and a high voltage (VHI) is 1.0 V, while a low voltage (VLO) is 0 V. A sweeping voltage (Vsrc) is applied between VLO and a data storage node that transitions from 0 V to 1.0 V, and then back again.
FIG. 15B is a graph showing a response of the 6-T SRAM cell of FIG. 15 under the noted simulation conditions. FIG. 15B shows two response variation ranges 1564 and 1562 showing responses to different sweep directions of Vsrc. As shown, responses 1564 and 1562 include “eye” regions 1566-0 and 1566-1 that indicate stable switching between states.
Referring now to FIGS. 16A and 16B, a response of a 6-T SRAM cell is shown. FIG. 16A is a schematic diagram showing a 6-T cell that, unlike the embodiment of FIGS. 15A and 15B, includes transistors with doped channels. Such doped channels are subject to RDF, as noted above, resulting in greater response variation. The conventional 6-T SRAM cell is subject to the same simulation conditions as FIG. 15A, and the transistors may also have gate lengths of 28 nm.
FIG. 16B is a graph showing a response of a conventional 6-T SRAM cell under the same simulation conditions as FIG. 15B. Like FIG. 15B, FIG. 16B shows two response variation ranges 1664 and 1662 to the sweeping of Vsrc. However, due to RDF, resulting threshold voltage variations translate into a wider range of responses, resulting eye regions 1666-0 and 1666-1 substantially smaller than those of FIG. 15B.
As noted in conjunction with FIG. 2A, a DDC transistor may take various forms. A DDC transistor according to one very particular embodiment will now be described with reference to FIG. 17. Such a transistor may be included in any of the embodiments shown above, or equivalents.
Referring to FIG. 17, a DDC according to a very particular embodiment is shown in a side cross sectional view.
As shown in FIG. 17, a DDC transistor 1760 may include a gate 1712 separated from a substrate 1724 by a gate insulator 1722. A gate 1712 may include insulating sidewalls 1768 formed on its sides. Source and drain regions may include a lightly doped drain (LDD) structures 1776 formed over deep source/drain diffusions 1774 to extend towards each other under a portion of the gate. A DDC stacked channel structure may be formed by a substantially undoped channel layer 1714, a threshold voltage (Vt) set layer 1770 formed by epitaxial growth and implant, or alternatively, by controlled out-diffusion from a screening layer 1716 positioned below the undoped channel layer 1714. The screening layer 1716 acts to define termination of the depletion zone below the gate, while the Vt set layer 1770 adjusts Vt to meet transistor design specifications. In the embodiment shown, screening layer 1716 may be implanted into body/bulk region 1718 so that it extends between and in contact with the source and drain diffusions 1774.
In a very particular embodiment, a DDC transistor 1760 may be an n-channel transistor having a gate length 1778 of 28 nm or less. The screening layer 1716 may have a carrier concentration of greater than about 5×1018 donors/cm3, while an overlying Vt set layer 1770 may have a carrier concentration of about 5×1017 to about 5×1018 donors/cm3. A substantially undoped channel region 1714 may have a carrier concentration of less than about 5×1017 donors/cm3. It is understood that the above noted carrier concentrations are provided by way of example only and alternate embodiments may include different concentrations according to desired performance in a digital circuit.
A DDC transistor according to a further embodiment is shown in FIG. 18, and designated by the general reference character 1860. A DDC transistor 1860 may include items like those shown in FIG. 17B, and like items are referred to by the same reference character. DDC transistor 1860 differs from that of FIG. 17B in that screening layer 1716 may be implanted into body/bulk region 1718 so that it extends below the gate without contacting the source and drain diffusions 1774. The above DDC transistors are but particular implementations of a DDC transistor, and should not construed as unduly limiting the circuit elements included within the various digital circuit embodiments shown herein.
As noted above, some embodiments may include DDC transistors and conventional doped channel transistors. FIG. 19 shows a portion of an IC substrate containing such two different types of transistors.
Referring to FIG. 19, an IC device 1900 may include DDC transistor active regions (one shown as 1982-0) and conventional transistor active regions (one shown as 1982-1) formed in a substrate and separated from one another by isolation structure 1972. A DDC transistor active region 1982-0 may include stacked channel structures formed below a control gate, as described herein, to form one or more DDC transistors (one shown as 1906). A conventional transistor active region 1982-1 may include a doped channel formed below a control gate to form one or more conventional transistors (one shown as 1980). Both types of transistors (e.g., 1982-0 and 1982-1) may form all or part of a digital circuit as described herein, or equivalents.
In this way, an IC device having digital circuits may include both DDC transistors and non-DDC transistors. Alternatively, selective masking to block out areas of a die for manufacture of DDC or non-DDC transistors can be employed, or any other conventional technique for manufacturing die having at least some DDC transistors. This is particularly useful for mixed signal die having multiple transistors types, including high speed digital logic and analog transistors, as well as power efficient logic and/or memory transistors.
Digital circuits according to embodiments shown herein, and equivalents, may provide improved performance over conventional circuits by operating with transistors (e.g., DDC transistors) having lower threshold voltage (Vt) variability. Possible improvements may include faster signal propagation times, as noted above.
Improved performance may translate into reductions in device size. As but one example, digital circuit transistors may be sized with respect to one another to achieve a particular response. Such sizing may take into account expected variations in Vts. Because DDC transistors have lower Vt variation, less sizing margin may be necessary to achieve a desired response. As but one very particular example, SRAM cells may have a predetermined sizing between access transistors and driver transistors. SRAM cells according to the embodiments may lower a relative size scaling between such devices, relative to comparably sized conventional transistors. As SRAM cells may be repeated thousands, or even millions of times in a device, reductions in size by extend beyond expected limits presented by conventional arrays incorporating SRAM cells with doped channels.
In addition, such improvements may include lower operating voltages. In the embodiments, digital circuit switching voltages, established by transistor Vts, may be subject to less variability. Accordingly, a “worst” switching point may be lower, allowing for an operating voltage to be correspondingly lower. In some embodiments, operating voltages (Vsupply) may be no greater than 1 V, and a threshold voltage may be no greater than 0.6*Vsupply.
As noted above, in some embodiments digital circuits may include DDC transistors body bias connections driven with a bias voltage different than a logic high or low voltage. A screening layer within such transistors may enable higher body effect for modulating threshold voltage. In such embodiments, a variation in threshold utilizing a body effect may be achieved with a lower body bias voltage than conventional transistors. Body effect modulation may enable bodies to be driven to reduce threshold voltage, and hence reduce leakage.
Digital circuits according to embodiments may have lower power consumption than circuits employing conventional doped channels. As noted above, because a worst case threshold voltage variation may be low, a power supply voltage may be reduced, which may reduce power consumption. In addition, substantially undoped channels in DDC devices may have improved mobility as compared to some conventional transistors, and hence provide lower channel resistance.
It should be appreciated that in the foregoing description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.
It is also understood that the embodiments of the invention may be practiced in the absence of an element and/or step not specifically disclosed. That is, an inventive feature of the invention may be elimination of an element.
Accordingly, while the various aspects of the particular embodiments set forth herein have been described in detail, the present invention could be subject to various changes, substitutions, and alterations without departing from the spirit and scope of the invention.

## Claims

1. A digital logic circuit, comprising:
a plurality of MOSFET transistors coupled by way of a plurality of controllable current paths to at least a first and second logic node to produce a predefined logic function: wherein the plurality of transistors includes at least one precharge transistor that couples a precharge node to the first logic node in response to a first clock signal;
wherein the plurality of transistors includes at least one evaluation transistor that couples a discharge node to the second logic node in response to a second clock signal;
wherein the plurality of transistors are configured to selectively couple an output node to the first or second logic node in response to at least one input signal, the plurality of transistors having gates coupled to receive at least one input signal and source-drain paths coupled to the at least one output node;
wherein each of the plurality of transistors has a deeply depleted channel formed below its gate that includes a substantially undoped channel region formed fully over a relatively highly doped screening layer formed fully over and in contact with a doped body region.

2. The circuit of claim 1, wherein at least some of the plurality of transistors have a strong body coefficient sufficient to receive a bias signal to a transistor body.

3. The circuit of claim 2, wherein each of the plurality of transistors further includes a heavily doped screen layer below the blanket epitaxial layer.

4. The circuit of claim 3, wherein the digital logic circuit is configured to provide and receive signals from a static random access memory cell.

5. The circuit of claim 1, wherein each of the plurality of transistors includes a well having a preselected dopant profile, the dopant profile configured so that a predefined portion of the channel remains substantially undoped, the preselected dopant profile having a notched profile in the continuum of dopant concentration.