U.S. patent application number 14/268183 was filed with the patent office on 2016-08-11 for integrated circuits with asymmetric and stacked transistors.
The applicant listed for this patent is Altera Corporation. Invention is credited to Andy L. Lee, Shih-Lin S. Lee, Jun Liu, Srinivas Perisetty, Irfan Rahim, Albert Ratnakumar, Shankar Sinha, Jeffrey Xiaoqi Tung, Jeffrey T. Watt, Qi Xiang, Yanzhong Xu.
Application Number | 20160232952 14/268183 |
Document ID | / |
Family ID | 54355696 |
Filed Date | 2016-08-11 |
United States Patent
Application |
20160232952 |
Kind Code |
A9 |
Liu; Jun ; et al. |
August 11, 2016 |
Integrated Circuits with Asymmetric and Stacked Transistors
Abstract
Asymmetric transistors may be formed by creating pocket implants
on one source-drain terminal of a transistor and not the other.
Asymmetric transistors may also be formed using dual-gate
structures having first and second gate conductors of different
work functions. Stacked transistors may be formed by stacking two
transistors of the same channel type in series. One of the
source-drain terminals of each of the two transistors is connected
to a common node. The gates of the two transistors are also
connected together. The two transistors may have different
threshold voltages. The threshold voltage of the transistor that is
located higher in the stacked transistor may be provided with a
lower threshold voltage than the other transistor in the stacked
transistor. Stacked transistors may be used to reduce leakage
currents in circuits such as memory cells. Asymmetric transistors
may also be used in memory cells to reduce leakage.
Inventors: |
Liu; Jun; (Milpitas, CA)
; Xu; Yanzhong; (Santa Clara, CA) ; Sinha;
Shankar; (San Jose, CA) ; Lee; Shih-Lin S.;
(San Jose, CA) ; Tung; Jeffrey Xiaoqi; (Palo Alto,
CA) ; Ratnakumar; Albert; (San Jose, CA) ;
Xiang; Qi; (San Jose, CA) ; Rahim; Irfan;
(Milpitas, CA) ; Lee; Andy L.; (San Jose, CA)
; Watt; Jeffrey T.; (Palo Alto, CA) ; Perisetty;
Srinivas; (Santa Clara, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Altera Corporation |
San Jose |
CA |
US |
|
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Prior
Publication: |
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Document Identifier |
Publication Date |
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US 20150318029 A1 |
November 5, 2015 |
|
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Family ID: |
54355696 |
Appl. No.: |
14/268183 |
Filed: |
May 2, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13923276 |
Jun 20, 2013 |
8750026 |
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14268183 |
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12629831 |
Dec 2, 2009 |
8482963 |
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13923276 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G11C 7/1051 20130101;
H01L 29/1045 20130101; H01L 29/66659 20130101; G11C 5/06 20130101;
G11C 11/412 20130101; H01L 29/4983 20130101; H01L 21/266 20130101;
H01L 27/1104 20130101; H01L 21/82345 20130101; H01L 21/26586
20130101; H01L 29/6659 20130101; H01L 29/66492 20130101 |
International
Class: |
G11C 7/10 20060101
G11C007/10; G11C 5/06 20060101 G11C005/06 |
Claims
1. An integrated circuit, comprising: an array of memory cells
loaded with respective data bits, each memory cell having an output
at which a static output signal is produced based on the data bit
loaded into that memory cell; and programmable circuitry including
a plurality of programmable transistors, each programmable
transistor having a gate that is electrically connected to a
respective one of the outputs to receive a respective one of the
static output signals, wherein the memory cells each include at
least one asymmetric transistor that exhibits a strong mode of
operation and a weak mode of operation.
2. The integrated circuit defined in claim 1 wherein the asymmetric
transistor of each memory cell comprises an address transistor
having first and second source-drains of a first doping type and
having an energy-barrier-inducing implant at the second source
drain, wherein the energy-barrier-inducing implant has a second
doping type opposite to the first doping type.
3. The integrated circuit defined in claim 2 wherein the first
doping type comprises n-type and wherein the second doping type
comprises p-type.
4. The integrated circuit defined in claim 1 wherein the memory
cells each include at least one stacked transistor.
5. The integrated circuit defined in claim 1 wherein each of the
memory cells includes two inverters, wherein each of the inverters
has a p-channel transistor connected in series with a pair of
n-channel transistors, and wherein each pair of n-channel
transistors includes first and second series-connected n-channel
transistors with different threshold voltages.
6. The integrated circuit defined in claim 1 wherein each of the
memory cells includes a bistable element formed from four
interconnected inverter-like circuits.
7. The integrated circuit defined in claim 6 wherein each
inverter-like circuit includes a p-channel transistor and an
n-channel transistor connected in series.
8. The integrated circuit defined in claim 1 wherein each of the
memory cells includes a bistable element having at least eight
transistors.
Description
[0001] This application is a divisional of patent application Ser.
No. 13/923,276, filed Jun. 20, 2013, which is a divisional of
patent application Ser. No. 12/629,831, filed Dec. 2, 2009, which
are hereby incorporated by reference herein in their entirety.
BACKGROUND
[0002] This invention relates to integrated circuits, and more
particularly, to circuitry such as memory circuitry that may
incorporate asymmetric transistors and stacked transistors.
[0003] There is a trend with each successive generation of
integrated circuit technology to scale transistors to smaller
sizes, smaller threshold voltages, and smaller power supply
voltages. Made properly, these adjustments allow improved
performance and lowered costs. Care must be taken, however, to
avoid issues such as excessive power consumption.
[0004] One aspect of lowering power consumption on an integrated
circuit relates to transistor leakage currents. Leakage currents
are undesired currents that flow between the terminals of a
transistor during normal operation. An ideal transistor would
exhibit no leakage. In the real world, however, leakage currents
are unavoidable and must be minimized as best possible. If leakage
currents are too high, a circuit may exhibit unacceptably large
static power consumption. Particularly in circuits with large
numbers of transistors, such as modern integrated circuits that
include memory cells, leakage current minimization can be highly
beneficial. Although leakage currents can be reduced by limiting
power supply voltages and using less leaky transistors such as
transistors with increased threshold voltages and reduced gate
widths, these approaches tend to slow circuit speed and may not be
acceptable for many designs.
[0005] It would therefore be desirable to be able to provide
transistor structures that help improve the tradeoffs that exist
between transistor performance and leakage current reduction.
SUMMARY
[0006] An integrated circuit may be provided with asymmetric
transistors and stacked transistors. Memory cells and other
circuits in the integrated circuit may be provided with asymmetric
transistors. For example, asymmetric transistors may be used as
address transistors. Memory cells and other circuits in the
integrated circuit may also be provided with stacked transistors.
For example, stacked transistors may be used to replace some or all
of the transistors used in forming bistable elements for memory
cells.
[0007] Asymmetric transistors may be formed by creating an energy
barrier at one source-drain terminal of a transistor and not the
other. The energy barrier may be formed by creating a dopant
implant of opposite doping type to that of the source-drain.
Asymmetric transistors may also be formed using dual-gate
structures having first and second gate conductors of different
work functions.
[0008] Stacked transistors may be formed by stacking two
transistors of the same channel type in series (i.e., two n-channel
devices or two p-channel devices). One of the source-drain
terminals of each of the two transistors is connected to a common
node. The gates of the two transistors are also connected together.
The two transistors may have different threshold voltages. The
threshold voltage of the transistor that is located higher in the
stacked transistor may be provided with a lower threshold voltage
than the other transistor in the stacked transistor.
[0009] Further features of the invention, its nature and various
advantages will be more apparent from the accompanying drawings and
the following detailed description of the preferred
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a diagram of an illustrative integrated circuit
that may contain transistor structures in accordance with an
embodiment of the present invention.
[0011] FIG. 2 is a diagram of an illustrative array of memory cells
in accordance with an embodiment of the present invention.
[0012] FIGS. 3, 4, and 5 are cross-sectional side views of an
asymmetric transistor that has an energy-barrier-inducing pocket
implant during three successive stages of fabrication in accordance
with an embodiment of the present invention.
[0013] FIG. 6 is a cross-sectional side view of an asymmetric
transistor having a dual gate structure in accordance with an
embodiment of the present invention.
[0014] FIG. 7 is a band diagram of an asymmetric transistor showing
how an energy barrier is formed at one of the source-drains of the
transistor in accordance with an embodiment of the present
invention.
[0015] FIG. 8 is a band diagram showing how an asymmetric
transistor operates in a strong mode when driving a logic zero onto
a logic one to overwrite the one with the zero in accordance with
an embodiment of the present invention.
[0016] FIG. 9 is a band diagram showing how an asymmetric
transistor operates in a weak mode when driving a logic one onto a
zero to overwrite the zero with the one in accordance with an
embodiment of the present invention.
[0017] FIG. 10 is a graph showing how the amount of current that
flows through an asymmetric transistor as a function of applied
gate voltage varies as a function of whether the asymmetric
transistor is being operated in a strong mode or a weak mode in
accordance with an embodiment of the present invention.
[0018] FIG. 11 is a diagram of an illustrative memory cell with a
single-ended writing and single-ended reading configuration in
accordance with an embodiment of the present invention.
[0019] FIG. 12 is a diagram of an illustrative memory cell with a
differential writing and single-ended reading configuration in
accordance with an embodiment of the present invention.
[0020] FIG. 13 is a diagram of an illustrative n-channel transistor
formed from a pair of stacked transistors in accordance with an
embodiment of the present invention.
[0021] FIG. 14 is a diagram of an illustrative p-channel transistor
formed from a pair of stacked transistors in accordance with an
embodiment of the present invention.
[0022] FIG. 15 is a graph showing how leakage currents can be
minimized using a stacked transistor in accordance with an
embodiment of the present invention.
[0023] FIG. 16 is a diagram of an illustrative memory cell based on
four interconnected inverter-like circuits of the type that may
include stacked transistors and asymmetric transistors in
accordance with an embodiment of the present invention.
[0024] FIG. 17 is a diagram of a memory cell showing how the
n-channel inverter transistors may be implemented using stacked
transistors and how the address transistors can be implemented
using asymmetric transistors in accordance with an embodiment of
the present invention.
DETAILED DESCRIPTION
[0025] An illustrative integrated circuit that may contain
asymmetric and stacked transistors is shown in FIG. 1. As shown in
FIG. 1, integrated circuit 10 may include memory elements (cells)
20.
[0026] Memory elements 20 can be used in any suitable integrated
circuits that use memory. These integrated circuits may be memory
chips, digital signal processing circuits with memory arrays,
microprocessors, application specific integrated circuits with
memory arrays, programmable integrated circuits such as
programmable logic device integrated circuits in which memory
elements are used for configuration memory, or any other suitable
integrated circuit. For clarity, the use of memory elements 20 is
sometimes be described in the context of programmable integrated
circuits such as programmable logic device integrated circuits.
This is, however, merely illustrative. Memory cells 20 and the
asymmetric and stacked transistors in such memory cells may be used
in any suitable circuits.
[0027] On integrated circuits such as memory chips or other
circuits in which memory is needed to store processing data, memory
elements 20 can be used to perform the functions of static
random-access memory (RAM) cells and are sometimes referred to as
SRAM cells. In the context of programmable logic device integrated
circuits, memory elements 20 can be used to store configuration
data and are therefore sometimes referred to in this context as
configuration random-access memory (CRAM) cells.
[0028] As shown in FIG. 1, device 10 may have input/output
circuitry 12 for driving signals off of device 10 and for receiving
signals from other devices via input/output pins 14.
Interconnection resources 16 such as global and local vertical and
horizontal conductive lines and busses may be used to route signals
on device 10. Interconnection resources 16 include fixed
interconnects (conductive lines) and programmable interconnects
(i.e., programmable connections between respective fixed
interconnects). Programmable logic 18 may include combinational and
sequential logic circuitry. The programmable logic 18 may be
configured to perform a custom logic function. The programmable
interconnects associated with interconnection resources may be
considered to be a part of programmable logic 18.
[0029] Integrated circuit 10 may contain volatile memory elements
20. For example, integrated circuit 10 may be a programmable
integrated circuit such as a programmable logic device integrated
circuit that can be loaded with configuration data (also called
programming data) using pins 14 and input/output circuitry 12. Once
memory elements 20 are loaded in this way, the memory elements each
provide a corresponding static control output signal that controls
the state of an associated logic component in programmable logic
18. If desired, memory elements 20 may be used in SRAM-type memory
arrays (e.g., to store data for processing circuitry during
operation of device 10).
[0030] Each memory element 20 may be formed from a number of
transistors configured to form a bistable circuit (i.e., a
latch-type circuit). True and complement data storage nodes in the
bistable circuit element can store corresponding true and
complement versions of a data bit.
[0031] A bistable circuit element may be based on any suitable
number of transistors. For example, the bistable portion of each
memory element may be formed from cross-coupled inverters, from
groups of multiple inverter-like circuits (e.g., in a distributed
configuration that provides enhanced immunity from soft-error-upset
events, etc.).
[0032] Arrangements with bistable elements formed from
cross-coupled inverter pairs are sometimes described herein as an
example. This is, however, merely illustrative. Memory elements 20
may be formed using any suitable memory cell architecture.
[0033] With one suitable approach, complementary
metal-oxide-semiconductor (CMOS) integrated circuit technology is
used to form the memory elements 20, so CMOS-based memory element
implementations are described herein as an example. If desired,
other integrated circuit technologies may be used to form the
memory elements and the integrated circuit in which the memory
elements are used to form memory arrays.
[0034] The memory elements may be loaded from any suitable source
of data. As an example, memory elements 20 may be loaded with
configuration data from an external erasable-programmable read-only
memory and control chip or other suitable data source via pins 14
and input/output circuitry 12. Loaded CRAM memory elements 20 may
provide static control signals that are applied to the terminals
(e.g., gates) of circuit elements (e.g., metal-oxide-semiconductor
transistors) in programmable logic 18 to control those elements
(e.g., to turn certain transistors on or off) and thereby configure
the logic in programmable logic 18. The circuit elements may be
transistors such as pass transistors, parts of multiplexers,
look-up tables, logic arrays, AND, OR, NAND, and NOR logic gates,
etc.
[0035] The memory elements 20 may be arranged in an array pattern.
In a typical modern integrated circuit such as a programmable
integrated circuit, there may be millions of memory elements 20 on
each chip. During programming operations, the array of memory
elements is provided with configuration data by a user (e.g., a
logic designer). Once loaded with configuration data, the memory
elements 20 produce static control signals at their outputs that
selectively control portions of the circuitry in the programmable
logic 18 and thereby customize its functions so that it will
operate as desired.
[0036] The circuitry of device 10 may be organized using any
suitable architecture. As an example, the logic of device 10 may be
organized in a series of rows and columns of larger programmable
logic regions each of which contains multiple smaller logic
regions. The logic resources of device 10 may be interconnected by
interconnection resources 16 such as associated vertical and
horizontal conductors. These conductors may include global
conductive lines that span substantially all of device 10,
fractional lines such as half-lines or quarter lines that span part
of device 10, staggered lines of a particular length (e.g.,
sufficient to interconnect several logic areas), smaller local
lines, or any other suitable interconnection resource arrangement.
If desired, the logic of device 10 may be arranged in more levels
or layers in which multiple large regions are interconnected to
form still larger portions of logic. Still other device
arrangements may use logic that is not arranged in rows and
columns.
[0037] When memory elements 20 are arranged in an array, horizontal
and vertical conductors and associated loading circuitry may be
used to load the memory elements with configuration data. Any
suitable memory array architecture may be used for memory elements
20. One suitable arrangement is shown in FIG. 2.
[0038] As shown in FIG. 2, control circuitry 24 may be used to read
and write data from memory cells 20 in memory cell array 28. When
array 28 is being used as regular SRAM, data write operations may
be performed when it is desired to store processing results and
data read operations may be performed when it is desired to
retrieve stored data. When array 28 is being used as CRAM, data
write operations may be performed to load configuration data and
data read operations may be performed to confirm that configuration
data loading operations have been performed successfully. In CRAM
environments and certain SRAM environments, read speeds may be less
critical than in conventional SRAM environments, because read
operations may be performed relatively infrequently (e.g., mostly
when confirming that configuration data has been written properly
into a CRAM array). During normal operation of a circuit contains
an array of CRAM cells (i.e., when CRAM cells are neither being
written to or read from), the static output of each CRAM cell can
be used to control a respective programmable logic component such
as a transistor. For example, the output signal OUT from each cell
20 may be provided to the gate G of a corresponding pass transistor
such as n-channel metal-oxide-semiconductor transistor 40 (i.e.,
programmable logic transistors such as transistor 40 in blocks of
programmable logic 18).
[0039] Array 28 may include rows and columns of memory cells 20. In
the example of FIG. 2, there are three rows and three columns of
memory elements 20 in array 28. This is merely illustrative.
Integrated circuits such as integrated circuit 10 may have any
suitable number of memory elements 20. A typical memory array
might, as an example, have thousands or millions of memory elements
20 arranged in hundreds or thousands or rows and columns.
[0040] Lines such as lines 32, 34, 36, and 38 may be used to
distribute signals in array 28. One or more lines per column such
as lines 34 may be used to load data into cells 20 and may be used
to read data out from cells 20. Lines 34 may sometimes be referred
to as data lines or bitlines. One or more lines 34 per row may be
used to convey address signals and may sometimes be referred to as
address lines or word lines. In some array configurations, the
cells of the array may be cleared (e.g., during power-up
operations). Array 28 may be cleared by writing zeros into the
array using through the data lines or by asserting one or more
global clear signals using a global (or nearly global) network of
clear lines such as clear lines 36.
[0041] During data writing operation, write drivers in circuitry 24
may supply data to array 28 on lines 34 (e.g., in appropriate
columns of the array) while appropriate address lines are asserted
to define the desired array location (i.e., the appropriate rows of
the array) to which the data is to be written. During read
operations, appropriate address lines are asserted to define the
desired array location from which data is to be read (i.e.,
appropriate rows) while the outputs of appropriate data lines 34
are monitored (e.g., using sense amplifiers). Single-ended and
differential schemes may be used for reading and/or writing. In
differential write schemes, a pair of true and complement data
lines are used. In differential read schemes, a differential sense
amplifier may be used in reading signals from a pair of true and
complement data lines.
[0042] In circuits such as circuit 10, asymmetric transistors
and/or stacked transistors can be used to improve the tradeoff
between transistor performance (i.e., drive strength and therefore
speed) and leakage current (i.e., static power consumption). These
arrangements may be used to improve transistor speed while
maintaining existing levels of leakage current performance or even
somewhat relaxing leakage current performance. These arrangements
may also be used to improve leakage current performance while
maintaining or even somewhat reducing transistor performance.
Arrangements in which leakage current and transistor performance
are simultaneously improved are also possible.
[0043] A metal-oxide-semiconductor (MOS) transistor has four
terminals: a source, a drain, a gate that overlies a channel
region, and a body. The source and drain are sometimes collectively
referred to as source-drain terminals. By convention, the drain of
an MOS transistor is typically the source-drain terminal that is
biased high, whereas the source is grounded or biased at a lower
voltage. Because the labels "source" and "drain" may therefore be
context-sensitive, it may sometimes be clearest to refer to the
both the source and the drain of a MOS transistor as being
"source-drain" terminals or "source-drains."
[0044] In a symmetric transistor, the source-drain terminals of the
transistors are substantially identical. It therefore does not
matter whether the source-drain terminals of a symmetrical
transistor are reversed, as performance will not significantly
change. In an asymmetric transistor, however, there is an energy
barrier at one of the source-drain terminals that is not present at
the other of the source-drain terminals. This leads to different
performance characteristics depending on how the transistor is
operated.
[0045] Asymmetric transistors may be formed by adjusting the sizes,
shapes, and materials of the structures that make up the
transistor. With one suitable arrangement, an asymmetric transistor
is formed by making a one-sided energy-barrier-inducing pocket
implant. This type of approach is shown in FIGS. 3, 4, and 5.
[0046] Initially, as shown in FIG. 3, a gate conductor GC is formed
on a gate insulator GI and a low density ion implantation process
is used to implant dopant to form source-drains SD1 and SD2 in
substrate SUB. Gate conductor GC may be formed from doped
polysilicon, metal, or other suitable conductive materials. Gate
insulator GI may be formed from silicon dioxide, a high-K material
such as a hafnium-based oxide, other oxides, nitrides, and
oxynitrides, or other suitable insulating material. Substrate SUB
may be formed from a semiconductor such as silicon (e.g., silicon
in a bulk silicon wafer, a layer of epitaxial silicon on a bulk or
silicon-on-insulator wafer, etc.). Substrates of other
semiconductors may also be used.
[0047] As shown in FIG. 4, a blocking layer BL may be formed over
one of the source-drain terminals. Blocking layer BL may be formed
from any suitable type of material. For example, blocking layer BL
may be formed from a layer of photolithographically patterned
photoresist. During a subsequent ion implantation step, ions I1 are
not blocked and form energy-barrier-inducing pocket implant P,
whereas ions I2 are blocked and do not form an energy-barrier
inducing pocket implant.
[0048] The doping type of the pocket implant P in an asymmetric
transistor is of the opposite doping type to that of the
source-drain implants. For example, if the source-drains of a given
transistor are n-type, the pocket implant in that transistor will
be p-type (i.e., p+). In n-channel metal-oxide-semiconductor (NMOS)
asymmetric transistors, the source-drain terminals are n-type and
doping type of semiconductor channel region is p-type. The pocket
implant in an NMOS asymmetric transistor will therefore be p-type.
In p-channel metal-oxide-semiconductor (PMOS) asymmetric
transistors, the source-drain terminals are p-type. The pocket
implant in a p-channel asymmetric transistor will therefore be
n-type.
[0049] To complete formation of the source-drain terminals, a high
density implant operation may be performed. Spacers SP (FIG. 5) may
help define the locations of the high density implant. Spacers SP
may be formed from silicon oxide (as an example). Following the
high-density implant, the source-drains of the transistor appear as
shown in FIG. 5. Because the pocket implant P is only present at
source-drain SD1, the operation of the transistor is not
symmetrical.
[0050] Another way in which to form an asymmetric transistor
involves the use of a dual gate structure. As shown in FIG. 6, for
example, gate conductor GC may be formed from a gate metal GC1 that
has a different work function than gate metal GC2.
[0051] Over the portion of the channel region that would
conventionally contain an energy-barrier-inducing pocket implant,
the gate may be formed from a metal with a relatively high work
function. In an n-channel metal-oxide-semiconductor transistor,
this metal may, for example, have a work function of about 5.1 eV,
which makes its electrical performance comparable to that of a
heavily doped p-type gate conductor such as a p+ polysilicon gate
conductor. Over the remaining portion of the channel region in the
re-channel transistor, the gate may be formed from a metal that has
a relatively low work function. This portion of the gate may, for
example, have a work function of about 4.2 eV, which makes its
electrical performance comparable to that of a heavily doped n-type
gate conductor such as an n+ polysilicon gate conductor. Other
arrangements may also be used such as arrangements in which the
metal work functions for different gate conductors differ by
different amounts (e.g., by less than 0.3 eV, by 0.3 eV or more, by
at least 0.6 eV, by at least 0.9 eV, etc.). P-channel
metal-oxide-semiconductor asymmetric transistors may also be formed
using mixed gates (i.e., gate conductors with different work
functions).
[0052] Both the asymmetric transistor of FIG. 5 and the asymmetric
transistor of FIG. 6 may be characterized by an asymmetric band
diagram exhibiting an energy barrier EB at only one of the two
source-drain terminals.
[0053] The asymmetric performance of an asymmetric transistor may
be understood with reference to the energy band diagrams of FIGS. 8
and 9.
[0054] When writing a logic zero from source-drain SDA to
source-drain SDB, electrons initially need not overcome barrier EB.
Rather, electrons may accelerate due to the electric field present
in region A. After accelerating in region A, the electrons can
surmount energy barrier EB with relative ease. The configuration of
FIG. 8 therefore represents a configuration in which the asymmetric
transistor is relatively strong (i.e., the transistor is operating
in a "strong mode").
[0055] When writing a logic one from source-drain SDA to
source-drain SDB, however, electrons are initially required to
surmount energy barrier EB, before reaching region A. This is a
less favorable condition than the situation in FIG. 7 and results
in a lower current I for a given applied gate voltage than the
situation in FIG. 7. The configuration of FIG. 9 therefore
represents a configuration in which the asymmetric transistor is
weaker than in the configuration of FIG. 8 (i.e., the transistor is
operating in a "weak mode").
[0056] A graph showing how more drain-source current flows through
an asymmetric transistor when operated in the strong mode rather
than the weak mode is shown in FIG. 10. Because more current flows
in the strong mode than in the weak mode for a given gate voltage,
the performance of the asymmetric transistor varies depending on
how the transistor is being used. This property may be exploited in
circuits such as memory cell circuits, because the asymmetric
transistors can be used as address transistors that are configured
to operate in their strong mode during data loading operations
while operating in their weak mode during holding operations. The
enhanced strength of the asymmetric transistors when used in strong
mode can improve write margin (i.e., the asymmetric transistor in
this situation is operating as if its threshold voltage had been
decreased). The weakness of the asymmetric transistors during hold
mode can result in lower leakage current (i.e., the asymmetric
transistor in this situation is operating as if its threshold
voltage had been increased).
[0057] In addition to serving as address transistors, asymmetric
transistors may be used in other portions of a memory cell or in
other circuits on device 10. In some configurations, stacked
transistors that are formed from multiple series-connected
transistors, may be used alone or in combination with asymmetric
transistors. For example, memory cells and other circuits may be
provided with both stacked transistors and asymmetric
transistors.
[0058] An illustrative memory cell circuit in which asymmetric
transistors and stacked transistors may be used is shown in FIG.
11.
[0059] As shown in FIG. 11, memory cell 20 may include a bistable
element BE that stores a data bit (i.e., a "1" or a "0"). Bistable
element BE may be formed from any suitable number of transistors.
In the example of FIG. 11, bistable element BE is formed from a
pair of cross-coupled inverters. A first of these inverters is
formed by transistors M1 and M2. A second of these inverters is
formed from transistors M3 and M4. These inverters are said to be
"cross-coupled," because the output of the first inverter is
connected to the input of the second inverter and vice versa. The
transistors of bistable element BE may be powered using any
suitable power supply voltages. For example, bistable element BE
may be powered by a positive power supply voltage Vcc (e.g., a
voltage in the range of 0.7 to 1.1 volts as an example) and a
ground power supply voltage (e.g., 0 volts or other suitable ground
voltage).
[0060] Cell 20 may include true and complement data storage node.
True data storage node ND may hold a true version of the data bit
that is being stored in cell 20. Complementary data storage node
CND may store an inverted (complementary) version of the data bit
on node ND. These stored values are therefore complementary to each
other, but both represent the same stored data. Either the data on
node ND or the data on node CND may be used as an output. In the
example of FIG. 11, the data bit on node ND is used as an output
signal OUT on line 38.
[0061] Cell 20 may receive address signals such as read address
signal RADD and write address signal WADD on address lines (word
lines) 32. Write address signal WADD and read address signal RADD
may be asserted independently. During write operations, write
control signal WADD is systematically asserted (e.g., taken high)
in each row into which data is being written, while all read
address signals RADD in the memory array are maintained low (e.g.,
by deasserting all RADD lines). During read operations, all WADD
signals in the array are deasserted, while read control signal RADD
is systematically asserted in all rows of memory array from which
it is desired to read data.
[0062] Data may be routed to and from memory cell 20 using data
lines (bit lines) 34. Data that is to be written into cell 20 may
be provided to cell 20 on as write data signal WDATA. Data read
from cell 20 (RDATA) may be read out from bistable element BE using
an address transistor or, as shown in the example of FIG. 11, using
a read assistance circuit made up of transistors such as
transistors M7 and M8. During reading, signal RADD is asserted
(e.g., taken high) to turn on read access transistor M7, while the
signal RDATA is monitored to detect the value of the current bit
stored on node ND. If ND is high, M8 will be turned on and RDATA
will be connected to Vss (e.g., a ground voltage of 0 V). If the
bit stored on node ND of cell 20 is low, transistor M8 of the read
assistance circuit will be turned off and RDATA will remain at a
precharged value (e.g., Vcc) during read operations.
[0063] Clear signals may be supplied to clear transistor M6 during
power-up operations (i.e., signal CLR may be asserted when cell 20
is powered up, thereby clearing cell 20 and ensuring that output
signal OUT on output line 38 will initially be low).
[0064] Transistor M5 may be formed from an asymmetric transistor.
With this type of arrangement, the asymmetric performance of
transistor M5 can help improve the tradeoff between leakage and
performance for cell 20.
[0065] During power-up, cells 20 are cleared. This places a logic
zero on the data storage node ND in each cell (i.e., a ground
voltage Vss) and a logic high on each complementary data storage
node CND. When loading data into array 28 (FIG. 2), the states of
memory cells 20 therefore only need to be changed when it is
desired to overwrite the stored zero with a logic one (i.e., to
cause ND to flip from zero to one). The cells that are cleared and
that are supposed to remain in that cleared state after data
loading operations (i.e., the cells that are being "loaded" with
zeros) do not change state during data write operations.
[0066] A "flip" of node ND from zero to one is accomplished by
driving a logic zero onto complementary data node CND. In cleared
cells, CND is initially at a logic one value. Successful data
writing operations therefore involve overpowering bistable element
BE so that the initial logic one on node CND is overwritten with a
logic zero.
[0067] When driving a logic zero onto node CND to replace a logic
one on node CND, the asymmetric nature of transistor M5 assists the
writing process. As shown in FIG. 11, an energy-barrier-inducing
pocket implant P or the higher work function gate conductor in an
asymmetric transistor of the type shown in FIG. 6 is located in
source-drain SDB. Source-drain SDB is connected to node CND. Data
loading operations that involve overwriting a logic one on node CND
with a logic zero involve use of asymmetric transistor M5 in its
strong mode, as described in connection with FIG. 8. The use of an
asymmetric transistor for transistor M5 of cell 20 therefore can
improve the tradeoff between transistor strength and leakage
current. If, for example, increased write margin is desired, the
asymmetric nature of transistor M5 may be used to increase
transistor strength without requiring a reduction in the threshold
voltage of transistor M5 or an increase in the gate width for
transistor M5. Strength can thus be increased without increasing
leakage current. If, on the other hand, it is desired to maintain
an existing level of write margin in cell 20, the asymmetric nature
of transistor M5 may be used to maintain the write strength of
transistor M5 while the gate width for transistor M5 and/or the
threshold voltage for transistor M5 are reduced to lower leakage.
Balanced designs are also possible in which write margin is
somewhat increased while leakage current values are somewhat
decreased through the use of slightly smaller gate widths and/or
slightly higher threshold voltages.
[0068] When using an asymmetric transistor for transistor M5 of
cell 20, transistor M5 exhibits a weakened ability to drive a logic
zero from node CND onto a logic one on source-drain SDA. This is
acceptable in the cell arrangement of FIG. 11, because read
operations do not involve the use of transistor M5. It is therefore
not necessary for transistor M5 to be used to transfer data.
Rather, transistor M5 need only be used to hold data in bistable
element BE in its weak mode. Data may be read from node ND by
asserting read address signal RADD to control the read assistance
circuit made up of transistors M7 and M8 or other suitable read
circuitry.
[0069] In the example of FIG. 11, write operations involve use of a
single-ended approach to write data onto node CND and read
operations involve use of a single-ended approach to read data from
node ND. Clear operations are also performed by controlling the
voltage on only one of the two complementary data nodes in bistable
element BE (i.e., by asserting signal CLR to turn on transistor M6
and thereby short node ND to ground).
[0070] If desired, differential schemes may be used for writing,
reading, and/or clearing operations. For example, a pair of address
transistors may be used for performing differential write
operations and the same pair of address transistors may be used for
performing differential read operations. With this type of
arrangement, clear operations may be performed by loading cells
with logic zeros. Asymmetric transistors may be used for one or
both of the address transistors with this type of approach.
[0071] Another illustrative arrangement is shown in FIG. 12. As
shown in FIG. 12, a differential access scheme may be used for
write operations (and clear operations, which involve the writing
of zeros into the cell), whereas a single-ended arrangement may be
used for read operations (using, in this example, read circuit
M7/M8). To ensure that the asymmetric nature of address transistors
M5 and M6 assists during differential write operations, the
source-drain with the energy-barrier-inducing pocket implant P for
transistor M5 (SDB) may be connected to node CND, whereas the
source-drain with the energy-barrier-inducing pocket implant for
transistor M6 (SDB) may be connected to the complementary write
data line NWDATA.
[0072] Leakage currents in memory cells 20 and other circuits on
device 10 may also be reduced using stacked transistors. An
illustrative n-channel metal-oxide-semiconductor (NMOS) stacked
transistor TN is shown in FIG. 13. An illustrative p-channel
metal-oxide-semiconductor (PMOS) stacked transistor TP is shown in
FIG. 14. The use of NMOS stacked transistors is sometimes described
herein as an example. This is, however, merely illustrative. Both
NMOS and PMOS transistors may, in general, be implemented using
stacked transistor configurations.
[0073] As shown in the example of FIG. 13, stacked transistor TN
may be formed from two (or more) individual transistors of the same
channel type (i.e., both having p-type doping in their channels
because they are both re-channel devices) that are connected in
series and that share a common gate terminal G. In particular,
transistor TN may have a first NMOS transistor T1 and a second NMOS
transistor T2, each of which has an n-type source-drain terminal
coupled to common node N. The other n-type source-drain terminals
of transistors T1 and T2 (i.e., SDU and SDL) may be connected to
other circuitry on device 10. Each of the transistors that make up
stacked transistor TN may have four terminals (two source-drain
terminals, a gate, and a body B).
[0074] There are two contributions to static leakage-body leakage
current Iboff and source-drain leakage current Isoff. As shown in
FIG. 13, transistor T1 is characterized by leakage currents Iboff1
and Isoff1, whereas transistor T2 is characterized by Iboff2 and
Isoff2. Because of the series connection between transistors T1 and
T2, the currents Isoff1 and Isoff2 are equal. This can help limit
the amount of leakage current exhibited by transistor TN relative
to regular (non-stacked) transistor designs.
[0075] The way in which leakage currents are limited in stacked
transistors can be understood with reference to the graph of FIG.
15. In the graph of FIG. 15, leakage currents Iboff and Isoff are
plotted as a function of drain-source voltage Vds. As shown in FIG.
15, the value of Isoff is exponentially proportional to the value
of Vds. In a stacked transistor such as transistor TN of FIG. 13,
however, the value of Isoff1 cannot exceed the value of Isoff2,
because T1 and T2 are connected in series.
[0076] Consider, as an example, a scenario in which terminal SDU is
biased at 1.0 volts and terminal SDL is biased at 0 volts. Gate G
is biased at 0 volts to turn off transistor TN. With gate G at 0
volts, the gates of both transistors T1 and T2 are at 0 volts. The
voltage at node N in this type of situation will tend to
equilibrate at a value near to the voltage at SDL. For example, the
voltage at node N might be 0.15 volts (as an example). In a
conventional (not stacked) transistor, the drain-source voltage of
the transistor would be 1.0 volts and the transistor would exhibit
leakage currents of Isoffr and Iboffr. In a stacked transistor such
as stacked transistor TN of FIG. 13, however, the drain-source
voltage of transistor T2 would be only 0.15 volts. As a result, the
value of Isoff for transistor T2 and, because of the series
connection between transistors T1 and T2, the value of Isoff for
transistor T1, would be smaller (see, e.g., currents Isofff1 for T1
and Isofff2 for T2 in the graph of FIG. 15). The amount of
source-drain leakage in a stacked transistor is therefore
significantly lower than that of an unstacked transistor.
[0077] Iboff for transistor T2 will be relatively low (see, e.g.,
Ibofff2 in FIG. 15), due to the reduced value of Vds for transistor
T2. The leakage current Ibofff1 for transistor T1 in stacked
transistor TN will also be lower than Iboffr.
[0078] Further reductions in leakage current may be achieved by
lowering the threshold voltage of transistor T1 relative to that of
transistor T2. The value of Iboff tends to decrease with reductions
in threshold voltage and tends to increase with increases in
threshold voltage. To reduce Iboff1, the threshold voltage VT1 of
transistor T1 may therefore be lowered. For example, threshold
voltage VT1 of transistor T1 may be lowered relative to threshold
voltage VT2 of transistor T2. When VT1 is reduced relative to VT2,
Iboff1 will be reduced, thereby reducing the overall Iboff of
stacked transistor TN. Isoff1 will not be able to rise in response
to the reduction in VT1, because Isoff1 is pinned at the value of
Isoff2 due to the series connection of T1 and T2.
[0079] In a typical semiconductor fabrication process, it may be
possible to fabricate different types of transistors on the same
integrated circuit die. For example, the process may allow a
circuit designer to choose between two, three, or more than three
different transistor types, each of which may have a different
threshold voltage. A process may, for example, allow a first set of
transistors to be fabricated with a threshold voltage of about 0.05
volts to 0.15 volts (so-called low VT or LVT transistors), a second
set of transistors to be fabricated with a threshold voltage of
about 0.15 volts to 0.3 volts (so-called high VT or HVT
transistors), and a third set of transistors to be fabricated with
a threshold voltages of about 0.3 to 0.5 volts (so-called
ultra-high VT or UHVT transistors). In this type of illustrative
environment, transistor T1 may be implemented with an LVT
transistor (e.g., VT1=0.1 volts) and transistor T2 may be
implemented with a UHVT transistor (e.g., VT2=0.4 volts), as an
example. Other illustrative configurations for T1/T2 that may be
implemented in this type of process environment are LVT/HVT and
LVT/UHVT pairs. Stacked transistors with T1/T2 configurations of
LVT/LVT, HVT/HVT, or UHVT/UHVT may also be used, although these
configurations will typically exhibit somewhat larger Iboff values
then configurations in which VT1 is less than VT2.
[0080] Stacked transistors such as transistor TN of FIG. 13 and
transistor TP of FIG. 14 may be used as pull-down or pull-up
transistors in an interconnect path (e.g., interposed between pass
transistors and buffers), in memory cells 20, or in other circuitry
on device 10. Asymmetric transistors may also be used in these
circuits.
[0081] As an example, transistors such as transistors M1 and M3
(and, if desired, transistors M2 and M4) in bistable elements BE of
cells 20 (see, e.g., FIGS. 11 and 12) may be formed using stacked
transistors (e.g., stacked transistors in which transistor T1 has a
lower threshold voltage than transistor T2). This type of stacked
transistor memory cell circuit may be used with or without one or
more asymmetric transistors (e.g., asymmetric address transistor M5
of FIGS. 11 and 12, an optional asymmetric transistor for clear
transistor M6 of FIG. 11, asymmetric transistor M6 of FIG. 12,
etc.).
[0082] If desired, stacked transistors and/or asymmetric
transistors may be used in memory cells with bistable elements that
are formed from more than four transistors (i.e., more transistors
than are contained in a pair of cross-coupled inverters). For
example, it may be desirable to form a bistable element from four
interconnected inverter-like circuits to provide enhanced immunity
to soft error upset events due to radiation strikes. In this type
of arrangement, eight transistors may be used to implement bistable
element BE.
[0083] FIG. 16 is a diagram of an illustrative memory cell 20 that
has a bistable element BE that is based on four interconnected
inverter-like circuits. As shown in FIG. 16, each inverter-like
circuit includes a p-channel transistor and an n-channel
transistor. The first inverter-like circuit includes p-channel
transistor PO and n-channel transistor NO, the second inverter-like
circuit includes p-channel transistor P1 and n-channel transistor
N1, the third inverter-like circuit includes p-channel transistor
P2 and n-channel transistor N2, and the fourth inverter-like
circuit includes p-channel transistor P3 and n-channel transistor
N3.
[0084] Data can be conveyed over true and complement data lines
DATA and NDATA. Address signal ADD may be used to control address
transistors TA. Memory cell 20 may be cleared by loading logic
zeros into cell 20. Differential reading and writing operations may
be performed using differential data lines DATA and NDATA.
[0085] There are four labeled nodes in memory element 20 of FIGS.
16: X0, X1, X2, and X3. Any of these nodes may be used as output 38
of FIG. 2. For example, node X2 may be connected to output 38 of
FIG. 2.
[0086] Memory element 20 exhibits bistable operation. When memory
element has been loaded with a "1," the values of X0, X1, X2, and
X3 will be "1," "0,", "1," and "0," respectively. When memory
element has been loaded with a "0," the values of X0, X1, X2, and
X3 will be "0," "1,", "0," and "1," respectively. The architecture
of memory element 20 of FIG. 16 is more distributed than a
conventional memory element based on a pair of cross-coupled
inverters. This distributed nature tends to make cells of the type
shown in FIG. 16 more immune to radiation-induced upset events, at
the expense of a somewhat higher transistor count. If desired,
different types of circuits may be used for clearing cell 20, for
reading data from cell 20, and for writing data into cell 20. For
example, different numbers of address transistors, clear
transistors, and other transistors may be used in accessing cell
20. The arrangement of FIG. 16 is merely an example.
[0087] In the illustrative configuration of FIG. 16, data may be
loaded into memory element 20 by asserting address signal ADD.
[0088] Consider, as an example, the situation in which it is
desired to load a 1 onto node X2. If node X2 is already high, no
transitions will take place during data loading operations. If,
however, the current state of node X2 is low, transitions will
occur.
[0089] When loading a logic one onto a low node X2, data signal
DATA will be 1 (high) and its complement NDATA will be 0 (low).
While DATA is high, address line ADD is taken high to turn on
transistors TA. When transistors TA are turned on by the high ADD
signal, transistors TA drive logic one values onto nodes X2 and X0
and drive logic zero values onto nodes X1 and X3. When loading a
logic zero, these signal values are reversed. During read
operations, the values of the signals on lines DATA and NDATA are
monitored using sense amplifier circuitry while address signal ADD
is asserted.
[0090] In memory cell 20 of FIG. 16, transistors N0, N1, N2, and N3
may be implemented as stacked transistors. For example, transistor
NO may be implemented using transistor TN of FIG. 13, transistor N1
may be implemented using transistor TN of FIG. 13, transistor N2
may be implemented using transistor TN of FIG. 13, and transistor
N3 may be implemented using transistor TN of FIG. 13. The p-channel
transistors P0, P1, P2, and P3 may also be implemented using
stacked transistors. For example, transistors P0, P1, P2, and P3
may each be implemented using a stacked transistor such as stacked
transistor TP of FIG. 14.
[0091] Memory cell 20 of FIG. 16 may use asymmetric transistors.
For example, transistors TA may be provided with
energy-barrier-inducing pocket implants P, as shown in FIG. 16 or
may be provided with mixed gates. Stacked transistor configurations
may be used for a memory cell of the type shown in FIG. 16 without
using asymmetric transistors or may be used in a memory cell of the
type shown in FIG. 16 that has one or more asymmetric transistors.
For example, transistors N0, N1, N2, and N3 may be implemented as
stacked transistors and transistors TA may all be asymmetric
transistors. If desired, transistors TA may be implemented using
asymmetric transistors without using any stacked transistors in
cell 20 of FIG. 16. In general, any suitable combination of stacked
and asymmetric transistors may be used. These are merely
illustrative examples.
[0092] FIG. 17 is a diagram of a memory cell 20 showing how
n-channel inverter transistors M1 and M3 in bistable element BE may
be implemented using stacked transistors. The p-channel transistors
in memory cell 20 of FIG. 17 may be implemented as stacked
transistors or as non-stacked transistors.
[0093] Memory cell 20 of FIG. 17 uses a differential data writing
scheme and a differential data reading scheme. During data writing
operations, address signal ADD is asserted, while data to be loaded
is placed in true and complement form on respective true and
complement data lines DATA and NDATA. During data reading
operations, the signals DATA and NDATA are monitored (e.g., using
sense amplifier circuitry) while address signal ADD is
asserted.
[0094] If desired, address transistors M5 and M6 may be implemented
using asymmetric transistors. For example, transistors M5 and M6
may be provided with pocket implants P as shown in FIG. 17 or
transistors M5 and M6 may be provided with dual gate structures to
produce energy barriers EB.
[0095] The foregoing is merely illustrative of the principles of
this invention and various modifications can be made by those
skilled in the art without departing from the scope and spirit of
the invention.
* * * * *