16-07-2014, 12:23 PM
MEMRISTOR
[attachment=65883]
ABSTRACT
Every person with an electronics background will be familiar with the three fundamental circuit elements - the resistor, the capacitor, and the inductor. These three elements are defined by the relation between two of the four fundamental circuit variables- current, voltage, charge and flux.
In 1971, Leon Chua reasoned on the grounds of symmetry that there should be a fourth fundamental circuit element which gives the relationship between flux and charge. He named
this circuit element the memristor, which is short for memory resistor. In May 2008, researchers at HP Labs published a paper announcing a model for the physical realization of the memristor.
It is proposed that memory storage devices that has very high data density and computers
that require no time for boot up can be developed using memristor based hardware. A new
physical quantity which is also introduced associated with memristor. It also solves some
unexplained voltage current characteristics observed in certain materials at atomic levels.
This report focuses on the memristor and reviews its properties. The HP model for the
memristor is also discussed, and few of the potential applications of the memristor are
presented.
INTRODUCTION
For nearly 150 years, the known fundamental passive circuit elements were limited to
the capacitor (discovered in 1745), the resistor (1827), and the inductor (1831). Anyone
familiar with electronics knows the trinity of fundamental components: the resistor, the
capacitor, and the inductor. Typically when most people think about electronics they may
initially think of products such as cell phones, radios, laptop computers, etc. Others, having
some engineering background, may think of resistors, capacitors, transistors, etc. which are
the basic components necessary for electronics to function. Such basic components are fairly
limited in number and each has their own characteristic function. For example,
resistors perform the function of electrical energy dissipation, capacitors perform the function
of electrical energy storage, and transistors perform the functions of electrical
energy amplification and switching. The arrangement of these few fundamental circuit
components form the basis of almost all of the electronic devices we use in our everyday life.
In a brilliant but underappreciated 1971 paper, Leon Chua, a professor of electrical
engineering at the University of California, Berkeley, predicted the existence of a fourth
fundamental device, which he called a memristor. He proved that memristor behavior could
not be duplicated by any circuit built using only the other three elements, which is why the
memristor is truly fundamental. Memristor is a contraction of ―memory resistor,‖ because
that is exactly its function: to remember its history.
MEMRISTOR FEATURES
Memristor is passive two-terminal element that maintains functional relation between
charge flowing through the device (i.e. time integral of current) and flux or A memristor is a
two-terminal semiconductor device whose resistance depends on the magnitude and polarity
of the voltage applied to it and the length of time that voltage has been applied. When you
turn off the voltage, the memristor remembers its most recent resistance until the next time
you turn it on, whether that happens a day later or a year later.
PIPE AND CURRENT ANALOGY
A common analogy to describe a memristor is similar to that of a resistor.
Think of a resistor as a pipe through which water flows. The water is electric charge. The
resistor’s obstruction of the flow of charge is comparable to the diameter of the pipe: the
narrower the pipe, the greater the resistance. For the history of circuit design, resistors have
had a fixed pipe diameter. But a memristor is a pipe that changes diameter with the amount
and direction of water that flows through it. If water flows through this pipe in one direction,
it expands (becoming less resistive). But send the water in the opposite direction and the pipe
shrinks (becoming more resistive). Further, the memristor remembers its diameter when
water last went through. Turn off the flow and the diameter of the pipe
MEMRISTOR AND RESISTOR
This new circuit element shares many of the properties of resistors and shares the
same unit of measurement (ohms). However, in contrast to ordinary resistors, in which the
resistance is permanently fixed, memristance may be programmed or switched to different
resistance states based on the history of the voltage applied to the memristance material. This
phenomena can be understood graphically in terms of the relationship between the current
flowing through a memristor and the voltage applied across the memristor. In ordinary
resistors there is a linear relationship between current and voltage so that a graph comparing
current and voltage results in a straight line. However, for memristors a similar graph is a
little more complicated.
ADVENT OF HP LABS
Even though Memristance was first predicted by Professor Leon Chua, Unfortunately,
neither he nor the rest of the engineering community could come up with a physical
manifestation that matched his mathematical expression.
Thirty-seven years later, a group of scientists from HP Labs has finally built real working
memristors, thus adding a fourth basic circuit element to electrical circuit theory, one that will
join the three better-known ones: the capacitor, resistor and the inductor.
Interest in the memristor revived in 2008 when an experimental solid state version was
reported by R. Stanley Williams of Hewlett Packard. HP researchers built their memristor
when they were trying to develop molecule-sized switches in Teramac (tera-operation-per second
multi architecture computer). Teramac architecture was the crossbar, which has since
become the de facto standard for nanoscale circuits because of its simplicity, adaptability, and
redundancy.
A solid-state device could not be constructed until the unusual behavior of nanoscale
materials was better understood. The device neither uses magnetic flux as the theoretical
memristor suggested, nor do stores charge as a capacitor does, but instead achieves a
resistance dependent on the history of current using a chemical mechanism.
The HP team’s memristor design consisted of two sets of 21 parallel 40-nm-wide wires
crossing over each other to form a crossbar array, fabricated using nanoimprint lithography.
A 20-nm-thick layer of the semiconductor titanium dioxide (TiO2) was sandwiched between
the horizontal and vertical nanowires, forming a memristor at the intersection of each wire
pair. An array of field effect transistors surrounded the memristor crossbar array, and the
memristors and transistors were connected to each other through metal traces.
The crossbar is an array of perpendicular wires. Anywhere two wires cross, they are
connected by a switch. To connect a horizontal wire to a vertical wire at any point on the
grid, you must close the switch between them. Note that a crossbar array is basically a storage
system, with an open switch representing a zero and a closed switch representing a one. You
read the data by probing the switch with a small voltage. Because of their simplicity, crossbar
arrays have a much higher density of switches than a comparable integrated circuit based on
transistors.
APPEARANCE OF MEMRISTOR
HP Labs' memristor has Crossbar type memristive circuits contain a lattice of 40-
50nm wide by 2-3nm thick platinum wires that are laid on top of one another perpendicular
top to bottom and parallel of one another side to side. The top and bottom layer are separated
by a switching element approximately 3-30nm in thickness. The switching element consists
of two equal parts of titanium dioxide (TiO2). The layer connected to the bottom platinum
wire is initially perfect TiO2 and the other half is an oxygen deficient layer of TiO2
represented by TiO2-x where x represents the amount of oxygen deficiencies or vacancies.
The entire circuit and mechanism cannot be seen by the naked eye and must be viewed under
a scanning tunneling microscope, as seen in Figure 6, in order to visualize the physical set up
of the crossbar design of the memristive circuit described in this section.
Fig 6: figure showing crossbar architecture and magnified memristive switch having platinum
electrodes and
TRANSISTOR VERSUS MEMRISTOR
The first transistor was a couple of inches across which was developed about 60 years
ago. Today, a typical laptop computer uses a processor chip that contains over a billion
transistors, each one with electrodes separated by less than 50 nm of silicon. This is more
than a 1000 times smaller than the diameter of a human hair. These billions of transistors are
made by ―top down‖ methods that involve depositing thin layers of materials, patterning
nano-scale stencils and effectively carving away the unwanted bits. This approach has
become overly successful. The end result is billions of individual components on a single
chip, essentially all working perfectly and continuously for years on end. No other
manufactured technology comes close in reliability or cost.
Still, miniaturization cannot go on forever, because of the basic properties of matter. We are
already beginning to run into the problem that the silicon semiconductor, copper wiring and
oxide insulating layers in these devices are all made out of atoms. Each atom is about 0.3 nm
across.
NON-VOLATILE MEMORY
Non-volatile memory is the dominant area being pursued for memristor technology. Of
course most of the companies listed (with the exception of Hewlett Packard) do not refer to
their memory in terms of the memristor and rather use a variety of acronyms (i.e. RRAM,
CBRAM, PRAM, etc.) to distinguish their particular memory design. While these acronyms
do represent real distinctions in terms of the materials used or the mechanism of resistance
switching employed, the materials are still all memristors because they all share the same
characteristic voltage-induced resistance switching behavior covered by the mathematical
memristor model of Chua. Flash memory currently dominates the semiconductor memory
market. However, each memory cell of flash requires at least one transistor meaning that
flash design is highly susceptible to an end to Moore’s law. On the other hand, memristor
memory design is often based on a crossbar architecture which does not require transistors in
the memory cells. Although transistors are still necessary for the read/write circuitry, the total
number of transistors for a million memory cells can be on the order of thousands instead of
millions and the potential for addressing trillions of memory cells exists using only millions
(instead of trillions) of transistors. Another fundamental limitation to conventional memory
architectures is Von Neumann’s bottleneck which makes it more difficult to locate
NEUROMORPHIC ELECTRONICS
Neuromorphics has been defined in terms of electronic analog circuits that mimic neurobiological architectures. Since the early papers of Leon Chua it was noted that the equations of the memristor were closely related to behavior of neural cells. Since memristors integrate aspects of both memory storage and signal processing in a similar manner to neural
synapses they may be ideal to create a synthetic electronic system similar to the human brain
capable of handling applications such as pattern recognition and adaptive control of robotics
better than what is achievable with modern computer architectures.
MANUFACTURING
Manufacturers could make memristors in the same chip fabrication plants used now,
so companies would not have to undertake expensive retooling or new construction. And
memristors are by no means hard to fabricate. The titanium dioxide structure can be made in
any semiconductor fab currently in existence. The primary limitation to manufacturing hybrid
chips with memristors is that today only a small number of people on Earth have any idea of
how to design circuits containing memristors
One of the key fabrication advantages of the crossbar architecture is that the structure is a
well ordered, periodic and simple structure. However, to achieve Nanoscale resolutions the
standard lithography approaches are insufficient. The manufacturing techniques for the
Nanoscale crossbar devices developed by Hewlett-Packard include nanoimprint lithography,
which uses a stamp-like structure with nanometer resolution to transfer a pattern of
Nanoscale resolution to a substrate
CONCLUSION
Thus the discovery of a brand new fundamental circuit element is something not to be
taken lightly and has the potential to open the door to a brand new type of electronics. Memristor
will change circuit design in the 21st century as radically as the transistor changed it in the 20th.
Note that the transistor was lounging around as a mainly academic curiosity for a decade until
1956, when a revelutionary app—the hearing aid—brought it into the marketplace.
By redesigning certain types of circuits to include Memristors, it is possible to obtain the same
function with fewer components, making the circuit itself less expensive and significantly
decreasing its power consumption.
In fact, it can be hoped to combine Memristors with traditional circuit-design elements to
produce a device that does computation. The Hewlett-Packard (HP) group is looking at
developing a memristor-based nonvolatile memory that could be 1000 times faster than magnetic
disks and use much less power.
Memristor open door to a wide area of research in the field of computer hardware and
memory storage devices that has much higher data density.As rightly said by the originators of
memristor, Leon Chua and R.Stanley Williams, ―memristors are so significant that it would be
mandatory to re-write the existing electronic engineering textbooks.”
[attachment=65883]
ABSTRACT
Every person with an electronics background will be familiar with the three fundamental circuit elements - the resistor, the capacitor, and the inductor. These three elements are defined by the relation between two of the four fundamental circuit variables- current, voltage, charge and flux.
In 1971, Leon Chua reasoned on the grounds of symmetry that there should be a fourth fundamental circuit element which gives the relationship between flux and charge. He named
this circuit element the memristor, which is short for memory resistor. In May 2008, researchers at HP Labs published a paper announcing a model for the physical realization of the memristor.
It is proposed that memory storage devices that has very high data density and computers
that require no time for boot up can be developed using memristor based hardware. A new
physical quantity which is also introduced associated with memristor. It also solves some
unexplained voltage current characteristics observed in certain materials at atomic levels.
This report focuses on the memristor and reviews its properties. The HP model for the
memristor is also discussed, and few of the potential applications of the memristor are
presented.
INTRODUCTION
For nearly 150 years, the known fundamental passive circuit elements were limited to
the capacitor (discovered in 1745), the resistor (1827), and the inductor (1831). Anyone
familiar with electronics knows the trinity of fundamental components: the resistor, the
capacitor, and the inductor. Typically when most people think about electronics they may
initially think of products such as cell phones, radios, laptop computers, etc. Others, having
some engineering background, may think of resistors, capacitors, transistors, etc. which are
the basic components necessary for electronics to function. Such basic components are fairly
limited in number and each has their own characteristic function. For example,
resistors perform the function of electrical energy dissipation, capacitors perform the function
of electrical energy storage, and transistors perform the functions of electrical
energy amplification and switching. The arrangement of these few fundamental circuit
components form the basis of almost all of the electronic devices we use in our everyday life.
In a brilliant but underappreciated 1971 paper, Leon Chua, a professor of electrical
engineering at the University of California, Berkeley, predicted the existence of a fourth
fundamental device, which he called a memristor. He proved that memristor behavior could
not be duplicated by any circuit built using only the other three elements, which is why the
memristor is truly fundamental. Memristor is a contraction of ―memory resistor,‖ because
that is exactly its function: to remember its history.
MEMRISTOR FEATURES
Memristor is passive two-terminal element that maintains functional relation between
charge flowing through the device (i.e. time integral of current) and flux or A memristor is a
two-terminal semiconductor device whose resistance depends on the magnitude and polarity
of the voltage applied to it and the length of time that voltage has been applied. When you
turn off the voltage, the memristor remembers its most recent resistance until the next time
you turn it on, whether that happens a day later or a year later.
PIPE AND CURRENT ANALOGY
A common analogy to describe a memristor is similar to that of a resistor.
Think of a resistor as a pipe through which water flows. The water is electric charge. The
resistor’s obstruction of the flow of charge is comparable to the diameter of the pipe: the
narrower the pipe, the greater the resistance. For the history of circuit design, resistors have
had a fixed pipe diameter. But a memristor is a pipe that changes diameter with the amount
and direction of water that flows through it. If water flows through this pipe in one direction,
it expands (becoming less resistive). But send the water in the opposite direction and the pipe
shrinks (becoming more resistive). Further, the memristor remembers its diameter when
water last went through. Turn off the flow and the diameter of the pipe
MEMRISTOR AND RESISTOR
This new circuit element shares many of the properties of resistors and shares the
same unit of measurement (ohms). However, in contrast to ordinary resistors, in which the
resistance is permanently fixed, memristance may be programmed or switched to different
resistance states based on the history of the voltage applied to the memristance material. This
phenomena can be understood graphically in terms of the relationship between the current
flowing through a memristor and the voltage applied across the memristor. In ordinary
resistors there is a linear relationship between current and voltage so that a graph comparing
current and voltage results in a straight line. However, for memristors a similar graph is a
little more complicated.
ADVENT OF HP LABS
Even though Memristance was first predicted by Professor Leon Chua, Unfortunately,
neither he nor the rest of the engineering community could come up with a physical
manifestation that matched his mathematical expression.
Thirty-seven years later, a group of scientists from HP Labs has finally built real working
memristors, thus adding a fourth basic circuit element to electrical circuit theory, one that will
join the three better-known ones: the capacitor, resistor and the inductor.
Interest in the memristor revived in 2008 when an experimental solid state version was
reported by R. Stanley Williams of Hewlett Packard. HP researchers built their memristor
when they were trying to develop molecule-sized switches in Teramac (tera-operation-per second
multi architecture computer). Teramac architecture was the crossbar, which has since
become the de facto standard for nanoscale circuits because of its simplicity, adaptability, and
redundancy.
A solid-state device could not be constructed until the unusual behavior of nanoscale
materials was better understood. The device neither uses magnetic flux as the theoretical
memristor suggested, nor do stores charge as a capacitor does, but instead achieves a
resistance dependent on the history of current using a chemical mechanism.
The HP team’s memristor design consisted of two sets of 21 parallel 40-nm-wide wires
crossing over each other to form a crossbar array, fabricated using nanoimprint lithography.
A 20-nm-thick layer of the semiconductor titanium dioxide (TiO2) was sandwiched between
the horizontal and vertical nanowires, forming a memristor at the intersection of each wire
pair. An array of field effect transistors surrounded the memristor crossbar array, and the
memristors and transistors were connected to each other through metal traces.
The crossbar is an array of perpendicular wires. Anywhere two wires cross, they are
connected by a switch. To connect a horizontal wire to a vertical wire at any point on the
grid, you must close the switch between them. Note that a crossbar array is basically a storage
system, with an open switch representing a zero and a closed switch representing a one. You
read the data by probing the switch with a small voltage. Because of their simplicity, crossbar
arrays have a much higher density of switches than a comparable integrated circuit based on
transistors.
APPEARANCE OF MEMRISTOR
HP Labs' memristor has Crossbar type memristive circuits contain a lattice of 40-
50nm wide by 2-3nm thick platinum wires that are laid on top of one another perpendicular
top to bottom and parallel of one another side to side. The top and bottom layer are separated
by a switching element approximately 3-30nm in thickness. The switching element consists
of two equal parts of titanium dioxide (TiO2). The layer connected to the bottom platinum
wire is initially perfect TiO2 and the other half is an oxygen deficient layer of TiO2
represented by TiO2-x where x represents the amount of oxygen deficiencies or vacancies.
The entire circuit and mechanism cannot be seen by the naked eye and must be viewed under
a scanning tunneling microscope, as seen in Figure 6, in order to visualize the physical set up
of the crossbar design of the memristive circuit described in this section.
Fig 6: figure showing crossbar architecture and magnified memristive switch having platinum
electrodes and
TRANSISTOR VERSUS MEMRISTOR
The first transistor was a couple of inches across which was developed about 60 years
ago. Today, a typical laptop computer uses a processor chip that contains over a billion
transistors, each one with electrodes separated by less than 50 nm of silicon. This is more
than a 1000 times smaller than the diameter of a human hair. These billions of transistors are
made by ―top down‖ methods that involve depositing thin layers of materials, patterning
nano-scale stencils and effectively carving away the unwanted bits. This approach has
become overly successful. The end result is billions of individual components on a single
chip, essentially all working perfectly and continuously for years on end. No other
manufactured technology comes close in reliability or cost.
Still, miniaturization cannot go on forever, because of the basic properties of matter. We are
already beginning to run into the problem that the silicon semiconductor, copper wiring and
oxide insulating layers in these devices are all made out of atoms. Each atom is about 0.3 nm
across.
NON-VOLATILE MEMORY
Non-volatile memory is the dominant area being pursued for memristor technology. Of
course most of the companies listed (with the exception of Hewlett Packard) do not refer to
their memory in terms of the memristor and rather use a variety of acronyms (i.e. RRAM,
CBRAM, PRAM, etc.) to distinguish their particular memory design. While these acronyms
do represent real distinctions in terms of the materials used or the mechanism of resistance
switching employed, the materials are still all memristors because they all share the same
characteristic voltage-induced resistance switching behavior covered by the mathematical
memristor model of Chua. Flash memory currently dominates the semiconductor memory
market. However, each memory cell of flash requires at least one transistor meaning that
flash design is highly susceptible to an end to Moore’s law. On the other hand, memristor
memory design is often based on a crossbar architecture which does not require transistors in
the memory cells. Although transistors are still necessary for the read/write circuitry, the total
number of transistors for a million memory cells can be on the order of thousands instead of
millions and the potential for addressing trillions of memory cells exists using only millions
(instead of trillions) of transistors. Another fundamental limitation to conventional memory
architectures is Von Neumann’s bottleneck which makes it more difficult to locate
NEUROMORPHIC ELECTRONICS
Neuromorphics has been defined in terms of electronic analog circuits that mimic neurobiological architectures. Since the early papers of Leon Chua it was noted that the equations of the memristor were closely related to behavior of neural cells. Since memristors integrate aspects of both memory storage and signal processing in a similar manner to neural
synapses they may be ideal to create a synthetic electronic system similar to the human brain
capable of handling applications such as pattern recognition and adaptive control of robotics
better than what is achievable with modern computer architectures.
MANUFACTURING
Manufacturers could make memristors in the same chip fabrication plants used now,
so companies would not have to undertake expensive retooling or new construction. And
memristors are by no means hard to fabricate. The titanium dioxide structure can be made in
any semiconductor fab currently in existence. The primary limitation to manufacturing hybrid
chips with memristors is that today only a small number of people on Earth have any idea of
how to design circuits containing memristors
One of the key fabrication advantages of the crossbar architecture is that the structure is a
well ordered, periodic and simple structure. However, to achieve Nanoscale resolutions the
standard lithography approaches are insufficient. The manufacturing techniques for the
Nanoscale crossbar devices developed by Hewlett-Packard include nanoimprint lithography,
which uses a stamp-like structure with nanometer resolution to transfer a pattern of
Nanoscale resolution to a substrate
CONCLUSION
Thus the discovery of a brand new fundamental circuit element is something not to be
taken lightly and has the potential to open the door to a brand new type of electronics. Memristor
will change circuit design in the 21st century as radically as the transistor changed it in the 20th.
Note that the transistor was lounging around as a mainly academic curiosity for a decade until
1956, when a revelutionary app—the hearing aid—brought it into the marketplace.
By redesigning certain types of circuits to include Memristors, it is possible to obtain the same
function with fewer components, making the circuit itself less expensive and significantly
decreasing its power consumption.
In fact, it can be hoped to combine Memristors with traditional circuit-design elements to
produce a device that does computation. The Hewlett-Packard (HP) group is looking at
developing a memristor-based nonvolatile memory that could be 1000 times faster than magnetic
disks and use much less power.
Memristor open door to a wide area of research in the field of computer hardware and
memory storage devices that has much higher data density.As rightly said by the originators of
memristor, Leon Chua and R.Stanley Williams, ―memristors are so significant that it would be
mandatory to re-write the existing electronic engineering textbooks.”