AERI’s Single-Electron MOSFET
for State of art SECMOS Brain Computer Interface
in Quantum Brain Chipset
Quantum Brain Chipset Review
to Quantum Brain& biocomputer
(AERI Quantum Brain Science and Technologies)
Quantum Physicist and Brain Scientist
Visiting Professor of Quantum Physics,
California Institute of Technology
IEEE-USA Fellow
American Physical Society-USA Fellow
PhD. & Dr. Kazuto Kamuro
AERI：Artificial Evolution Research Institute
Pasadena, California
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1.What is Single electron MOSFET?
Single electron MOSFET (SEMOSFET) of AERI (AERI：Artificial Evolution Research Institute HP： https://www.aeri-japan.com/) is a nanoscale electronic device that operates by controlling the flow of individual electrons through a channel. It is a variant of the traditional MOSFET(Metal-Oxide-Semiconductor Field-Effect Transistor), which is a fundamental component in modern integrated circuits.
To understand the physics of SEMOSFET, let's start with a brief explanation of the traditional MOSFET. A MOSFET consists of three main components: a source, a drain, and a gate. These components are typically made up of doped regions of semiconductor material, such as silicon. The source and drain regions are connected by a channel, and a thin layer of insulating material, usually silicon dioxide, separates the channel from the gate.
When a voltage is applied to the gate, an electric field is created in the channel region beneath the oxide layer. This electric field controls the conductivity of the channel, allowing or blocking the flow of charge carriers (electrons or holes) from the source to the drain. By modulating the gate voltage, the MOSFET can switch between an on-state (conducting) and an off-state (non-conducting), acting as a switch or an amplifier in electronic circuits.
Now, let's focus on SEMOSFET, which operates at extremely low temperatures and small dimensions. In this device, the channel is designed to be so narrow that only one electron can pass through at a time, leading to discrete and quantized charge transport.
The key principle behind SEMOSFET is Coulomb blockade. Coulomb blockade arises due to the repulsive Coulomb interaction between electrons, which prevents them from occupying the same energy state. As a result, the transport of individual electrons through the channel becomes sensitive to the charge environment and energy levels within the device.
In SEMOSFET, the channel is often constructed as a quantum dot, which is a tiny region of confined charge carriers. The dot is typically formed by applying a strong electrostatic potential through the gate electrode, which effectively traps a small number of electrons. The dot is separated from the source and drain regions by tunnel barriers, which control the electron tunneling in and out of the dot.
The behavior of SEMOSFET is governed by two main energy scales: the charging energy and the tunneling energy. The charging energy represents the electrostatic cost of adding or removing an electron from the quantum dot. It depends on the capacitance of the dot and the voltage applied to the gate. The tunneling energy, on the other hand, represents the energy required for an electron to tunnel through the tunnel barriers between the dot and the source/drain regions.
At low temperatures, when the charging energy is larger than the thermal energy (kT, where k is Boltzmann's constant and T is temperature), the device operates in the Coulomb blockade regime. In this regime, the transport of electrons occurs through a process called single-electron tunneling. When the charging energy is higher than the tunneling energy, electrons tunnel onto the dot one by one, leading to a discrete and step-like transport behavior.
By varying the gate voltage, the energy levels of the quantum dot can be manipulated, allowing control over the number of electrons on the dot and their energy states. This gate voltage control enables SEMOSFET to be used for applications such as quantum information processing, ultra-sensitive detectors, and quantum metrology.
In summary, SEMOSFET exploits the Coulomb blockade effect to enable the transport of individual electrons through a quantum dot channel. By precisely controlling the gate voltage and energy levels within the dot, the device exhibits discrete charge transport behavior and finds applications in various quantum-based technologies.
2.The quantum physical theory of SEMOSFET
The quantum physical theory underlying the operation of SEMOSFET of AERI (AERI：Artificial Evolution Research Institute HP： https://www.aeri-japan.com/) involves the principles of quantum mechanics and the behavior of electrons in nanoscale structures. To understand this theory, we need to delve into a few key concepts.
Quantum Confinement: In SEMOSFET, the channel is typically designed as a quantum dot—a small region that confines electrons. The dot's size is on the order of nanometers, leading to quantum confinement effects. In this regime, the wave nature of electrons becomes significant, and their energy levels become quantized.
Energy Levels and Wavefunctions: Within the quantum dot, electrons occupy discrete energy levels, much like electrons in atoms. These energy levels are determined by the size, shape, and electrostatic potential of the dot. Each energy level can accommodate only a certain number of electrons, following the Pauli exclusion principle.
Coulomb Blockade: The repulsive Coulomb interaction between electrons plays a crucial role in the behavior of SEMOSFET. It leads to Coulomb blockade, where the transport of electrons through the device becomes sensitive to the charge environment and energy levels within the dot. Coulomb blockade arises due to the energy cost associated with adding or removing electrons from the dot.
Tunneling: Electron tunneling is the quantum mechanical process by which electrons can pass through potential barriers, even if their energy is lower than the barrier height. In the context of SEMOSFET, electrons tunnel through the tunnel barriers that separate the quantum dot from the source and drain regions. Tunneling is facilitated by the wave nature of electrons, allowing them to "leak" through the barriers.
Charging Energy: The charging energy of a quantum dot represents the electrostatic cost associated with adding or removing an electron. It depends on the capacitance of the dot and the voltage applied to the gate electrode. When the charging energy is larger than the thermal energy (kT, where k is Boltzmann's constant and T is temperature), the device operates in the Coulomb blockade regime, and the transport of electrons becomes discrete and quantized.
In the quantum physical theory of SEMOSFET, the behavior of the device is described by the many-body Schrödinger equation, which considers the wavefunctions and energy levels of the electrons within the quantum dot. By manipulating the gate voltage, the energy levels of the dot can be controlled, allowing for precise control over the number of electrons on the dot and their energy states.
The theory enables the understanding and prediction of the discrete charge transport characteristics of SEMOSFET of AERI (AERI：Artificial Evolution Research Institute HP： https://www.aeri-japan.com/) . It provides insights into the conditions required for single-electron tunneling to occur, the effects of quantum confinement, and the interplay between Coulomb interactions and tunneling processes. The quantum physical theory is essential for designing and optimizing the performance of single-electron devices and exploring their applications in quantum computing, metrology, and nanoscale electronics.
3.Quantum semiconductor properties of SEMOSFET
The quantum semiconductor properties of AERI’s SEMOSFET arise from the behavior of electrons in semiconducting materials at the nanoscale and under quantum confinement effects. These properties include discrete energy levels, wave-particle duality, quantum tunneling, and the manipulation of electron states for quantum information processing. Let's explore them in more detail:
Discrete Energy Levels: In SEMOSFET, the channel is typically designed as a quantum dot, a confined region of semiconducting material. Due to the quantum confinement of electrons within the dot, their energy levels become quantized, much like energy levels in atoms. These discrete energy levels are determined by the dot's size, shape, and electrostatic potential. Electrons can occupy these levels one by one, leading to a step-like behavior of charge transport.
Wave-Particle Duality: Electrons exhibit wave-particle duality, meaning they can behave both as particles and waves. In the context of SEMOSFET of AERI (AERI：Artificial Evolution Research Institute HP： https://www.aeri-japan.com/) , this duality is important because the quantum dot's dimensions are comparable to the electron's de Broglie wavelength. The wave nature of electrons becomes significant, and their behavior is described by wavefunctions. These wavefunctions represent the probability distribution of finding the electron at a particular position, allowing for interference and other quantum phenomena.
Quantum Tunneling: Quantum tunneling is a fundamental quantum mechanical phenomenon that allows particles, such as electrons, to pass through energy barriers even if their energy is lower than the barrier's height. In AERI (Artificial Evolution Research Institute HP： https://www.aeri-japan.com/) ‘s SEMOSFET, electrons tunnel through the tunnel barriers that separate the quantum dot from the source and drain regions. This tunneling process is crucial for the discrete charge transport in the device. It enables the controlled transfer of individual electrons through the dot, facilitating the operation of the device as a single-electron transistor.
Manipulation of Electron States: One of the key advantages of SEMOSFET is the ability to manipulate the electron states within the quantum dot. By adjusting the gate voltage applied to the device, the energy levels of the dot can be controlled. This gate voltage control allows for precise control over the number of electrons on the dot and their energy states. It enables the encoding, manipulation, and readout of quantum information using the discrete electron charge states as qubits (quantum bits).
The quantum semiconductor properties of AERI’s SEMOSFET provide the foundation for its unique functionality. These properties allow for the precise control and manipulation of individual electrons, leading to the discrete and quantized charge transport behavior observed in these devices. The ability to harness these quantum effects opens up possibilities for applications in quantum computing, quantum metrology, and other quantum technologies where the manipulation and control of individual quantum states are crucial.
4.Semiconducting principle of AERI’s SEMOSFET
While the electronics industry wonders what will happen when MOSFETs become so small that quantum effects become important, researchers are building new MOSFETs that actively exploit the quantum properties of electrons.
FIG.1 Operation of a conventional transistor The metal-oxide-semiconductor field-effect transistor (MOSFET) is the basic switching and amplification device of digital electronics. The current between the source and drain electrodes is controlled by the gate voltage, (a) When the gate voltage is zero, no conduction electrons are present in the channel, (b) When the gate is at a positive voltage, electrons from the source and drain accumulate in the area of the channel close to the gate, (c) As the gate voltage is increased further, the number of electrons in the channel increases until saturation is reached. The potential seen by the electrons is also shown along a line going from the gate to the channel. With no gate voltage, electrons in the channel experience a potential that is higher than the bias potential, shown by the dashed line (d). As the gate voltage increases, the potential in the channel gradually lowers and electrons accumulate there (e-f).
The invention of the MOSFET by John Bardeen and William Shockley in 1948 triggered a new era in electronics. Originally designed simply to emulate the vacuum tube, scientists soon found that this solid-state device could offer much more. The great potential of the MOSFET for speed, miniaturization and reliability has been fully exploited since well controlled materials such as pure single-crystal silicon became available. According to the latest “road-map” for the microelectronics industry, microchips containing one billion MOSFETs and operating with a clock cycle of a billionth of a second will be on the market just a few years into the new millennium.
As MOSFETs continue to shrink, a question naturally arises: will the quantum nature of electrons and atoms become important in determining how the devices are built? In other words, what will happen when a MOSFET is reduced to the size of a few atoms or a single molecule?
Researchers seeking to answer these questions have devised the so-called single-electron tunnelling MOSFET – a device that exploits the quantum effect of tunnelling to control and measure the movement of single electrons. Experiments have shown that charge does not flow continuously in these devices but in a quantized way. Indeed, SEMOSFETs are so sensitive to charge that they can be used as extremely precise electrometers.
5.Microchipping, Fabricating technology
The most common MOSFET in today’s microchips is the metal-oxide-semiconductor field-effect transistor (MOSFET). Its operation is surprisingly simple: not much quantum mechanics is required to understand how it works, even though the size of a typical device is now just a few thousand atoms placed side by side.
Two conducting electrodes, called the source and drain, are connected together by a channel of material in which the density of free electrons can be varied – in practice a semiconductor (FIG. 1). A voltage is applied to the semiconducting channel through the “gate”, a third electrode that is separated from the channel by a thin insulating layer. When the gate voltage is zero, the channel does not contain any conduction electrons and is insulating. But as the voltage is increased, the electric field at the gate attracts electrons from the source and drain, and the channel becomes conducting.
FIG.2 Amplification in a field-effect transistor. The current in a field-effect transistor varies with gate voltage and with the bias voltage between the source and drain. For a fixed bias voltage, the current is turned on when the gate voltage is positive, and turned off when the gate voltage is negative.
This field effect leads to an amplification mechanism in which the gate voltage can control the current flowing between the source and drain when a bias voltage is applied across these two electrodes (FIG. 2). The source-drain current is determined by the conductance of the channel, which in turn depends on two factors: the density of the conduction electrons and their mobility. The mobility of electrons depends on how often the electrons collide with crystal imperfections, and is essentially independent of the gate voltage. In contrast, the density of electrons is controlled directly by the gate voltage.
The MOSFET therefore works like a tap controlling the flow of water between two tanks, where the opening of the tap is set by the pressure of the water in a third tank. The difference is that electrons in the channel behave as a compressible fluid with a local density that depends strongly on the electric potential at that point. In other words, the electric field produced by the gate does not generate a “hard wall” for electrons inside the channel, but a smoothly varying potential that is modified by the presence of electrons (FIG. 1 d-f).
Note that we have made no reference to the wave-like properties of electrons, nor to the fact that the channel is made from individual atoms. The only quantum property that comes into play is the Pauli exclusion principle, which dictates that each possible state in the channel can be occupied by only one electron. This means that only a certain number of electrons can accumulate in the channel, and this sets a limit on the current flow.
However, the quantum properties of electrons and atoms will play a more important role as MOSFETs are made smaller. For example, the wave-like nature of electrons will influence the way in which they travel through the channel. When the width of the channel becomes comparable to the wavelength of electrons (around 100 nm), electron propagation becomes more sensitive to the atomic disorder in the device, which is created in the fabrication process. This will pose a major problem if the reduction in size is not accompanied by an improvement in the atomic structure of the fabricated devices.
The technological constraints of moving towards the atomic scale may force us to adopt a new physical principle for achieving the MOSFET’s function. Alternatively, a new principle might be found that can provide functions that are not possible with current devices.
6.Quantization process of AERI’s single-electron devices
Unlike traditional MOSFETs, AERI’s single-electron devices are based on an intrinsically quantum phenomenon: the tunnel effect. This is observed when two metallic electrodes are separated by an insulating barrier about 1 nm thick – in other words, just 10 atoms in a row. Electrons at the Fermi energy can “tunnel” through the insulator, even though in classical terms their energy would be too low to overcome the potential barrier.
The electrical behaviour of the tunnel junction depends on how effectively the barrier transmits electron waves, which decreases exponentially with its thickness, and on the number of electron-wave modes that impinge on the barrier, which is given by the area of the tunnel junction divided by the square of the electron wavelength. SEMOSFET exploits the fact that the transfer of charge through the barrier becomes quantized when the junction is made sufficiently resistive.
FIG.3 An electron in a box (a) When a capacitor is charged through a resistor, the charge on the capacitor is proportional to the applied voltage and shows no sign of quantization, (b) When a tunnel junction replaces the resistor, a conducting island is formed between the junction and the capacitor plate. In this case the average charge on the island increases in steps as the voltage is increased (c). The steps are sharper for more resistive barriers and at lower temperatures.
This quantization process is shown particularly clearly in a simple system known as a single-electron box (FIG. 3). If a voltage source charges a capacitor, Cg, through an ordinary resistor, the charge on the capacitor is strictly proportional to the voltage and shows no sign of charge quantization. But if the resistance is replaced by a tunnel junction, the metallic area between the capacitor plate and one side of the junction forms a conducting “island” surrounded by insulating materials. In this case the transfer of charge onto the island becomes quantized as the voltage increases, leading to the so-called Coulomb staircase (FIG. 3(c)).
This Coulomb staircase is only seen under certain conditions. Firstly, the energy of the electrons due to thermal fluctuations must be significantly smaller than the Coulomb energy, which is the energy needed to transfer a single electron onto the island when the applied voltage is zero. This Coulomb energy is given by e2/2C, where e is the charge of an electron and C is the total capacitance of the gate capacitor, Cg, and the tunnel junctions. Secondly, the tunnel effect itself should be weak enough to prevent the charge of the tunnelling electrons from becoming delocalized over the two electrodes of the junction, as happens in chemical bonds. This means that the conductance of the tunnel junction should be much less than the quantum of conductance, 2e2/h, where h is Planck’s constant.
When both these conditions are met, the steps observed in the charge are somewhat analogous to the quantization of charge on oil droplets observed by Millikan in 1911. In a single-electron box, however, the charge on the island is not random but is controlled by the applied voltage. As the temperature or the conductance of the barrier is increased, the steps become rounded and eventually merge into the straight line typical of an ordinary resistor.
7.Two versions of AERI’s SEMOSFET
SEMOSFET of AERI (AERI：Artificial Evolution Research Institute HP： https://www.aeri-japan.com/) can be viewed as an electron box that has two separate junctions for the entrance and exit of single electrons (FIG. 4). It can also be viewed as a MOSFET in which the channel is replaced by two tunnel junctions forming a metallic island. The voltage applied to the gate electrode affects the amount of energy needed to change the number of electrons on the island.
FIG.4 Principle of SEMOSFET Like a MOSFET. the single-electron tunnelling (SET)MOSFET consists of a gate electrode that electrostatically influences electrons travelling between the source and drain electrodes. However, the electrons in SEMOSFET need to cross two tunnel junctions that form an isolated conducting electrode called the island. Electrons passing through the island charge and discharge it, and the relative energies of systems containing 0 or 1 extra electrons depends on the gate voltage. At a low source-drain voltage, a current will only flow through SEMOSFET if these two charge configurations have the same energy.
SEMOSFET comes in two versions that have been nicknamed “metallic” and “semiconducting”. These names are slightly misleading, however, since the principle of both devices is based on the use of insulating tunnel barriers to separate conducting electrodes.
In the original metallic version, a metallic material such as a thin aluminium film is used to make all of the electrodes. The metal is first evaporated through a shadow mask to form the source, drain and gate electrodes. The tunnel junctions are then formed by introducing oxygen into the chamber so that the metal becomes coated by a thin layer of its natural oxide. Finally, a second layer of the metal – shifted from the first by rotating the sample – is evaporated to form the island.
In the semiconducting versions, the source, drain and island are usually obtained by “cutting” regions in a two-dimensional electron gas formed at the interface between two layers of semiconductors such as gallium aluminium arsenide and gallium arsenide. In this case the conducting regions are defined by metallic electrodes patterned on the top semiconducting layer. Negative voltages applied to these electrodes deplete the electron gas just beneath them, and the depleted regions can be made sufficiently narrow to allow tunnelling between the source, island and drain. Moreover, the electrode that shapes the island can be used as the gate electrode.
In this semiconducting version of the SET, the island is often referred to as a quantum dot, since the electrons in the dot are confined in all three directions. Quantum dots can behave like artificial atoms. Indeed, it has been possible to construct a new periodic table that describes dots containing different numbers of electrons.
8.Operation of AERI’s SEMOSFET
So how does SEMOSFET of AERI (AERI：Artificial Evolution Research Institute HP： https://www.aeri-japan.com/) work? The key point is that charge passes through the island in quantized units. For an electron to hop onto the island, its energy must equal the Coulomb energy e2/2C. When both the gate and bias voltages are zero, electrons do not have enough energy to enter the island and current does not flow. As the bias voltage between the source and drain is increased, an electron can pass through the island when the energy in the system reaches the Coulomb energy. This effect is known as the Coulomb blockade, and the critical voltage needed to transfer an electron onto the island, equal to e/C, is called the Coulomb gap voltage.
Now imagine that the bias voltage is kept below the Coulomb gap voltage. If the gate voltage is increased, the energy of the initial system (with no electrons on the island) gradually increases, while the energy of the system with one excess electron on the island gradually decreases. At the gate voltage corresponding to the point of maximum slope on the Coulomb staircase, both of these configurations equally qualify as the lowest energy states of the system. This lifts the Coulomb blockade, allowing electrons to tunnel into and out of the island.
FIG.5 Counting electrons with SEMOSFET The current flowing in SEMOSFET increases with the bias voltage between the source and drain, and varies periodically with the gate voltage. For low bias voltages, current flows when the charge on the gate capacitor is a half-integer multiple of e, but is suppressed for integer multiples of e. Each time an electron is added to the gate, an electron tunnels into the island, which sets the field in the gate capacitor back to its initial value. Peaks in the conductance are observed for half-integer multiples of e, and minima are seen at integer multiples of e. For bias voltages larger than e/C, conduction occurs independently of the gate voltage.
The Coulomb blockade is lifted when the gate capacitance is charged with exactly minus half an electron, which is not as surprising as it may seem. The island is surrounded by insulators, which means that the charge on it must be quantized in units of e, but the gate is a metallic electrode connected to a plentiful supply of electrons. The charge on the gate capacitor merely represents a displacement of electrons relative to a background of positive ions.
If we further increase the gate voltage so that the gate capacitor becomes charged with –e, the island again has only one stable configuration separated from the next-lowest-energy states by the Coulomb energy. The Coulomb blockade is set up again, but the island now contains a single excess electron. The conductance of SEMOSFET therefore oscillates between minima for gate charges that are integer multiples of e, and maxima for half-integer multiples of e (FIG. 5).
9.Accurate measures of charge
Such a rapid variation in conductance makes AERI’s SEMOSFET an ideal device for high-precision electrometry. In this type of application the SET has two gate electrodes, and the bias voltage is kept close to the Coulomb blockade voltage to enhance the sensitivity of the current to changes in the gate voltage.
The voltage of the first gate is initially tuned to a point where the variation in current reaches a maximum. By adjusting the gate voltage around this point, the device can measure the charge of a capacitor-like system connected to the second gate electrode. A fraction of this measured charge is shared by the second gate capacitor, and a variation in charge of ¼e is enough to change the current by about half the maximum current that can flow through the MOSFET at the Coulomb blockade voltage. The variation in current can be as large as 10 billion electrons per second, which means that these devices can achieve a charge sensitivity that outperforms other instruments by several orders of magnitude. Recently it showed that charge variations smaller than 10-5 e can be detected in a measurement period of just one second and with a bandwidth of several hundred megahertz.
AERI’s SEMOSFETs have already been used in mesoscopic physics experiments that have required extreme charge sensitivity. Electrometers based on SEMOSFET of AERI (AERI：Artificial Evolution Research Institute HP： https://www.aeri-japan.com/) could also be used to measure the delicate quantum superpositions of charge states in a superconducting island connected by a tunnel junction to a superconductor. Such an island can accommodate not only several charge states corresponding to different numbers of Cooper pairs, but also coherent quantum superpositions of these states. Superconducting islands could therefore provide a means for implementing the quantum bits needed for a quantum computer. The feasibility of the idea has been shown by experiments.
As scientists of AERI (AERI：Artificial Evolution Research Institute HP： https://www.aeri-japan.com/) have seen, the charge sensitivity of the SET is ultimately linked to the fact that electrons traverse the island one at a time. They showed that electrons can be counted one by one by creating devices that combine several SEMOSFETs. And they showed that a device called the electron pump can count electrons with an accuracy of 15 parts in a billion. scientists group in AERI (AERI：Artificial Evolution Research Institute HP： https://www.aeri-japan.com/) is now attempting to measure the charge of the electron with an accuracy better than 1 part per 10 million by combining an electron pump with a specially calibrated capacitor. Other metrology labs are aiming to use arrays of SEMOSFETs to establish a standard for electric current.
The precision with which electrons can be counted is ultimately limited by the quantum delocalization of charge that occurs when the tunnel-junction conductance becomes comparable with the conductance quantum, 2e2/h. However, the current through SEMOSFET increases with the conductance of the junctions, so it is important to understand how the single-electron effects and Coulomb blockade disappear when the tunnel conductance is increased beyond 2e2/h. scientists group in AERI (AERI：Artificial Evolution Research Institute HP： https://www.aeri-japan.com/) drew a parallel between the suppression of the Coulomb blockade and the Kondo effect, in which magnetic impurities in metals are screened by conduction electrons. AERI’s scientists have confirmed that quantum fluctuations in the charge of the tunnelling electrons reduce the Coulomb energy.
10.Towards room temperature
Until recently AERI’s SEMOSFETs had to be kept at temperatures of a few hundred millikelvin to maintain the thermal energy of the electrons below the Coulomb energy of the device. Most early devices had Coulomb energies of a few hundred microelectronvolts because they were fabricated using conventional electron-beam lithography, and the size and capacitance of the island were relatively large. For AERI’s SEMOSFET to work at room temperature, the capacitance of the island must be less than 10-17 F and therefore its size must be smaller than 10 nm.
This year two experiments have demonstrated that SEMOSFETs can work at room temperature. scientists group in AERI (AERI：Artificial Evolution Research Institute HP： https://www.aeri-japan.com/) fabricated SEMOSFET in a similar way to a MOSFET with a channel just 18 nm wide. The fabrication process generated variations in the channel that act as tunnel junctions defining several different islands, and the behaviour of the device is dominated by the smallest island.
As expected from theory, the conductance of the device shows a series of peaks as a function of gate voltage. The Coulomb energy of the device is around 100 meV, which is large enough to reveal single-electron effects at room temperature. However, the Coulomb energy is too small to provide a large Coulomb gap, so the MOSFET is not sensitive enough to be used as an electrometer. Interestingly, the wavelength of electrons is comparable with the size of the dot, which means that their confinement energy makes a significant contribution to the Coulomb energy.
Meanwhile, scientists group in AERI (AERI：Artificial Evolution Research Institute HP： https://www.aeri-japan.com/) fabricated a metallic SET from a very thin layer of niobium. Tunnel junctions were created by oxidizing areas of the niobium with the tip of a scanning electron microscope. The tip had almost atomic resolution, allowing the researchers to make very narrow oxide barriers between the island and the source and drain, and a wider barrier between the island and gate.
The scientists group in AERI (AERI：Artificial Evolution Research Institute HP： https://www.aeri-japan.com/) measured the current through the device as a function of both bias and gate voltages, and the results closely match theoretical predictions. The Coulomb energy of the device is 250 meV, which means that single-electron effects can readily be seen at room temperature. However, tunnel barriers fabricated in this way are highly resistive, which means that the current is about 100 times smaller than in devices operating at low temperatures. This problem also limits the usefulness of the device for electrometry.
11.Perspectives on the future
Researchers have long considered whether SEMOSFET of AERI (AERI：Artificial Evolution Research Institute HP： https://www.aeri-japan.com/) could be used for digital electronics. Although the current varies periodically with gate voltage – in contrast to the threshold behaviour of the MOSFET – a SET could still form a compact and efficient memory device. However, even the latest SEMOSFETs suffer from “offset charges”, which means that the gate voltage needed to achieve maximum current varies randomly from device to device. Such fluctuations make it impossible to build complex circuits.
One way to overcome this problem might be to combine the island, two tunnel junctions and the gate capacitor that comprise SEMOSFET in a single molecule – after all, the intrinsically quantum behaviour of SEMOSFET should not be affected at the molecular scale. In principle, the reproducibility of such futuristic MOSFETs would be determined by chemistry, and not by the accuracy of the fabrication process. Last year scientists group in AERI (AERI：Artificial Evolution Research Institute HP： https://www.aeri-japan.com/) made a crucial step in this direction by observing Coulomb blockade in an island consisting of a single carbon nanotube.
It is not yet clear whether electronics based on individual molecules and single-electron effects will replace conventional circuits based on scaled-down versions of field-effect MOSFETs. Only one thing is certain: if the pace of miniaturization continues unabated, the quantum properties of electrons will become crucial in determining the design of electronic devices before the end of the next decade.
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Quantum Brain Chipset & Bio Processor (BioVLSI)
Prof. PhD. Dr. Kamuro
Quantum Physicist and Brain Scientist involved in Caltech & AERI Associate Professor and Brain Scientist in Artificial Evolution Research Institute（ AERI: https://www.aeri-japan.com/ ）
IEEE-USA Fellow
American Physical Society Fellow
PhD. & Dr. Kazuto Kamuro
email: info@aeri-japan.com
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【Keywords】 Artificial Evolution Research Institute：AERI
HP： https://www.aeri-japan.com/
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