U.S. patent application number 16/586090 was filed with the patent office on 2020-01-23 for super-saturation current field effect transistor and trans-impedance mos device.
The applicant listed for this patent is Circuit Seed, LLC. Invention is credited to Robert C. Schober, Susan Marya Schober.
Application Number | 20200027880 16/586090 |
Document ID | / |
Family ID | 59057176 |
Filed Date | 2020-01-23 |
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United States Patent
Application |
20200027880 |
Kind Code |
A1 |
Schober; Susan Marya ; et
al. |
January 23, 2020 |
SUPER-SATURATION CURRENT FIELD EFFECT TRANSISTOR AND
TRANS-IMPEDANCE MOS DEVICE
Abstract
The present invention relates to an improvement to a current
field effect transistor and trans-impedance MOS devices based on a
novel and inventive compound device structure, enabling a
charge-based approach that takes advantage of sub-threshold
operation, for designing analog CMOS circuits. The present
invention further relates to a super-saturation current field
effect transistor (xiFET), having a source, a drain, a diffusion, a
first gate, and a second gate terminals, in which a source channel
is defined between the source and diffusion terminals, a drain
channel is defined between the drain and diffusion terminals. The
first gate terminal is capacitively coupled to the source channel;
and the second gate terminal is capacitively coupled to said drain
channel. The diffusion terminal receives a current causing change
in diffused charge density throughout said source and drain
channel. The xiFET provides a fundamental building block for
designing various analog circuits.
Inventors: |
Schober; Susan Marya;
(Newport Beach, CA) ; Schober; Robert C.;
(Huntington Beach, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Circuit Seed, LLC |
Newport Beach |
CA |
US |
|
|
Family ID: |
59057176 |
Appl. No.: |
16/586090 |
Filed: |
September 27, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16394578 |
Apr 25, 2019 |
10446547 |
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16586090 |
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15748908 |
Jan 30, 2018 |
10283506 |
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PCT/US2016/044800 |
Jul 29, 2016 |
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16394578 |
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62267174 |
Dec 14, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 21/823814 20130101;
H03F 1/086 20130101; H03F 3/04 20130101; H01L 27/0207 20130101;
H03F 3/165 20130101; H01L 27/092 20130101; H03F 3/45179 20130101;
H03F 2203/45246 20130101 |
International
Class: |
H01L 27/092 20060101
H01L027/092; H01L 21/8238 20060101 H01L021/8238; H01L 27/02
20060101 H01L027/02; H03F 1/08 20060101 H03F001/08; H03F 3/04
20060101 H03F003/04; H03F 3/45 20060101 H03F003/45; H03F 3/16
20060101 H03F003/16 |
Claims
1. A field effect transistor, comprising: a semiconductor substrate
of a conductivity type, comprising: a source, a drain, a diffusion,
a first gate, and a second gate; a source channel is defined
between said source and diffusion, a drain channel is defined
between said drain and diffusion; wherein said first gate is
capacitively coupled to said source channel; and said second gate
is capacitively coupled to said drain channel.
2. The field effect transistor as recited in claim 1, wherein a
trans-resistance of the field effect transistor is established by a
ratio of: a. the ratio of the width and the length of the drain
channel segment; and b. the ratio of the width and the length of
the source channel segment.
3. The field effect transistor as recited in claim 2, wherein a
current is applied at said diffusion is converted to a voltage by
said trans-resistance.
4. The field effect transistor as recited in claim 1, wherein said
conductive type is N-type.
5. The field effect transistor as recited in claim 1, wherein said
conductive type is P-type.
6. A composite transistor, comprising: N-type field effect
transistor (NxiFET) and a P-type field effect transistor (PxiFET),
each comprising: a semiconductor substrate of a corresponding
conductivity type, comprising: a source, a drain, a diffusion, a
first gate, and a second gate; a source channel is defined between
said source and diffusion, a drain channel is defined between said
drain and diffusion; said first gate is capacitively coupled to
said source channel; and said second gate is capacitively coupled
to said drain channel; wherein said source of said NxiFET receives
negative supply voltage; said source of said PxiFET receives
positive supply voltage; said drain of said NxiFET and said drain
of said PxiFET are coupled together to form an output; and the
second gate of said NxiFET and the second gate of said PxiFET are
coupled together to form an input.
7. A focal plane array read out, comprising: a P-type current field
effect transistor (PiFET) and an N-type current field effect
transistor (NxiFET), said PiFET comprising a semiconductor
substrate of a corresponding conductivity type, comprising: a
source, a drain, a diffusion, and a gate; a source channel is
defined between said source and said diffusion, a drain channel is
defined between said drain and said diffusion; said gate is
capacitively coupled to said source channel and said drain channel;
said NxiFET comprising a semiconductor substrate of a corresponding
conductivity type, comprising: a source, a drain, a diffusion, a
first gate, and a second gate; a source channel is defined between
said source and diffusion, a drain channel is defined between said
drain and diffusion; said first gate is capacitively coupled to
said source channel; and said second gate is capacitively coupled
to said drain channel; wherein said drain of said PiFET and said
drain of said NxiFET are connected together to form an output; said
source of said PiFET is coupled to a positive power supply; said
source of said NxiFET is coupled to a negative power supply; said
diffusion of said PiFET receives a gain control signal; said gate
of said PiFET receives a bias voltage; a switch operable by a
select signal comprising first and second phases; and a capacitor
for storing a pixel voltage from a photodiode, comprising a first
and second terminals, said second terminal of said capacitor is
coupled to said negative power supply and said first terminal of
said capacitor receives said pixel voltage from said photodiode,
and said first terminal is coupled to said second gate of said
NxiFET; wherein, during said first phase of said select signal,
said switch causes said bias voltage to couple with said first gate
of said NxiFET; during said second phase of said select signal
causes said negative power supply to couple with said first
gate.
8. A latch current comparator, comprising: a differential amplifier
for amplifying difference between a first input and a second input,
each of said first and second inputs comprising negative and
positive polarity inputs, comprising: a first complementary pair of
a first n-type current field-effect transistor (NiFET) and a first
p-type current field-effect transistor (PiFET); a second
complementary pair of a second NiFET and a second PiFET; and a
third complementary pair of a third NiFET and a third PiFET; and a
comparator, comprising: a fourth complementary pair of a fourth
NiFET and a fourth PiFET; a fifth complementary pair of a fifth
NiFET and a fifth PiFET; a plurality of switches operable on a
control signal that alternates enable and setup phases; a first
capacitor and a second capacitor, each has a first terminal and a
second terminal; wherein each of said NiFETs and PiFETs comprises:
a source, a drain, a gate, and a diffusion, defining a source
channel between said source and said diffusion, and a drain channel
between said drain and said diffusion, and said gate is
capacitively coupled to said source channel and said drain channel;
wherein said source of said NiFET of said each complimentary pair
is connected to negative power supply and said source of said PiFET
of said each pair is connected to positive power supply, and drains
of said NiFET and said PiFET of said each complimentary pair are
connected together to form an output; and wherein said gates of
said first complimentary pair, said gates of said second
complementary pair and said gates of said third complementary pair
are connected with said output of said second complementary pair
for generating a bias voltage output; said diffusion of said third
PiFET receives said negative polarity input of said first input;
said diffusion of said first PiFET receives said positive polarity
input of said first input; said diffusion of said third NiFET
receives said negative polarity input of said second input; said
diffusion of said first NiFET receives said positive polarity input
of said second input; and said output of said first complementary
pair forms positive voltage output of said differential amplifier;
said output of said third complementary pair forms negative voltage
output of said differential amplifier; wherein said output of said
fourth complementary pair is capacitively coupled to said input of
said fifth complementary pair through said second capacitor; said
second terminal of said first capacitor is coupled to said input fo
said fourth complementary pair; during said setup phase of said
control signal, said plurality of switches cause said positive
voltage output of said differential amplifier to be coupled with
said first terminal of said first capacitor, said fourth
complementary pair to be self-biased by connecting said output of
said fourth complementary pair to said input of said fourth
complementary pair, and said fifth complementary pair to be
self-biased by connecting said output of said fifth complementary
pair to said input of said fifth complementary pair; during said
enable phase of said control signal, said plurality of switches
cause said negative voltage output of said differential amplifier
to be coupled to said first terminal of said first capacitor, and
said output of said fifth complementary pair to said input of said
fourth complementary pair.
9. The latch current comparator recited in claim 8, wherein, for
said each complimentary pair, said gate of said PiFET and said gate
of said NiFET are connected together to form a common gate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of the
non-provisional application Ser. No. 16/394,578, filed on Apr. 25,
2019, which is a continuation of Ser. No. 15/748,908, filed on Jan.
30, 2018, which is 35 U.S.C. .sctn. 371 National Phase Entry of,
and claims priority to PCT International Application No.
PCT/2016/044800 filed on Jul. 29, 2016, which claims the benefit of
and priority to U.S. Provisional Application No. 62/267,174 filed
on Dec. 14, 2015, the contents of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] The present invention relates to an improvement to a current
field effect transistor and trans-impedance MOS devices based on a
novel and inventive compound device structure, enabling a
charge-based approach that takes advantage of sub-threshold
operation, for designing analog CMOS circuits. The present
invention further relates to super-saturation current field effect
transistors.
Description of Related Art
[0003] The new millennium brings with it a demand for connectivity
that is expanding at an extremely rapid pace. By the end of year
2015, the number of global network connections will exceed two
times the world population and it is estimated that in 2020 more
than 30 billion devices will be wirelessly connected to the cloud
forming the Internet of Things (or "IoT"). Enabling this new era
are the revolutionary developments in mobile computing and wireless
communication that have arisen over the last two decades. Following
Moore's Law, development of highly-integrated and cost-effective
silicon complementary metal oxide semiconductor (CMOS) devices
allowed incorporation of digital and analog system elements, such
as bulky Analog-to-Digital converters or transceivers, into a more
cost effective single chip solution.
[0004] In the last few years, however, while digital circuits have
largely followed the predicted path and benefited from the scaling
of CMOS technology into ultra-deep submicron (sub-.mu.m), analog
circuits have not been enabled to follow the same trend, and may
never be enabled without a paradigm shift in analog design. Analog
and radio frequency (or "RF") designers still struggle to discover
how to make high-performance integrated circuits (or "ICs") for
ultra-deep sub-.mu.m feature sizes without losing the benefits of
shrinking size; including reduced power, compact footprint, and
higher operational frequencies. Truly a paradigm shift is needed to
break through the established science of analog design to meet the
system on chip (SoC) demands of the new millennium.
PRIOR ART
[0005] The core building block of analog circuits is the amplifier.
Discrete component amplifiers are free to use resistors,
capacitors, inductors, transformers, and non-linear elements as
well as various types of transistors. Unwanted parasitics between
various components are normally negligible. However, in order to
build amplifiers within an integrated circuit, the normal analog
circuit components are not readily available, and often take
special IC process extensions to obtain these circuit elements if
at all. The parasitics on integrated circuit amplifiers are severe
due to their close proximity and being coupled together through the
silicon wafer they are integrated into. Moore's law IC process
advancements are focused on digital, microprocessor, and memory
process development. It takes a generation (.about.18 months) or
two to extend the IC process to incorporate analog components, thus
analog functionality is generally not included on the latest
process single chip systems. These "mixed-mode" IC processes are
less available, vender dependent, and more expensive as well as
being highly subject to parametric variation. It takes substantial
engineering to include sparse analog functionality on any IC which
becomes specific to its IC vender and process node. Because analog
circuitry is carefully and specifically designed or arranged for
each process node, such analog circuitry is highly non-portable.
Reprobating this limitation, analog circuit design engineers are
becoming scarce and are slowly retiring without adequate
replacements.
[0006] Operational Amplifiers (or OpAmps) are the fundamental IC
analog gain block necessary to process analog information. OpAmps
make use of a very highly matched pair of transistors to form a
differential pair of transistors at the voltage inputs. Matching is
a parameter that is readily available on an integrated circuit, but
to approach the required level of matching, many considerations are
used: like centroid layout, multiple large devices, well isolation,
and physical layout techniques among many other considerations.
Large area matched sets of transistors are also used for current
mirrors and load devices. OpAmps require current sources for
biasing. OpAmps further require resistor and capacitor (or RC)
compensation poles to prevent oscillation. Resistors are essential
for the "R" and the value of the RC time constant is relatively
precise. Too big value for a resistor would make the amplifier too
slow and too small results in oscillation. Constant "bias" currents
add to the power consumed. In general, these bias currents want to
be larger than the peak currents required during full signal
operation.
[0007] As IC processes are shrunk, the threshold voltages remain
somewhat constant. This is because the metal-oxide-semiconductor
(or MOS) threshold cutoff curve does not substantially change with
shrinking of the IC processes and the total chip OFF leakage
current must be kept small enough to not impact the full-chip power
supply leakage. The threshold and saturation voltage tends to take
up the entire power supply voltage, not leaving sufficient room for
analog voltage swings. To accommodate this lack of signal swing
voltage, OpAmps were given multiple sets of current mirrors,
further complicating their design, while consuming more power and
using additional physical layout area. This patent introduces
amplifier designs that operate even better as power supply voltages
are shrunk far below 1 volt.
[0008] The conventional MOS amplifier gain formation is an input
voltage driving a trans-conductance (g.sub.m) which converts the
input voltage into an output current. This output current then
drives an output load which is normally the output of a current
source for the purpose of establishing a high load resistance. This
high resistance load converts the output current back into an
output voltage. The equivalent output load resistance is actually
the parallel combination of the load current source transistor and
the amplifier output transistors. In order to keep this equivalent
load resistance high to provide the required voltage gain, these
load transistors must be very long, but to drive enough current
these transistors must be very wide also, thus very large
transistors are necessary. It also might be noted that the load
resistance the amplifier output drives is additional parallel
resistance that reduces the voltage gain. It should also be noted
that a load capacitance interacts with the amplifiers output
resistance, modifying the AC performance. What is actually needed
is exactly the inverse operating principle, which the present
invention is about. FIG. 1a is a transistor level schematic diagram
of a high-quality MOS IC OpAmp as a baseline reference (from the
Wiley textbook: Analysis and Design of Analog Integrated Circuits
by Gray and others, 4.sup.th edition pg. 482) which is used for
comparison in the description of the amplifiers illustrated
herein.
[0009] The baseline comparisons are (all made in an 180 nm IC
process) in the form of performance plots as in: a Bode Gain-Phase
plot over frequency FIG. 1b, when V.sub.dd=1.8 Volts and
R.sub.cmp=700 ohms. Wherever possible all the axis scales for each
of these three comparison plots are kept the same. A readily
available 180 nm process was selected for comparison of all the
comparative examples in this document because the conventional
prior art amplifiers work best and have had the most usage and have
mature mixed-mode IC process extensions offered which are required
for conventional analog. Also as the IC process is shrunk and the
power supply voltage is decreased, this is where the
implementations of the present invention become highly
beneficial.
[0010] Normally MOS amplifiers operate within a square-law
relationship due to the strong inversion MOS transistor square-law
characteristics; these are not very well defined or predictably
stable to the degree that analog circuits need. Exponential-law
operation, like bipolar transistors operation is higher gain,
stable, and well defined. At very weak operating conditions, MOS
transistors convert to exponential operation, but they are too slow
to be of very much use. Furthermore, the "moderate-inversion"
transition between these two operating mode provide non-linarites
that lower the quality of analog MOS circuits. At the threshold
voltage, where MOS transistors operate around, is where 50% of the
current is square-law and the other 50% is exponential. This is the
definition of threshold voltage in the latest MOS simulation
equations. Full exponential MOS operation at high speed would
provide higher gain that is predictable, stable, and well defined.
This patent is about amplifiers that operate in the exponential
mode.
[0011] To understand the prior art, let's begin with a discussion
of Weak vs. Strong inversion. Referring to FIGS. 1e and 1f, weak
inversion is the range where most designers would consider the
transistor to be OFF: [0012] Drain to Source voltage is small (on
the order of 100 mV); [0013] The gate G (or 17s) is at a similar
small potential (typically less than 300 mV); [0014] This creates a
surface conduction layer, of uniform depth from source S to drain
D; [0015] The conductivity of this surface layer is exponential
with respect to the Gate G voltage; [0016] This allows operation
over many decades (about 6) of dynamic range; [0017] The channel
appears as a moderate value resistor (100.sup.+s of K-Ohms); and
[0018] The uniform depth conduction channel promotes an
exponentially higher gain but with a speed penalty (due to low
charge density in the conduction channel).
[0019] Strong inversion (referring to FIGS. 1g & 1h) is
characterized by a graduated conduction channel, deeper near the
Source and shallow near at the Drain: [0020] Drain to Source
voltage is larger than the Gate to Source voltage Vg of FIG. 1g and
threshold V.sub.threshold in FIG. 1h (typically in excess of 400
mV); [0021] The Gate 17u is operated above its threshold voltage
V.sub.threshold; [0022] This creates a conduction channel that is
deeper at the Source and tapers to near pinch-off at the Drain 12u;
[0023] The resulting conduction layer behaves with a Square-law
response to the gate voltage at the Gate 17u; [0024] Dynamic range
is limited to about 3 decades as compared to weak inversion; [0025]
The channel 12g appears as an adjustable current source (high value
resistor); and [0026] The wedge shape of the conduction channel 12g
provides higher speed than weak inversion because of higher charge
density in the conduction channel.
[0027] Now, referring back to FIG. 1e, which shows the channel 12e
development under weak inversion conditions. The conduction channel
12e has a relatively even distribution of carriers over its entire
length and width. Note that the conduction depth 10s of the entire
channel is the same as the pinch-off area 12u on the right side of
FIG. 1g. This thin conduction layer contributes a significant
amount of noise because the channel current travels along the
surface where charge carrier defect traps are concentrated. The
Gate 17s to channel voltage V.sub.g in FIG. 1e has a strong
(exponential) effect on the density of carriers in this conduction
layer.
[0028] FIG. 1g shows the channel 12u development under Strong
inversion conditions. The higher potential difference between
Source and Drain over the gate 17u causes "channel length
modulation" (the flat part of the channel 12u), resulting in
pinch-off near the drain diffusion where the channel reaches a thin
layer near 12u. The pinch-off region 12u (where the carriers are
forced to the top of the channel) imparts significant noise by
means of surface defect carrier traps. The higher the drain voltage
V.sub.d, the longer the pinch-off region and thus the higher the
contributed noise, thus it is desired to keep this voltage low for
low noise contribution to the channel current. Other effects such
as velocity saturation and hot electron jumping over into the gate
oxide are noted around this thin saturated pinch-off region, thus
it would be highly desirable to minimize this region by lower
voltage and semiconductor doping profiles.
[0029] FIG. 1h shows a characteristic plot which approaches a
"constant current" relationship between drain current I.sub.d and
drain voltage V.sub.ds with a fixed Gate voltage V.sub.g on the
gate G. It is to be noted that the drain voltage V.sub.ds spans a
large range of nearly the power supply voltage V.sub.dd, while
maintaining the same current as opposed to the limited drain
voltage range of FIG. 1f.
[0030] FIGS. 1i to 1k illustrate a prior art MOS structure,
commonly known as a CMOS inverter, that turns out to actually
combine both modes of operation. A pair of MOSFETs with opposite
conductivities, PFET and NFET, are complementary connected with
each other. For example, the input 10i, 10j, 10k is connected to
both the gate control terminal of PFET and the gate control
terminal of NFET, the source of PFET is connected to power supply
(+), while the source terminal of NFET is connected to power supply
(-); and the drain of PFET and the drain of NFET connected together
for V.sub.out 19i.
[0031] FIG. 1j shows the structure related to a physical layout
abstraction shown in FIG. 1k, which is 2.times.strength CMOS or
two-finger inverter of prior art. As stated above, gate terminals
of PFET and NFET are connected together to receive V.sub.input 10j
and 10k and the drain terminals of PFET and NFET are connected
together for producing V.sub.output 19j and 19k. The layout shown
in FIG. 1k structurally corresponds to that of FIG. 1i. As it can
be seen, to minimize various shortcoming from the conventional FET
layouts, such as minimizing parasitic output capacitance, the
source terminal of PFET, for example, is split into two source
terminals S+ and S+, and the drain terminal D+ 12k is displaced
therebetween for forming a pair of parallel channels 14k and 16k
between S+ and D+ 12k; p-channel region of the gate G covers the
parallel channels 14k and 16k. Divided by the well border WB, NFET
is also provided with a pair of source terminals, S- and S-, and
the drain terminal thereof, D- 11k is displaced therebetween for
forming a pair of parallel channels 13k and 15k between S- and D-;
n-channel region of the gate G further covers the parallel channels
13k and 15k. Drains 12k and 11k are connected therebetween through
metal work 18k and forms V.sub.output 19k.
[0032] A 3-dimensional prospective view of this MOS transistor
structure is shown in FIG. 1m, while cross sectional view at
section AA in FIG. 1m is shown in FIG. 1n. This structure is
inherent in a 2.times. or two-finger inverter as shown in FIGS. 1j
and 1k. As it can be seen therein at the parallel channels 14k and
16k in PFET and the parallel channels 13k and 15k, all of these
channels taper from the drains D+, D- to the sources S+, S-.
[0033] Although similar MOS structures appear in prior art, no
significant exploitation of many of its unique properties are known
or published. In addition, proper biasing remains as a problem(s)
for its operation(s). A deeper understanding of the internal
mechanisms resulted in discovery of many desirable applications
(enabling superior operation at deep-sub-micron scale), including
an approach to proper biasing that takes advantage of natural
equilibrium. This natural equilibrium is the result of a "Band-Gap"
voltage reference mechanism, again functional at deep-sub-micron
scale.
[0034] Referring to FIGS. 1p and 1q, some references show a MOS
field effect transistor device with two identical regions 13p/13q
and 15p/15q of like "conductivity type" separated by a diffusion
region 11p (designated as Z for Low Impedance in the prior art).
Multiple papers by Bedabrata Pain/R Schober, Jet Propulsion Lab and
Jacob Baker/Vishal Saxena, Boise State University, including Pain,
Bedabrata et al., "A Self-Cascoding CMOS Circuit for Low-Power
Applications", Center for Space Microelectronics Technology Jet
Propulsion Laboratory, California Institute of Technology, contain
such references, but these references do not exploit any
opportunities as shown in this document, especially when
complementary devices like this are combined into a single
composite device as will be explained in this invention. Such
configurations have been called self-cascading or split-length
devices. The two regions of such a configuration are arranged
between source and a drain diffusions and have both a high
impedance common gate connection and a low impedance connection to
the mid channel regions. This low impedance mid channel control
input, when exploited as outlined in this document, enables an
entirely new set of analog design methods.
[0035] Although a cascade amplifier can be found in prior art, the
prior art does not contain a complementary pair of cascade
transistors connected as a totem-pole. With this simple compound
structure, feedback from the output to the input can be used to
self-bias the resulting inverter into its linear mode. As mentioned
above, biasing of an amplifier has always been problematic;
however, the novel and inventive self-biasing structure of the
present invention addresses such an issue. Advantages of the
configuration of the present invention (referred to as a
complementary iFET or CiFET) are many, including, but not limited
to: [0036] Gain of the single stage is maximum when the output is
at the midpoint (self-bias point); [0037] The gain of a single
CiFET stage is high (approaching 100), therefore, while the final
output may swing close to the rails, its input remains near the
midpoint. The stage before that, because of the high gain, operates
its input and output near the mid-point ("sweet-spot") where the
gain is maximized. So too for each of the preceding stages; [0038]
Slew rate and symmetry are maximized where the channel current is
highest (near the mid-point); [0039] Noise is minimized where the
channel current is highest (near the mid-point); and [0040]
Parasitic effects are negligible where the voltage swing is
small.
[0041] When the gate input signal moves in one direction, the
output moves in the inverse direction. For example; a positive
input yields a negative output, not so much because the N-channel
device is turned on harder, but rather because the P-channel device
is being turned off. Thevenin/Norton analysis shows that the
current through the P and N devices must be exactly the same,
because there is nowhere else for drain current in one transistor
to go except through the drain of the complementary transistor;
however, the voltage drop across those devices does not have to be
equal, but must sum to the power supply voltage. Due to the
super-saturated source channel, these voltages are tied together
exponentially. This is even more evident at low power supply
voltages where the voltage gain peaks. This means that the
gate-to-source voltage is precisely defined by the same and only
drain current going through both transistors. Exponentials have the
unique physical property like a time constant, or "half-life;" It
does not matter where we are at a given point of time, a time
constant later we will be a fixed percentage closer to the final
value. This is a "minds-eye" illustration of the primary
contributor to output movement in response to input change. This
same current balance of gate-to-source operating voltages also
indicates why the "sweet-spot" in the self-biased amplifier is so
repeatable. In effect it is used as a differential pair-like
reference point to the amplifier input signal.
[0042] It is to be noted that during the transition from vacuum
tubes to bipolar transistors the industry underwent a major
paradigm shift, learning to think in terms of current rather than
voltage. With the advent of FETs & MOSFETs the pendulum swing
is back toward thinking in terms of voltage, but much knowledge has
been lost or forgotten. Herein is contained the rediscovery of some
old ideas as well as some new ones, all applied to the up-coming
"current" state of the art. It is believed that the inherent
simplicity of the present invention speaks to their applicability
and completeness.
[0043] A first issue may be that there is always a need for a
little analog functionality, yet nearly all analog performance
metrics of a MOS transistor are remarkably poor as compared to that
of a Bipolar transistor. The industry has made MOS devices serve by
employing extensive "work-arounds." Conventional analog design is
constrained by one or more of the followings: [0044] Power supply
voltages sufficient to bias the stacked thresholds, and transistors
large enough to supply the necessary output drive current while
still providing the high output impedance required for linearity
and gain (g.sub.m*R.sub.L). [0045] Lack of analog IC process
extensions (unavailable at nanometer scale) are required for linear
signals, let alone with the enhanced performance demonstrated
herein. [0046] Resistors, inductors, and large capacitors are
mostly non-existent for analog designs in newer IC processes.
[0047] In contrast, bipolar transistors can be made to have high
gain (.beta.), wider bandwidth, wider dynamic range (many decades,
from near the rails down to the noise floor), better matching
(found in differential pairs), and band-gap references. Junction
FETs, which operate with sub-surface channel conduction below the
surface defects, have lower noise than bipolar transistors.
Likewise the CiFET super-saturated source channel operates
primarily below the defects at the channel surface underneath the
gate oxide.
[0048] MOS designs are poorer in the above areas but have their own
extreme advantages, including, but not limited to: [0049] MOS
devices are small and relatively simple [0050] highly scalable
[0051] high speed [0052] low power [0053] ultra-dense/high
functionality systems on a chip, where Bipolar designs cannot go
(deep sub-.mu.m scale).
[0054] Accordingly, building analog circuits on an IC has always
been problematic. Engineering around poorly performing analog
components has been the overriding objective for analog IC
designers since analog circuits have been integrated. This drove
the need for digital signal processing with algorithm development
yielding digital magic.
[0055] Today the real-world of analog circuit design, signals still
need to be converted on both the front and back end of signal
processing systems. This need has become a road-block at deep
sub-.mu.m scale.
[0056] Another problem may be that solid-state amplifiers have been
notoriously non-linear since their inception. To make them linear,
increased open loop gain (with levels significantly higher than is
ultimately needed) is traded for control over actual circuit gain
and linearity through the use of a closed loop (feedback). A closed
loop amplifier requires negative feedback. Most amplifier stages
are inverting, providing the necessary negative feedback. A single
stage inverter, with a closed loop, is stable (does not oscillate).
Increased loop gain requires stages be added such that there are
always an odd number of stages (sign is negative), to provide the
necessary negative feedback. While a single stage amplifier is
inherently stable, three stages and most definitely five stages are
unstable (they always oscillate--because they are ring
oscillators).
[0057] The problem then is how to properly compensate a multi-stage
closed loop amplifier while maintaining a reasonable gain-bandwidth
product. This is particularly difficult at deep-sub-micron scale
where circuit stages must be simple in their design. The severely
limited power supply voltages preclude the use of conventional
analog design approaches. Additionally, it is desirable to avoid
reliance upon analog extensions but rather to accomplish the
necessary analog functions using all digital parts, to improve
yields and decrease costs. Using all digital parts allows analog
functions at process nodes that do not yet have analog extensions,
and may never have them.
[0058] There is a long felt need for low-cost/high-performance
systems integrated on a single chip for affordable high-volume
devices such as the Internet of things, smart-sensors, and other
ubiquitous devices.
SUMMARY OF THE INVENTION
[0059] The present invention relates to circuits built out of a
novel and inventive compound device structure, which enables a
charge-based approach that takes advantage of exponential
relationships of a super-saturated source channel below which
possesses sub-threshold-like operation when used for analog CMOS
circuit designs. The present invention is an evolution of an
ordinary CMOS inverter, and further improvement of a current field
effect transistor (iFET). It provides extremely high precision,
speed, linearity, low noise, and a compact physical layout, using
an all-digital nanoscale or deep sub-.mu.m IC process. In addition
to the expected digital inverter function, five classes of analog
circuits are exemplified: a voltage input amplifier, a current
input amplifier, a current inverter as opposed to a current mirror,
an adjustable delay circuit, and a voltage or current reference
source. Take special note that analog functionality is realized, in
a digital IC process, using a single optimized digital logic
circuit cell.
[0060] According to another aspect of the present invention, it
provides a super-saturation iFET (or xiFET) device having a source
terminal, a drain terminal and a diffusion (or iPort) terminal,
defining a source channel between the source terminal and diffusion
terminal, and a drain channel between the drain terminal and the
diffusion terminal. Instead of a common gate that capacitively
coupled to the source and drain channels, the device has a first
gate that is capacitively coupled to the source channel and a
second gate that is capacitively coupled to the drain channel.
[0061] According to another aspect of the present invention, it
takes advantage of the Doping Profile and Rationing. Not everything
in optimizing a circuit has to do with the circuits' electrical
configuration. Proper device sizing and especially adjusting the
size relationship between complementary transistors provides
considerable performance benefits. The xiFET, being a compound
structure, offers extensive opportunity to establish impedance
matching and gain control through proper ratio of the physical
device parameters. Other important characteristics, like noise,
speed, and power, can be tailored through careful specification of
the physical construction and doping of the transistors, rather
than relying solely on circuit configuration.
[0062] According to yet another aspect of the present invention, it
provides certain noise advantages. In the end, it comes down to
signal-to-noise ratio. Low power supply voltage requirements in
ultra-deep-sub-micron IC processes limit the maximum signal swing
to a much smaller number than most analog designers are used to. So
with a smaller signal, the low-noise techniques embodied herein
must be employed in order to maintain the desired signal to noise
ratio.
[0063] Additional Advantages may be provided by the present
invention. The primary advantage delivered by this technology is
the ability to produce analog building blocks constructed entirely
from digital parts, without analog extensions. Equally important is
the fact that it actually operates at ultra-deep-sub-micron scale,
and operates best at reduced power supply voltages below one volt
as required for ultra-deep sub-.mu.m IC processes. These three
factors contribute to an unprecedented portability of designs
across process nodes. Entirely new circuit designs will be realized
because of a FET that has more than one control input. The CiFET
offers a high-impedance voltage control on the gate while
simultaneously offering a low-impedance current control at the
iPorts. These two inputs operate independently of each other and
their independent response is summed at the output.
[0064] According to yet another aspect of the present invention, it
provides a complementary xiFET compound device, which can be
configured as a current input amplifier or complementary current
low noise amplifier (CxiLNA). For example, the CxiFET compound
device, used as a current amplifier, offers an unexpectedly wider
dynamic range with great linearity over an extreme frequency range.
This CxiLNA provides constant gain and input resistance in its
passband. The CxiLNA of the present invention does not suffer from
phase shift in the passband. This CxiLNA operates with power
supplies down to the millivolt level with a reasonable bandwidth.
The CxiLNA is not dependent on operation above the normal MOS
threshold voltages. A skilled reader in the art would appreciate
and find it useful in many applications as a gain block that is
unencumbered by the surrounding parasitics, due to the fact that
CxiLNA of the present invention exhibits an incredibly higher
signal to noise ratio. Such a performance is seldom seen, even in
LNAs.
BRIEF DESCRIPTION OF FIGURES
[0065] FIG. 1a illustrates a high quality CMOS OpAmp prior art
transistor schematic from a prominent textbook "Analysis and Design
of Analog Integrated Circuits," 4.sup.th Ed, by Gray, Hurst Lewis
and Meyer, p 482 as a prior art amplifier for comparison;
[0066] FIGS. 1b to 1d are a baseline set of representative
performance plots illustrating frequency domain performance and
power supply dependency of the prior art OpAmp of FIG. 1a;
[0067] FIGS. 1e and 1g show cross-sectional views of prior art
MOSFET channel conduction in weak inversion and in strong
inversion, respectively, and FIGS. 1f and 1h show plots bearing
exponential relationship between drain current and gate voltage
when in weak inversion and quadratic relationship when in strong
inversion, respectively;
[0068] FIG. 1i shows a transistor schematic of two (2) finger
inverters of prior art;
[0069] FIGS. 1j and 1k show physical layout abstractions of the two
(2) finger inverters of prior art;
[0070] FIG. 1m shows a three (3) dimensional perspective view of
the two (2) finger inverters of prior art;
[0071] FIG. 1n shows cross-sectional view at Section AA shown in
FIG. 1m;
[0072] FIG. 1p shows a physical layout of a split channel CMOS
transistor of prior art;
[0073] FIG. 1q shows a 3D perspective view of an analog-sized
MOSFET of prior art;
[0074] FIG. 1r shows a physical layout plan of an analog-sized
array of Fin field effect transistors (FinFET) of prior art;
[0075] FIG. 1s shows a zoomed perspective view of inside the circle
Z shown in FIG. 1r, showing a perspective view of a FinFET of prior
art;
[0076] FIG. 1t shows a plot representing intrinsic gain scaling of
nMOS transistor of prior art;
[0077] FIG. 2a illustrates a three (3) dimensional prospective view
of a MOS field-effect transistor (or iFET) with a new mid-channel
bi-directional current port (iPort) of the present invention;
[0078] FIG. 2b illustrates a cross-sectional view of iFET of the
present invention with visualized channel charge distributions;
[0079] FIG. 2c shows a graph of drain voltage V.sub.ds and drain
current I.sub.s when there is no iPort injection current, while
FIG. 2d shows another graph when max iPort injection current is
provided;
[0080] FIG. 2e shows various iFET symbols of the present
invention;
[0081] FIG. 3a illustrates a schematic diagram of complimentary
pair of iFETs of the present invention;
[0082] FIGS. 3b and 3c illustrate a physical layout abstraction of
a complementary iFET (or CiFET) compound device of the present
invention;
[0083] FIG. 3d shows a three (3) dimensional perspective view of
the CiFET compound device; FIG. 3e illustrates cross-sectional view
at Section AA of FIG. 3d;
[0084] FIGS. 3f, 3g and 3h illustrate a CiFET operational modeling,
self-bias schematic and symbol therefor;
[0085] FIG. 4a illustrates a three (3) dimensional perspective view
of a super-saturation current field effect transistor (xiFET) of
the present invention;
[0086] FIG. 4b illustrates a symbol representation of the xiFET
shown in FIG. 4a;
[0087] FIG. 4c illustrates a cross-section view of the xiFET shown
in FIG. 4a;
[0088] FIG. 5 illustrates a schematic diagram of a complementary
pair of xiFET (or CxiFET) of the present invention;
[0089] FIG. 6 illustrates a physical layout abstraction of the
CxiFET shown in FIG. 5;
[0090] FIG. 7a illustrates a three (3) dimensional perspective view
of the CxiFET shown in FIG. 5;
[0091] FIG. 7b illustrates a cross-section view of the CxiFET shown
in FIG. 7a at Section AA;
[0092] FIG. 7c illustrates a power diagram for CxiFET in accordance
with the present invention;
[0093] FIG. 8a(1) shows a schematic diagram of a focal plane array
readout column of prior art;
[0094] FIG. 8a(2) shows a schematic diagram of one array pixel
portion of the focal plane array shown in FIG. 8a(1);
[0095] FIG. 8b shows a schematic diagram of a portion of readout
column of the focal plane array shown in FIG. 8a(1);
[0096] FIG. 8c(1) shows a schematic diagram of a focal plane array
readout column using CiFET in accordance with the present
invention;
[0097] FIG. 8c(2) shows a schematic diagram of a one array pixel
portion of the focal plane array shown in FIG. 8c(1);
[0098] FIG. 8d shows a schematic diagram of a portion of readout
column of the focal plane array shown in FIG. 8c(1);
[0099] FIG. 8e shows a schematic diagram of gain controllable bias
generator in accordance with the present invention; and
[0100] FIG. 9a shows a schematic diagram of latch current
comparator in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0101] A MOS structure referred to herein as an iFET, where the
letter "i" refers to a current and "FET" refers to a Field Effect
Transistor, is the enabling element of several high performance and
novel designs of the present invention. The present invention is
based on the addition of a direct connection to a mid-point in a
Field Effect Transistor (or FET) channel and the realization that
this is a low impedance port (current port, or herein referred to
as "iPort") providing a bidirectional current sink/source
mid-channel with a very low input impedance at a low saturation
voltage, and additionally connecting reciprocal iFETs pairs of
opposite "conductivity type" (P-type & N-type) interconnected
to take advantage of their complementary nature to operate as a
team and symmetry to self-bias near the midpoint between power
supplies. In addition, the relative strength of the first and
second channels of the iFETs can be adjusted (threshold choice,
relative sizing, and doping profiles) to tailor the gain, speed,
quiescent current and input impedance of such a complementary iFET
(or CiFET) compound device of the present invention.
[0102] The iFET, with its iPort provides an uncommon and unexpected
solution to the compensation problem, and then continues to provide
new or alternative solutions to other old problems, exceeding
industry expectations. The advantages of operating circuits in
"weak inversion" have long been known but, so also have the
problems. The CiFET enables circuits to exploit the high gain and
wider dynamic range available in "weak inversion," without
sacrificing superior speed performance. The CiFET compound device
provides a standard active IC gain device that is superior to
ordinary analog MOSETs making digital ICs host analog
functionality. It is not a tradeoff.
[0103] The following is a list of some of the unusual aspects of a
CiFET based circuit, including, but not limited to: [0104] Operates
at low power supply voltage; [0105] High gain; [0106] Extremely
linear; [0107] Very high speed (wide band); [0108] Self-Biasing;
[0109] Low noise; [0110] Quick recovery (DC); [0111] Uses all
digital parts and processes; [0112] iPorts respond to charge
(things in nature are charge based) rather than Volts across a
Resistance; and [0113] iPort has wide dynamic range with constant
gain in an open loop.
[0114] Referring to FIGS. 2a and 2b, according to a preferred
embodiment of the present invention, it provides a current FET (or
iFET) 200, which is comprised of substrate 26a or 26b, source
terminal 24a or 24b. and drain terminal 29a or 29b, defining
therebetween two channels 23a and 25a, or 23b and 25b on the
substrate 26a or 26b, respectively, typically the first (source
channel 23a, or 23b) is connected to the power supply (not shown)
while the second (drain channel 25a, or 25b) connects to the load
(not shown). The substrate 26a or 26b is N- or P-type. The two
channels, source and drain channels 23a and 25a, or 23b and 25b,
respectively, are connected to each other as shown in FIGS. 2a, and
2b, at the iPort control terminal 21a or 21b, and the channels 23a
and 25a, or 23b and 25b, share a common gate control terminal 27a
or 27b, respectively. This configuration means that the iFET 200
has more than one control input terminal.
[0115] The gate control terminal 27a or 27b operates like a
conventional MOSFET insulated gate, with its high input impedance
and a characteristic Trans-conductance (g.sub.m) transfer function.
Typical values of (g.sub.m) for a small-signal MOSFET transistor
are 1 to 30 millisiemens (1 millisiemen=1/1K-ohm) each, a measure
of Trans-conductance.
[0116] The iPort control terminal 21a or 21b is low impedance with
respect to the source terminal 24a or 24b, and has a transfer
function that looks more like beta (.beta.) of a bipolar
transistor, but is actually Trans-resistance (or r.sub.m), or more
generally, especially at high frequencies, Trans-impedance,
measured in K-ohms, where the output voltage is a consequence of an
input current. Depending on the channel sizing ratio of the CiFET
the typical resistance values (or values of r.sub.m) for a
small-signal iFET transistor 200 are from 1K.OMEGA. to 4M.OMEGA., a
measure of Trans-resistance. Current input to voltage output
(Trans-impedance) is the basis for the assertion that 1 .mu.A in
will yield an output of 100 mV (or a gain of 100,000:1) at a large
signal level, or 1 pA in will yield an output of 100 nanoV (or a
gain of 100,000:1) in a low noise amplifier (or LNA) (both results
from the same circuit and linear over this dynamic range).
[0117] These values have been shown to remain true for a single
minimum sized iFET, with inputs from 1 pico-ampere to 10
micro-amperes, using the same circuit in simulation. In 180 nm CMOS
construction the noise floor limits measurements below about 10
pico-amps. iFETS can be constructed with different length to width
proportions with very predictably differing results.
[0118] High gain, uncharacteristic or surprising results differing
from the state of the art designs, is the result of the "Weak
inversion" characteristics of the source channel 23b of the iFET
200 operating in a highly ionized super-saturation mode of FIG.
2b.
[0119] Speed in this super-saturated source channel 23b is not
limited by the transit time of carriers along the channel 23b, but
the high concentration of ionized charge carriers in the active
channel only have to push the surrounding charge a little as charge
is either added or removed from the channel 23b by means of the
iPort control terminal 21b, resulting in a diffusion current which
is defined by exponential relationship as has been realized when a
MOSFET is operated in weak-inversion. This is in contrast to an
electric field causing the charge to transit the channel, which is
a square-law function of the gate control voltage. In this
configuration, speed is faster than logic built from the same
fundamental transistors and unhampered by the "Weak inversion"
stage that has higher gains like bipolar transistors. As opposed to
bipolar transistors, control current can go either in or out of the
iPort control terminal 21b as well as operate with no iPort
current, which is useful for creating a self-bias operating
point.
[0120] Lower noise is facilitated by the self-biased operating
point. Here the potential at drain terminal 29a or 29b is the same
as potential at the gate control terminal 27a or 27b, greatly
reducing the pinch-off effect found in conventional analog circuit
designs.
[0121] The iFET 200, because of the common gate connection over the
source channel 23a/23b and the drain channel 25a/25b, places a
higher than expected voltage on the gate control terminal 27a/27b
(or GS) of the source terminal 24a/24b or source channel 23a/23b.
This higher than expected voltage is responsible for a much thicker
and deeper (lower resistance highly ionized) conduction layer,
allowing the majority of carriers to avoid the traps in the surface
of the crystal lattice, hence--much lower noise similar to the
manner in which a junction field effect transistor (or j-FET)
conduction channel is located below the surface.
[0122] Trans-resistance (r.sub.m) is the "dual" of
Trans-conductance (g.sub.m). When you look up Trans-resistance,
most of the references are to inductors and capacitors, suggesting
that the iFET may be useful in synthesizing inductors.
[0123] The iFET works in the following ways: A low noise amplifier
requires a low impedance channel. A low impedance channel is low in
voltage gain while high in current gain. To establish voltage gain,
a second stage, operating as a current to voltage converter, is
required. A cascaded pair provides such a configuration. Biasing
requirements for a cascaded pair preclude its use at low voltage
unless a solution for the biasing problem is found. The iFET
provides the solution to this problem through self-biasing of a
complementary pair. The impedance of the channel can be designed to
accommodate the impedance of the particular signal source driving
it (see later section on ratio).
[0124] Regarding FETs in general, carriers are attracted to the
surface by the gate field, a low gate voltage creates a thin
surface-layer on the channel (where the conductivity takes place)
while a higher gate voltage creates a thicker under-layer. The thin
layer of carriers is impeded by the non-uniform surface defects
resulting in electrical noise, while a thicker layer of carriers
finds a smoother path below the surface, thus reducing total
electrical noise. This indicates that higher gate voltage
translates to lower noise.
[0125] Referring to FIG. 2b, in the iFET 200, the electric field
created by the gate voltage Vg on the gate control terminal 27b
causes carriers to rise from the substrate 26b into the source
channel 23b region converting the semiconductor material to a
conductor with a relatively large number of carriers per volume or
at saturation, thus establishing a level of conductivity.
[0126] Injection current I.sub.inj introduced into the iPort
control terminal 21b increases the diffused charge (number of
carriers per volume) over and in the source channel 23b, thus
making the source channel 23b even more conductive. The rate of
conductivity change is exponential, similar to that found in "weak
inversion." This exponential rate of conductivity change is due to
the low voltage gradient along the source channel 23b (source
terminal 24b to iPort control terminal 21b voltage gradient).
[0127] The iFET exponential relationship between source channel 23b
charge and gate voltage 27b provides access to log functionality,
where the addition of two log functions is equivalent to
multiplication. A reverse anti-log, or reverse-exponential,
operation recovers the analog output through the opposing
complementary iFET channel. Such exponential relationship may be
used for various low noise amplifier applications. The exponential
relationship is also responsible for the wider dynamic range of
these iFET circuits.
[0128] Again, referring to the source region in FIG. 2a, removing
charge from the gate control terminal 27a or/and iPort control
terminal 21a (number of carriers per volume) results in reduced
conductivity of the semiconductor material in the source channel
23a. In this respect, the iPort control terminal 21a-to-source
terminal 24a connection operates in a manner similar to the
base-region of a bipolar transistor (which is exponential): the
more control current to the iPort control terminal 21a, the more
the device conductivity (g.sub.m).
[0129] The drain channel 25a of the iFET 200 of FIG. 2a operates
more like a conventional FET, in that the thickness of the drain
channel 25a is greater near the iPort control terminal 21a (same
thickness as the source channel 23a) and tapers as it reaches its
diffusion region around the drain terminal 29a (the decreasing
voltage differential between drain channel 25a and gate control
terminal 27a diminishes the field) establishing the output
resistance of the transistor as set by the gate voltage V.sub.g. A
lower drain voltage V.sub.g (close to the voltage found on the
gate), decreases the drain channel output resistance (thicker
channel at the drain diffusion). Along with a thicker conduction
layer, this lower drain channel resistance results in lower noise
and a high output drive capability to establish the desired drain
voltage at the drain 29a with a low impedance drive offered by the
thick conduction layer.
[0130] Diffusion regions around the source region 24a of the iFET
200, operating at a low voltage, has lower voltage gain but it also
has low noise. Diffusion region around the drain terminal 29a,
operating at a higher voltage, provides the desired voltage gain
with a minimal noise contribution, due to the drain voltage being
the same as the gate voltage V.sub.g. This voltage equality is
contributed by a unique biasing construct, to be explained
hereinafter.
[0131] FIG. 2b further shows iFET channel charge distributions,
according to the present invention, with their operating points or
iFET's characteristics without iPort injection current graphed in
FIG. 2c, where the source channel current level 24c and voltage
level 25c at the drain channel 25b with no input current is applied
to iPort control terminal 21b. Slope 26c represents drain channel
25b am dots transresistance r.sub.m, while slope 23c is for source
channel 23b which is super saturated, and iPort input resistance
R.sub.in. FIG. 2d illustrates iFET's V-I characteristics with iPort
injection current, where slop 26d represents drain channel 25b and
its transresistance r.sub.m, while slope 23d is for source channel
23b which is super saturated, and iPort input resistance R.sub.in.
It is to be noted that how a small amount 21d of iPort current
greatly disturbs the drain channel output voltage V.sub.out 25d. As
it can be seen, V.sub.out can swig to almost at full power supply
(V.sub.dd). This is the converse or dual of a normal voltage
controlled current source use of the MOS device where large changes
in the drain-to-source voltage yield minimal changes in the drain
current during saturation as shown in FIG. 2d. This gives an analog
IC designer insight as to the usefulness of the iFET as an
amplifier which does not require a typical large, bulky analog
planar transistor for the needed transconductance to obtain gain.
Instead, the NiFET in a current-controlled voltage source
configuration uses transresistance to boost the gain of the
MOS-based device to new heights.
Non-Inverting Nature
[0132] Regarding the iPort control terminal, in the case of both
the N-channel and P-channel devices, a positive current into the
iPort control terminal displaces an equivalent current coming in
through the drain channel, causing the drain (output) connection to
move in a positive voltage direction--thus the non-Inverting nature
of the iPort input.
[0133] The iPorts also operate as a current inverter as opposed to
a conventional current mirror.
[0134] Interestingly, unlike other semiconductor devices, a
negative current can be extracted from the iPort, causing a drain
(output) shift in the negative direction. Zero input current is
also valid.
Proper Bias
[0135] An iFET 200 (as shown in FIG. 2b) has both gates connected
together and requires a proper bias voltage on the gate to
establish the desired operating point.
Symmetry
[0136] A P-channel device can be constructed and behaves in a
similar fashion to its N-channel counterpart.
[0137] It should be emphasized that while the gate input is
inverted with respect to the drain, the iPort is NOT inverted.
The CiFET Amplifier is the Basic Analog-in-DIGITAL Building
Block:
[0138] While a single iFET has interesting characteristics on its
own, a complementary pair of iFETs (or CiFET) prove to be much more
beneficial. Using the opposite semiconductor type iFET as a load
device conveniently provides the opposing iFET its bias and in
addition has the advantage of balancing out (linearizing) the
inherent non-linarites of MOSFET operation. For instance, the
high-gain exponential characteristics of the source channel's
super-saturated operation are linearized over an extremely wide
dynamic range.
[0139] The resulting complementary device (the seminal CiFET cell)
is arguably the highest possible power gain-bandwidth MOSFET
amplifier stage possible. For instance, looking into either iPort,
the super-saturated source channel input impedance is a relatively
low number constant resistance. This converts any input current
into a small input voltage, which calculates out to be a very high
voltage gain transfer function implemented by the high number
r.sub.m trans-resistance. In addition, the sub-surface operation of
the super-saturated source channel may operate with the lowest
noise possible for any MOS device. The drain channel also maximally
operates below its surface defects for low noise. In the end it is
all about signal-to-noise ratio.
[0140] FIG. 3a presents the seminal CiFET symbol and FIGS. 3b and
3c show a diagrammatically similar physical layout abstraction;
FIG. 3d shows three (3) dimensional perspective view and FIG. 3e
illustrates cross-sectional view at Section AA in FIG. 3d; and
FIGS. 3f and 3g illustrate iPort control terminal behavioral model
of a CiFET device of the present invention, self-bias schematic,
and symbol therefor of a complementary pair of iFET of the present
invention, which is a radical improvement from the state of the art
in high gain, high precision, small scale, analog primitive
building blocks. The complementary pairs of iFETs are built
entirely from logic components, without analog extensions, while
enabling scaling and portability. Both the footprint and the power
consumption per gain/bandwidth are drastically reduced from the
present state of the art, while retaining superior noise
performance.
[0141] Referring to FIG. 3a, the complementary pair of iFETs (or
CiFET) 300 comprises P-type iFET (or PiFET) 301 and N-type iFET (or
NiFET) 302, comprising input terminal 30a connected to both the
gate control terminal 37p of PiFET 301 and the gate control
terminal 37n of NiFET 302, function as the common gate terminal
30a. CiFET 300 receives power, Power - and Power +, where Power -
is connected to the source terminal of NiFET 302 and Power + is
connected to the source terminal of PiFET 301. Each of PiFET 301
and NiFET 302 comprises iPort control terminals (31a and 32a) for
receiving injection current. The drain terminal of PiFET 301 and
NiFET 302 are combined to provide output 39a.
[0142] FIG. 3b stretches out the CiFET 300 PiFET and NiFET devices
301 and 302 of FIG. 3a to visually correlate to the physical layout
abstract of FIG. 3c.
[0143] Referring to FIG. 3c, the CiFET 300 comprising PiFET 301 and
NiFET 302, laid out on the substrate (or body B+ and B-
respectively) like a mirror image along well border WB' shown
therein; PiFET 301 comprises source terminal S+, drain terminal D+,
and iPort control terminal Pi, defining source + channel 34c
between the source terminal S+ and the iPort control terminal Pi
diffusion region 32c, and drain channel 36c between the drain
terminal D+ and the iPort control terminal Pi diffusion region 32c.
NiFET 302 comprises source terminal S-, drain terminal D-, and
iPort control terminal Ni, defining source - channel 33c between
the source terminal S- and the iPort control terminal Ni diffusion
region 31c, and drain channel 35c between the drain terminal D- and
the iPort control terminal Ni diffusion region 31c. CiFET 300
further comprises a common gate terminal 30c over source + channel
34c, drain + channel 36c, source - channel 33c and drain - channel
35c. Accordingly, the common gate terminal 30c is capacitively
coupled to the channels 34c, 36c, 35c, and 33c.
[0144] FIG. 3d is a 3-Dimensional representation of the CiFET
physical layout of FIG. 3c and FIG. 3e is a cross-section AA of
FIG. 3d. The corresponding pinpoint numbers relate the same feature
between each of FIGS. 3a, 3b, 3c, 3d, 3e, 3f, and 3g where the same
feature is represented by the reference number with the figure
letter annexed. FIG. 3h shows a symbol diagram for a CiFET device
of the present invention. FIGS. 3d and 3e further points out the
active channel charge conduction regions 34d, 34e, 36d, 36e, 33d,
33e, 35d, and 35e that exist for the biased CiFET which has its
gate voltage at about half the difference between that on the S+
and S- terminals.
[0145] In many analog circuits, biasing is a problem. Using iFETs
in complementary pairs (31g & 32g) as shown in FIG. 3g allows
them to "self-bias" (38g), thus eliminating drift problems and
additionally, the amplifier finds the maximum gain point on its
operating curve.
[0146] In the "Behavioral Model" as shown in FIG. 3f, the current
at the iPort control terminals, NiPort 33f and PiPort 34f is
converted to a voltage by a trans-resistance (r.sub.m), whose value
determines the gain. Self-bias path 38f to V.sub.input 30f is
provided for eliminating drift problems. This "Trans-resistance"
(r.sub.m) is established by the ratio of the "drain channel" to
"source channel" strength, and remains constant throughout the
entire operational range. Here the iFET operation is derived from
different current densities in the source and drain channels, which
is similar to a dual of the band-gap method of generating a
reference voltage made by running the same current value through a
single transistor and a parallel combination of multiple instances
of an identical transistor. Simulation has shown this resistance
(r.sub.m) to typically be in the range of 1K.OMEGA. to 4M.OMEGA.
with a typical value of 100K.OMEGA., as set by the relative channel
sizing. r.sub.m is the dual of g.sub.m.
[0147] The output (V.sub.out 39f) is a low-impedance source
follower common-gate FET configuration that can deliver its voltage
with the necessary current to drive the following circuit.
[0148] The iPort input is a constant low resistance termination
(related to r.sub.m but much lower) with a constant offset voltage,
CTAT Ref, PTAT Ref, of about 1 mV to 100 mV from their respective
power supply rail. This offset voltage is a "bandgap" reference,
established by the ratio of the "drain channel" to "source channel"
strength.
[0149] A standard CiFET compound device cell can be physically
constructed and instantiated like a logic cell for designing
analog. Normally this is the only active circuit component needed.
Like a transistor, but the CiFET cell does everything needed for an
active component.
[0150] How then is the proper bias voltage produced? The simplest
way of generating the bias voltage is to use iFETs in complementary
pairs, NiFET 31g and PiFET 32g, creating an inverting device as
shown in FIG. 3g, and then using the output 39g to provide negative
feedback 38g to the input 30g. The CiFET compound device will
"self-bias" at a point between the power supplies, where the gain
is maximized and the speed or slew rate is poised for its most
rapid changes. At this self-bias voltage point, the current through
both of the complementary iFET devices 31g and 32g is exactly
equal, there is no other DC current path for the PiFET (32g) drain
except into the NiFET (31g) drain, and thus a specific gate voltage
is established for this equality of currents (or conductivity).
Also since both iFETs 31g and 32g have the same current, the
pull-up ability is exactly equal to the pull-down ability, which
defines the maximum slew rate bias point. The current at the iPort
control terminals, NiPort 33g and PiPort 34g is, then, converted to
a voltage by a trans-resistance (r.sub.m) (not shown), whose value
determines the gain.
[0151] Since the complementary pair of iFETs 31g and 32g is
self-biased, any parametric factors are auto-compensated, for
changes in operating environment. Because of inherent matching
between adjacent parts on an IC, the bias generator can be used to
bias other iFETs nearby. The real-time self-biasing circuit
corrects for parametric changes (in various forms).
[0152] Each of the transistors in an inverter of the present
invention acts as a "dynamic" load for its complement, allowing the
gate voltage to be significantly higher than the traditional bias
point of an analog circuit gate. With the complementary iFET
compound device's higher than normal gate voltage, the source
conduction channel is deep, yielding lower noise.
[0153] The dominant noise source in a traditional analog circuit is
related to "pinch-off." Biasing the drain (or output) at the same
voltage as the gate (zero differential) causes the drain conduction
channel to avoid the channel pinch-off (shallow channel) phenomena
usually encountered in analog circuits. Another way of stating this
is: a transistor gets noisier as the drain approaches its design
maximum voltage, the self-biased inverter operates its transistors
at around half the design maximum voltage and the gate is at the
same voltage as the drain (zero differential), therefore the
self-biased inverter is MUCH quieter.
[0154] The operation of the CiFET amplifier differs from the
operation of a conventional analog amplifier, with its current
mirror loads, in that:
[0155] The "Source" channel has an extremely small (.about.100 mv)
voltage from source terminal to iPort control terminal while the
"Gate terminal" is at .about.1/2 V.sub.supply. This puts the iFET
Source channel into "Super-Saturation," a condition similar to weak
inversion but with high Gate overdrive. Gate overdrives resulting
in an unusually thick conduction layer and along with a low Source
to iPort voltage resulting in that conduction layer remaining thick
all the way along the channel. Notice the differences in the
thickness between the conduction channel 10s in FIG. 1e from that
of 23b in FIG. 2b.
[0156] The "Drain" channel 25b operates with its' drain terminal
29b at .about.1/2 Vmax, greatly reducing the pinch-off (and DIBBL)
effect. This reduced pinch-off condition is further enhanced by the
fact that the "Gate terminal" is operated at .about.1/2
V.sub.supply (same as 1/2 Vmax), meaning no potential difference
between the Drain 29b and the Gate 27b.
[0157] Another important aspect of the CiFET compound device is its
current input that frees it from the speed robing effects of
parasitic capacitance.
[0158] This subtle but significant difference is one of the
enabling features that makes weak inversion work and gives the
complementary iFET amplifier its superior low noise, wider dynamic
range, and speed advantages.
[0159] MOSFETs do not make particularly good amplifiers compared to
equivalent bipolar circuits. They have limited gain, they are
noisy, and their high impedance makes them slow.
[0160] Bipolar Diff-Amps developed to the point where the input
offset is pretty good, but the move to CMOS never really delivered
as good a result.
[0161] It has long been known that superior performance can be had
from CMOS operated in weak inversion but complications arising from
high impedance, due to impractically low currents, preclude taking
advantage of the superior gain (equivalent to that of bipolar
transistors), dynamic range (exceeding that of bipolar
transistors), and logarithmic performance (allowing numerous
decades of amplification) found in weak inversion. Because of weak
inversion the CiFET brings the noise benefits of majority carriers
in a deep channel found in junction-FETs to the MOSFET.
[0162] While a MOSFET in weak inversion, working into a current
source load, delivers a logarithmic transfer function, the same
MOSFET working into an anti-log load cancels the nonlinearity,
yielding a perfectly linear transfer function. The CiFET amplifier
is such a circuit, i.e.: log input, antilog load, perfectly linear,
wide dynamic range, low noise. The low noise is a consequence of
the biasing, where the source channel gate potential is unusually
high and the potential across the source channel itself is
maintained at near zero volts. The drain channel is a level
shifter, maintaining a very low voltage on the source channel while
delivering high amplitude signal swings at the output.
[0163] The CiFET amplifier, implemented in a closed-loop,
sample-data block delivers near perfect performance in terms of
input offset because of its "flying capacitor" input. The CiFET
amplifier, implemented in an open-loop, delivers unexpected levels
of sensitivity (gain >1 million), even in the presence of high
levels of background, this because of the extreme dynamic
range.
Taking Advantage of the Doping Profile and Ratioing:
[0164] Traditionally engineers have avoided using digital logic in
an analog configuration because it was believed to be unacceptably
nonlinear and was difficult to bias. Digital logic also sacrifices
drive symmetry for compactness. Restoring the symmetry through
proper device ratioing (.about.3:1 p:n width) improves linearity,
increases noise immunity, and maximizes dynamic range. Self-biasing
solves the bias problem.
[0165] FIG. 1q depicts the basic symbol and 3-dimensional view of
the MOS transistor structure in saturation. The generic planar
MOSFET here is shown with a typical longer/wider channel used in
customary analog applications. The FET symbol and structure shown
applies to either n- or p-type planar transistors which can further
be related and applied to the wrapped-gate finFET structure as
desired. Note that the FET has four ports including the gate (g)
17q, drain (d) 19q, source (s) 14q, and body (b) 16q. Typically,
voltage is applied as input to the high-resistance gate port 17q,
while a voltage or current may be applied to the physically similar
(and interchangeable) drain 19q and source ports 14q. The bulk/body
port 16q is generally attached to the lowest (or low) voltage
potential for n-type FETs and highest (or high) voltage potential
for p-type FETs to control/prevent forward biasing of the
bulk-source junction and to give the lowest V.sub.gs relative to
the supply voltage for normal operation (although there are
exceptions and special uses for the bulk, they will not be covered
here). Additionally, the planar 3-dimensional MOSFET structure in
FIG. 1q is shown with a wider width, W, and longer length, L,
commonly used for analog circuits, along with a channel in the
pinch-off saturation region.
[0166] In order to maintain a high intrinsic gain, the MOSFET
requires a high output impedance. This is obtained through long
channel lengths necessary for high r.sub.o=R.sub.out. Since g.sub.m
is proportional to the W/L ratio of the MOSFET, in order to keep
g.sub.m high when the channel is long, the channel must also be
proportionally wider. Gain here is .about.g.sub.mR.sub.L/R.sub.out.
As the IC process shrinks, g.sub.m increases, but R.sub.out
decreases faster, ruling out short channel lengths for analog. This
is why as IC processes shrink analog transistors do not scale
accordingly in the newest double-digit CMOS technologies. Also, it
is to be noted that the analog channel current travels close to the
surface under the gate where the surface defect carrier traps
create the characteristic MOSFET 1/f noise.
[0167] FIG. 1r shows a physical layout plan of an array of Fin
field effect transistors (FinFET) of prior art. Sources 14r and
drains 19r are stacked and forms fins, and an array of gates 17r
are disposed therebetween to form FinFETs 12r. Zoomed in view of
the circle Z in FIG. 1r, which shows one of prior art three (3)
dimensional perspective view of FinFET 12r is shown in FIG. 1s.
[0168] FIG. 1t shows a plot representing intrinsic gain scaling of
nMOS transistor of prior art. As it can be seen, the steadily
decreasing intrinsic gain of nMOS transistors alerts analog
designers of impending difficulty that they face when attempting to
scale the design of an amplifier that may have run efficiently at
65 nm or 90 nm to the 14 nm CMOS process, where it will most likely
fail. Therefore, other methodologies which depart from conventional
procedures must be explored in order to find a viable tactic to
harness inherent transistor gain in the newer ultra-deep sub-.mu.m
CMOS technologies.
[0169] FinFETs have short nanoscale channel lengths that increase
g.sub.m while lowering the drain output resistance of the bare
field effect transistor. Higher g.sub.m provides better control
over channel conductance, but the close proximity of the drain to
the source makes them talk to each other making the output
resistance low. This yields a low intrinsic gain of the MOSFET at
nanoscale dimensions. Conversely the CiFET is a low output
resistance device and improves with deep scaling.
[0170] According to the preferred embodiment of the present
invention, noise figures can be particularly optimized on front end
amplifiers through proper ratioing. The iFET's electrical
characteristics can be enhanced by modifying the combined and
relative strength of the source and drain channels, without
modifying the available IC process (without analog extensions).
There are several approaches to realizing this optimization
(adjusting length, width, and threshold among others).
[0171] Nearly any source and drain channel size will make a
functional iFET, but varying the individual iFET channel size, both
relative and cumulative, increases the iFET performance depending
on the objective.
[0172] Fundamentally: [0173] Lower iPort impedance is achieved with
a lower current density (wider) source channel as compared to the
drain channel. [0174] Higher voltage gain is obtained through a
higher resistance (longer) drain channel as compared to the source
channel, which makes a higher output impedance looking into the
drain terminal (iFET Voltage gain=Drain channel resistance/Source
channel resistance). [0175] The power verses speed tradeoff is
controlled by the cumulative sum of all of the channel strengths
used to set the idle current through the complementary iFET
amplifier. This establishes the output slew rate (or output drive
capability).
[0176] To be clear, the strength of the iFET channels are a
function of the individual channel width and lengths, as well as
their thresholds. Each of the iFET channels can have individually
selected sizes and/or threshold relationships to the other
channel.
[0177] FIG. 2e shows various conventions/symbols for iFET devices
of the present invention. Symbols 22g and 24g for PiFET, and
symbols for NiFET 21g and 23g are shown. For example, NiFET 21g or
23g represents an n-type iFET (or NiFET) with shorter source
channel as previously described, and thus, as it can be seen,
NiPort is shown near the Source. An example sizing of the NiFET
device 21g may be, for drain channel with
W.sub.min/2.times.L.sub.min while the source channel is
2.times.W.sub.min/L.sub.min for a combined iFET drain:source ratio
of 4. This NiFET would allow for lower input iPort termination
resistance targeting current gain objectives, which is useful for
high gain current input trans-impedance amplifier applications.
Similarly, PiFET 22g or 24g is shown to have PiPort near the Source
as well which signifies wider source channel. Example sizing of the
PiFET device 22g may be, for drain channel with
3.times.W.sub.min/2.times.L.sub.min, while the source channel may
be 6.times.W.sub.min/L.sub.min. for combined iFET drain:source
ratio also of 4, but with 3.times.PiFET to NiFET ratio adjusting
for similar PiFET to NiFET overall strength, roughly balancing P to
N total channel conductance.
[0178] While iFET amplifiers can be constructed with minimum sized
devices which do provide ample current at the output for very fast
response and high accuracy, care must be exercised so that the
complementary iFET amplifier does not pass too much current,
subjecting it to mechanical failure. The physical layout requires
enough contacts and metal for the required DC and transient
currents.
[0179] Referring to a current field-effect transistor (iFET) in
FIGS. 2a and 2b; separating the source channel 23a/23b gate
s27a/s271) from the drain channel 25a125b gate diffusion region
d27a/d27b of the iFET 200, the super-saturation 28b level of the
iFET source channel 23a/23b can be electrically controlled via
independently adjusting the source gate s27a/s27b voltage over the
source channel s27a/s27b. In order to enable independent gate
control, the gate 27a/27b is split as shown in FIGS. 4a and 4e.
[0180] Super-saturation current field transistor (or xiFET)
includes a source terminal 44a, a drain terminal 49a, iPort (or
diffusion) terminal 41a on a body 46a. A drain channel 45a is
defined between iPort terminal 41a and the drain terminal 49a, and
source channel 43a between the iPort terminal 41a and the source
terminal 44a. A first gate terminal 47a1 is capacitively coupled
over the drain channel 45a; while the second gate terminal 47a2 is
capacitively coupled over the source channel 43a. This iFET
improvement in xiFET 400 provides, instead of the common gate
27a/27b or a method of the iFET illustrated in FIG. 2a/2b, means to
have its source channel 43a (as shown in FIGS. 4a and 4c) be at its
maximum saturation-limit charge distribution. This maximum
saturation limit provides its low impedance iPort control terminal
41a with the lowest possible input impedance for a given channel
sizing ratio. Here the source channel gate 47a2 voltage Vsg is
provided by an independent voltage source v47a2.
[0181] By adjusting voltage Vsg at the gate s47as with an
independent signal source v47a2, the iPort input impedance can be
electrically controlled. This is particularly useful for matching
the input impedance of an antenna or transmission line to minimize
reflections and maximize input power transfer into the iPort 41a.
This additional source channel gate input also can be used to
control the iPort trans-impedance gain, establishing a voltage
controlled amplifier gain application. Also the source channel gate
input 47a2 can be used to turn the xiFET 400 off and on rapidly to
cut off its drain current when not needed in order to save power.
The xiFET turn-on is nearly, instantaneous in that the bias point
at the iPort 41a is low impedance and does not need to settle as
compared with conventional analog current mirrors. When turned off,
the xiFET output is high impedance, thus when multiple xiFET
voltage outputs are tied together and individual xiFETs are
enabled, an array of xiFETs can be used as an analog
multiplexer.
[0182] This property is especially useful in focal plane array
(FPA) readouts. Here the FPA core is not changed at all, but only
the readout line drive voltages are controlled differently to
operate the existing two transistor pixel readout transistors as an
xiFET. The low noise, high speed, high dynamic range, low power
properties work in concert to create a high-quality FPA.
[0183] FIG. 4b shows its corresponding symbol s400 of the circuit
400 shown in FIGS. 4a and 4c, including source terminal s44, drain
terminal s49, iPort s41, body s46, first gate s47a and second gate
s47b.
[0184] FIG. 5 shows a schematic diagram of a complementary pair
xiFET (or CxiFET) 500 in accordance with the present invention.
CxiFET 500 includes n-type xiFET (or NxiFET) 501 and p-type xiFET
(or PxiFET) 502. The first gate terminals of NxiFET 501 and PxiFET
502 are connected together to receive input 50a, the drain
terminals of NxiFET 501 and PxiFET 502 are also connected together
to form output 59a. The source terminal of Nxi FET 501 receives
negative power supply and the source terminal of PxiFET 502
receives positive power supply. Providing this second gate control
inputs 57-a and 57+a in this complementary configuration 500 as
shown in FIG. 5 affords similar linearization to that of a CiFET
device structure as previously discussed.
[0185] When either of the second gate terminals 57-a, 57+a are
taken to their respective power supply rail, the CxiFET 501, 502 is
turned OFF, thus saving power when the amplifier is not in use.
Because of the large and uniform distribution of source channel
ionized charge carriers, and low channel/iPort voltage change
between OFF to saturated, and back OFF is extremely fast,
approaching logic speed for analog signals. The source channel
quickly cuts OFF or immediately reaches its stable bias point. The
low resistance and minimal voltage change drive rapid exponential
settling to a stable bias point.
[0186] In addition, when either or both second gates 57-a, 57+a are
taken to an intermediate voltage, the current through the CxiFET
device 500 from Power+ to Power- and trans-impedance r.sub.m gain
is controlled, making gain and power consumption verses speed
electrically controllable. When these source channel gates are
switched to the common gate voltage, the CxiFET becomes a
CiFET.
[0187] When the CxiFET is used for charge-coupled logic, inputs on
these second gates 57-a, 57+a can be used to turn the logic ON or
OFF as well as strobe the logic state with a clock as
appropriate.
[0188] It can also be a cross-coupled latch control where a pulse
of charge flips the latch and due to the low voltage operating
properties the latched state can be held with almost zero current
consumption as shown in FIG. 7c. The source channel voltages can be
used to electrically adjust the speed:power of the CxiFET operation
to optimize power in real time. The analog voltage signals on the
source channel gates are multiplied by the analog current signal on
the iPorts yielding an analog multiplier with a wide dynamic range
and the ability to sum the output derived from the PxiFET with the
output from the NxiFET.
[0189] In the "iFET," the source channel is tied to a common,
single gate. In the "xiFET," the source channel gate is taken to an
independent normally higher voltage than the drain channel gate
voltage, to increase the source channel's super-saturation level.
Thus the input resistance at the iPort is controlled by this
separate source channel's gate voltage input. This configuration is
tied to the proportional-to-absolute temperature (or
PTAT)/complementary-to-absolute temperature (or CTAT) coefficient
of the termination voltage of the iPort, as well as the magnitude
of the trans-impedance gain (r.sub.m). It can be modeled as two
separate FET transistors, with an iPort current input in the
connection between their series hookup. Using this as a
well-behaved linear analog bi-directional current/charge
input/output is unique, which leads to a methodology of creating a
trans-impedance MOSFET with separate electrically controllable
r.sub.m gain and input resistance and input termination voltage.
This is a method of adjusting the PTAT/CTAT iPort reference
voltages and temperature properties electrically.
[0190] In general, the iFET/CiFET and the xiFET/CxiFETs are a new
type of analog circuit element, or type, that operates in a
charge-injection mode, yielding a trans-resistance amplifier (or
more generally a trans-impedance amplifier) which creates a unique,
predictable, parametrically stable, and very useful analog gain
block. The advantage of this device is its stability, linearity,
high-gain, high-speed, low-noise, unprecedented low voltage
operation not limited by threshold voltage, high output drive, low
power, small, and built out of digital-only IC parts.
[0191] Various degrees of complementary cancellation result from
the opposite diffusion types of symmetry.
[0192] This yields an extremely linear transfer function obtained
by the same circuit in the opposite diffusion type creating the
complementary output signal drive. Thus, non-linearity and other
errors cancel out at the output. We do not know of any analog
circuit that considers this except a CMOS inverter with balanced
transistors when it is roughly optimized for analog
applications.
[0193] FIG. 6 is a physical layout abstraction of a CxiFET 500
including NxiFET 501 and PxiFET 502 joined together over well
border W, and FIG. 7a is a three dimensional representation
thereof, and FIG. 7b is a cross-sectional view at section AA in
FIG. 7a. This layout corresponds to the schematic diagram shown in
FIG. 5. Input 50d is connected to the common gate 57d, which is in
communication with the first gate 57d n of NxiFET 501,
capacitively. coupled to drain channel 55d between the drain
terminal d55d and NiPort 51d; and the first gate 57dp of PxiFET
502, which is capacitively coupled to drain channel 56d between the
drain terminal d56d and PiPort 52d. Second gate terminal 57-a of
NxiFET 501 is capacitively coupled to source channel 53d between
source terminal s53d and NiPort 51d; while second gate terminal
57+d of PxiFET 502 is capacitively coupled to source channel 54d
between source terminal s52d and PiPort 52d. Drain terminals d55d
and d56d of NxiFET 501 and. PxiFET 502 are connected together to
form voltage output 59d.
[0194] FIG. 7c shows power consumption of a CxiFET. Nanoscale
processes will be lower, primarily as a function of the small CMOS
geometry and threshold voltage reduction with IC process
shrink.
Noise Advantages:
[0195] In the end, it comes down to signal-to-noise ratio. Low
power supply voltage requirements in ultra-deep-sub-.mu.m IC
processes limit the maximum signal swing to a much smaller number
than most analog designers are used to. So with a smaller signal,
the noise must be equally small in order to maintain the desired
signal to noise ratio. It is imperative that noise issues be
reduced. This iFET amplifier technology not only reduces noise by
an amount as would be necessary, but performs far beyond
expectations, delivering ultra-quiet front ends.
[0196] 1/f noise in the source channel is reduced because the
self-bias scheme provides a high field strength on the source
channel's gate, forcing carriers in the channel to operate below
the surface where there is a smoother path (fewer obstructions)
than along the surface where crystal lattice defects interfere.
[0197] 1/f noise in the drain channel is also low. Unlike
conventional analog designs, the gate is self-biased at the
half-way point between the power supply rails as is the drain,
while the iPort is within .about.100 millivolts of the power rail.
With the high electric field along the drain channel, and the gate
voltage equal to the drain terminal voltage, the carriers are
constrained to flow mostly below the channel surface. This keeps
the drain channel out of pinched off conditions, where unwanted 1/f
noise would be generated.
[0198] Resistor noise is reduced because the self-bias
configuration puts the complementary pair at its lowest channel
resistance operating point. Resistance is caused by collisions,
between carriers and the surrounding atoms in the conductor. The
lower the resistance is, the fewer the collisions are.
[0199] Wide band noise (white-noise) would always be an issue in
high gain for high frequency circuits. While conventional designs
adjust the gate voltage to establish suitable operating point(s),
the designs of the present invention establish the gate voltage at
the optimum point (the "sweet-spot") and then adjust the load to
establish the desired operating point. This approach establishes a
higher quiescent current where (for reasons explained above) higher
current density circuits have lower wide band noise.
[0200] High common mode power supply rejection is inherent in the
complementary iFET circuit of the present invention. Signals are
with respect to the mid-point instead of being with respect to one
of the power supply rails, (similar to an op-amp with its "virtual"
ground). Power supply noise is from one rail to the other, equal
and opposite in phase with respect to each other; thus canceling
around the mid-point.
[0201] Ground-Loop noise is diminished because the circuit ground
is "virtual" (just like in many op-amp circuits), rather than
ground being one or the other power supply connections . . . . In
the closed-loop case, "Flying capacitors" (or "input voltage
sampling capacitors") are employed. With "flying capacitors" there
is no direct electrical connection between stages, so there is no
common ground; virtual or otherwise. The use of "differential
decoupling" (flying capacitors) offers transformer like isolation
between stages, with the compactness of integrated circuit
elements.
[0202] Coupled noise from "parasitic induced crosstalk" increases
by the square of the signal amplitude. Unintended capacitive
coupling with a 1 volt signal causes a lot more trouble than with a
100 mV signal, by a factor of 100:1 (square law effect). The small
voltage signals employed in the analog sections, reduce this
capacitive coupled interference substantially. Nearby Digital
signals will, by definition, be high amplitude (rail-to-rail). Good
layout practices are still the best defense against this digital
source of noise.
Additional Advantages:
[0203] There are a number of additional advantages. For example,
bi-directional control on the iPort means that current can flow
in-to as well as out of this connection; both directions having a
significant control effect on overall channel current. The iPort
has about five (5) orders of magnitude more dynamic control range
than the gate.
[0204] The iFET of the present invention yields an analog structure
that is significantly faster than logic using the same MOS devices.
This speed improvement is due to the fact that the complementary
structure expresses its maximum gain (and highest quiescent
current) at its natural self-bias point, midway between the power
supplies.
[0205] Since the iPort voltage does not significantly change, it is
immune to the R/C time constant effects of the surrounding
parasitics, thus the iPort (current) input responds faster than the
gate (voltage) input.
[0206] Since in most applications of the CiFET compound device of
the present invention, the output voltage (drain connection point)
does not vary greatly, and thus making the output immune to the R/C
time constant effects of the surrounding parasitics. A logic signal
is slower than analog here because logic signals have to swing from
rail to rail.
[0207] Drain-induced barrier lowering or (DIBL) threshold reduction
is avoided in the CiFET compound device operating in the analog
mode. When gain and threshold voltage is important, the drains are
operating around half of the power supply voltage, thus eliminating
the higher drain voltages where DIBL effects are prevalent.
[0208] FIG. 8a(1) shows a schematic diagram of a focal plane array
(pixel) 80a of prior art. FIG. 8a(2) shows a schematic diagram of
one array pixel portion PX80 of the focal plane array 80a shown in
FIG. 8a(1). FIG. 8b shows a schematic diagram of a portion 80b of
readout column of the focal plane array 80a shown in FIG. 8a(1),
including source follower Q82, row select control Q83 and SW80
(SW80 is controlled by row select signal SEL80), and current source
mirror Q84 and Q85.
[0209] The circuit 80a shows a single core pixel element PX80,
along with the current mirror Q84 and Q85 that pulls a current
80Iload down from each of the parallel columns to bias source
follower Q82 for applying the respective Pixel voltage to its
column ADC U80. The row of pixels is selected by logically turning
on the Row-Select MOSFET Switch Q83 by row select signal 80Rsel
inside each of the selected row pixels PX80. The respective column
bias currents are routed form Vdd in each pixel through the pixel
Source-Follower Q82; which in effect outputs the PhotoDiode node
PD80 voltage 80aVfd, minus the operating threshold of the source
follower Q82. Neither of these two Pixel transistors provide gain,
they just pass the Vfd pixel voltage 80aVfd to the long full array
readout column 80RC, making the pull-up relatively stiff, but
leaving the pull down weak, due to the high impedance current
source output, for driving the high capacitance column bus in one
direction, but not the other.
[0210] FIG. 8c(1) shows a schematic diagram of a focal plane array
(pixel) PX80c using Complementary iFET/xiFET structure in
accordance with the present invention; FIG. 8c(2) shows a schematic
diagram of a one array pixel portion PX80c of the focal plane array
80c shown in FIG. 8c(1); and FIG. 8d shows a schematic diagram of a
portion of readout column of the focal plane array shown in FIG.
8c(1). In particular, the circuit 80d shown in FIG. 8d is
functionally equivalent to the circuit 80b shown in FIG. 8b. When
the row select SW80d switch is at Vss, the pixel output is turned
off, but when the row select 80csel is switched to the CiFET bias
voltage 80cbias, the row select transistor channel now operates in
the CiFET mode providing low output impedance to the readout
column, which rapidly and accurately drives the column to a high
resolution pixel output voltage with low noise and wide dynamic
range.
[0211] As it can be seen in comparison of FIGS. 8a(1), 8a(2) and 8b
to 8c(1), 8c(2), and 8d, just a trivial change using CiFET and
CxiFET would allow eliminating the large bulky current mirrors and
replacing them with a minimum size iFETs. The pixel core PX80/PX80c
is unchanged, but just driven differently from outside the pixel
array core in that the current in the source follower and row
select transistors are reversed in direction. For this the pixel
core metal routing for the source follower Vdd is instead connected
to Vss, and the row select control line is switched to the CiFET
self-biased voltage instead of Vdd for selecting pixel row readout.
This assembles a fast low output impedance column driver with wide
dynamic signal range and low noise. Array readout gain control may
be also activated by trimming all of the FIG. 8c(1) PiPorts P84b
together including FIG. 8e bias generator 80e PiPort P86b. This
would provide various performance improvement over the prior
art.
[0212] In FIG. 8c(1), the circuit 80c includes PiFET P84, one array
pixel read out portion PX80c, and Analog to Digital converter (or
ADC) U81. In PiFET P84, its source terminal P84a receives positive
power supply Vdd, its iPort terminal P84b receives gain control
80cgain, gate terminal P84d receives bias 80cbias, and drain
terminal is connected to the input of ADC U81 and coupled to one
pixel readout circuit PX80c, which includes a photodiode PD80'', a
pair of FETs Q80'' and Q81'' and a xiFET pair Q82'' and Q83'' which
are connected together to receive Vrst 80cVrst, Initialization
signal 80cInit, exposure signal 80cExpo, and a row select signal
80cRsel for read out control. The circled regions in FIGS. 8a(2)
and 8c(2) highlight the connection differences between the prior
art and the current invention. As it can be seen in FIG. 8c(2), one
array pixel read out portion PX80c includes an hybrid xiFET in
control block 80d, which can be further and clearly seen in FIG. 8d
as N85.
[0213] The equivalent circuit 80d in FIG. 8d shows a hybrid pair of
a PiFET P84 and NxiFET N85 yielding CiFET amplification along with
output control functionality, which is at least equivalent to the
block 80b shown in FIG. 8b. Circuit 80d further includes switch
SW80d, functionally representing row-select logic which is operated
by a select signal 80cself for selectively coupling the gate
terminal N85d of NxiFET N85 to the CiFET bias 80cbias during row
activation or negative power supply Vss for deactivation. The
source gate terminal N85e of NxiFET N85 reads the PIXEL voltage
stored on capacitor C80d. The drain gate terminal N85d is switched
from Vss when the PIXEL row is unselected to FIG. 8e CiFET bias
voltage out80e for row selection. Additionally, gain can be
controlled by signal 80cgain which is received at iPort terminal
P84b of PiFET P84.
[0214] FIG. 8e shows a schematic diagram of gain controllable CiFET
bias generator 80e for generating out80e CiFET self-bias voltage to
feed 80cbias to the column pullup PiFET gates and also provides the
row select voltage level used during row select by switch SW80d
making the xiFET/PiFET operate similar to the seminal CiFET cell in
accordance with the present invention.
[0215] The gain controllable bias generator 80e includes a
complementary pair of NiFET N86 and PiFET P86, each of which
includes a source terminal N86a/P86a, iPort terminal N86b/P86b,
drain terminal N86c/P86c, and gate terminal N86d/P86d. The source
terminal N86a of NiFET N86 receives negative power supply Vss and
the source terminal P86a of PiFET P86 receives positive power
supply Vdd. The iPort terminal P86b of PiFET P86 receives gain
control signal gc80e. The gate terminals N86d and P86d, and the
drain terminals N86c and P86c are coupled together as output out80e
which is bias voltage generated by the present circuit 80e.
[0216] FIG. 9a shows a schematic diagram of a latching current
comparator 900 in accordance with the present invention. The
comparator 900 comprises a differential trans-impedance amplifier
(CiTIA) 900m and a comparator COMP. The CiTIA 900m includes three
pairs of hybrid complementary pairs of PiFET and NxiFET, including
first pair of PiFET 301a and NxiFET 501a, second pair of PiFET 301b
and NxiFET 501b, third pair of PiFET 301c and NxiFET 501c, and
receives current inputs 70m, 71m, 72m and 73m. Each of NxiFETs
501a, 501b and 501c further includes a second diffusion terminal
500ae/500be/500ce, which receives a strobe signal 900strb in order
to save power when the system is not in used. In a further
preferred embodiment of the present invention, PiFET 301a, 301b
and/or 301c may be replaced with PxiFET device(s) and receives the
strobe signal 900strb for saving power that may otherwise be
used.
[0217] The comparator COMP includes two CiFET 300a and 300b, each
of which are the same as CiFET 300 shown in FIGS. 3a, 3g, etc. A
first set of switches SS90a, SS90b, and SS90c close to pass through
during "SETUP" phase of a control signal, and a second set of
switches ES90a and ES90b, which close to pass through during
"ENABLE" phase of the control signal. The comparator COMP includes
a first capacitor COS90a, and a second capacitor COS90b, each
having a first terminal and second terminal. The second terminal of
the first capacitor, COS90a, is connected to the input of the first
CiFET 300a. The output of the first CiFET 300a is capacitively
coupled to the input of the second CiFET 300b. An output from the
second CiFET 300b forms output 900out of the circuit 900. This
capacitor COS90b stores the difference between the two CiFET 300a
and 300b self-bias voltages while capacitor COS90a stores the
difference between the self-bias voltage of CiFET 300a and the
positive reference voltage 79m of the differential TIA 710m for
comparison during the setup phase of operation.
[0218] During the "SETUP" phase of the control signal, the first
and second switches SS90a, SS90b, and SS90c and ES90a and ES90b
causes to couple the positive voltage output 79m of the CTIA 710m
with the first terminal of the first capacitor COS90a, causes the
first CiFET 300a to be self-biased by connecting its output to its
input, and further causes the second CiFET 300b to be self-biased
by connecting its output to its input, while the second capacitor
COS90b stores any slight offset voltage difference between these
two self-bias voltages.
[0219] At the beginning of the "ENABLE" phase of the control
signal, the first and second switches SS90a, SS90b, and SS90c and
ES90a causes the difference between the positive voltage 79m and
the negative voltage 78m to be instantly coupled to the first and
second terminals of the first capacitor COS90a, and through to the
input of the first latch CiFET 300a input. After a brief logic
buffer propagation delay, the output switch ES90b of the second
CiFET 300b is closed through to the input of the first CiFET 300a
forming a rapid AC positive feedback latching loop. This brief
logic buffer delay allows the CiFETs 300a and 300b to start in
their proper polarity direction as imposed by the difference of the
TIA outputs 79m and 78m. Ideally, once the output of CiFET 300b
begins to move, the positive feedback loop is closed driving the
comparator latch all the way to its logical state rapidly to be
passed onto the system which this comparator is embedded in.
[0220] For even greater comparator decision speed, the latching
current comparator 900 may optionally have a feedforward capacitors
CFF90p and CFF90n, which capacitively couple the input of the first
CiFET 300a to iPorts of the second CiFET 300b.
Definitions of Terms
[0221] iFET: A 4 terminal (plus body) device similar to a Field
Effect Transistor but with an additional control connection that
causes the device to respond to current input stimulus.
[0222] source channel: A semiconductor region between iPort
diffusion and the Source diffusion. Conduction in this region is
enabled by an appropriate voltage on the Gate.
[0223] drain channel: A semiconductor region between Drain
diffusion and the iPort diffusion. Conduction in this region is
enabled by an appropriate voltage on the Gate.
[0224] CiFET: A single stage, complementary iFET compound device
shown in FIG. 3a.
[0225] super-saturation: an exponential conduction condition
similar to weak inversion, but with high Gate overdrive and forced
low voltage along the conduction channel. FIG. 2b #23b.
[0226] feed-forward: A technique to present a signal on an output,
early on, in anticipation of the ultimate value.
[0227] self-biased: Unlike fixed-bias circuits, self-biased
circuits adjust to local conditions to establish an optimum
operating point.
[0228] dual: (of a theorem, expression, etc.) related to another by
the interchange of pairs of variables, such as current and voltage
as in "Trans-Conductance" to "Trans-Resistance."
[0229] trans-resistance: infrequently referred to as mutual
resistance, is the dual of Trans-conductance. The term is a
contraction of transfer resistance. It refers to the ratio between
a change of the voltage at two output points and a related change
of current through two input points, and is notated as r.sub.m:
g m = .DELTA. I out .DELTA. V in ##EQU00001## r m = .DELTA. V out
.DELTA. I in ##EQU00001.2##
[0230] The SI unit for Trans-resistance is simply the ohm, as in
resistance.
[0231] For small signal alternating current, the definition is
simpler:
g m = i out v in ##EQU00002## r m = v out i in ##EQU00002.2##
[0232] trans-impedance: similar to trans-resistance, but further
includes complex variables for high frequency applications.
[0233] trans-conductance is a property of certain electronic
components. Conductance is the reciprocal of resistance;
Trans-conductance is the ratio of the current variation at the
output to the voltage variation at the input. It is written as
g.sub.m. For direct current, Trans-conductance is defined as
follows:
g m = .DELTA. I out .DELTA. V in ##EQU00003## r m = .DELTA. V out
.DELTA. I in ##EQU00003.2##
[0234] For small signal alternating current, the definition is
simpler:
g m = i out v in ##EQU00004## r m = v out i in ##EQU00004.2##
[0235] Trans-conductance is a contraction of transfer conductance.
The old unit of conductance, the mho (ohm spelled backwards), was
replaced by the SI unit, the Siemens, with the symbol S (1
siemens=1 ampere per volt).
[0236] translinear circuit: translinear circuit is a circuit that
carries out its function using the translinear principle. These are
current-mode circuits that can be made using transistors that obey
an exponential_current-voltage characteristic--this includes BJTs
and CMOS transistors in weak inversion.
[0237] subthreshold conduction or subthreshold leakage or
subthreshold drain current is the current_between the source and
drain of a MOSFET when the transistor is in subthreshold region, or
weak-inversion region, that is, for gate-to-source voltages below
the threshold voltage. The terminology for various degrees of
inversion is described in Tsividis. (Yannis Tsividis (1999).
Operation and Modeling of the MOS Transistor_(Second Edition ed.).
New York: McGraw-Hill. p. 99. ISBN 0-07-065523-5.)
[0238] Subthreshold slope: in the subthreshold region the drain
current behavior--though being controlled by the gate_terminal--is
similar to the exponentially increasing current of a forward biased
diode. Therefore a plot of logarithmic drain current versus gate
voltage with drain, source, and bulk voltages fixed will exhibit
approximately log linear behavior in this MOSFET operating regime.
Its slope is the subthreshold slope.
[0239] Diffusion current: Diffusion current is a current_in a
semiconductor_caused by the diffusion_of charge carriers (holes
and/or electrons). Diffusion current can be in the same or opposite
direction of a drift current, that is formed due to the electric
field_in the semiconductor. At equilibrium in a p-n junction, the
forward diffusion current in the depletion region is balanced with
a reverse drift current, so that the net current is zero. The
diffusion current and drift current together are described by the
drift-diffusion equation.
[0240] Drain-induced barrier lowering: Drain-induced barrier
lowering or DIBL, is a short-channel effect in MOSFETs referring
originally to a reduction of threshold voltage_of the transistor at
higher drain voltages.
[0241] As channel length decreases, the barrier .phi..sub.B to be
surmounted by an electron from the source on its way to the drain
reduces.
[0242] As channel length is reduced, the effects of DIBL in the
subthreshold region (weak inversion) show up initially as a simple
translation of the subthreshold current vs. gate bias curve with
change in drain-voltage, which can be modeled as a simple change in
threshold voltage with drain bias. However, at shorter lengths the
slope of the current vs. gate bias curve is reduced, that is, it
requires a larger change in gate bias to effect the same change in
drain current. At extremely short lengths, the gate entirely fails
to turn the device off. These effects cannot be modeled as a
threshold adjustment.
[0243] DIBL also affects the current vs. drain bias curve in the
active mode, causing the current to increase with drain bias,
lowering the MOSFET output resistance. This increase is additional
to the normal channel length modulation effect on output
resistance, and cannot always be modeled as a threshold
adjustment.
[0244] PSSR stands for power supply rejection ratio, defined
as:
PSSR = 20 log 10 ( .DELTA. V dd .DELTA. V out A ) [ dB ]
##EQU00005##
[0245] where A is the gain of the circuit. Some manufacturers base
PSSR on the offset voltage to the amplifier input and others base
the PSSR on the voltage output as shown in the example
equation.
* * * * *