Since nanotechnology, information technology, material science, and biotechnology have been recognized as the four major sciences and technologies of the 21st century, a global wave of nanotechnology research and development has emerged, both internationally and domestically. In particular, China has taken a keen interest in "nano," making it a buzzword discussed in households across the nation. In response to this trend, Nanoscientific has proposed a development policy advocating for a comprehensive understanding of nanotechnology and its promotion towards healthy development. This policy emphasizes the critical importance of understanding nanodevices, particularly nanoelectronic devices, and stresses the need to vigorously advance their development. Questions such as what constitutes a nanodevice, the definition of a nanoelectronic device, how to categorize nanoelectronic devices, and the role of nanoelectronic devices within the broader field of nanotechnology, among others, warrant careful consideration. The aim of this article is to address these questions and introduce readers to the two primary types of nanoelectronic devices: solid nanoelectronic devices and molecular electronic devices, thereby fostering a comprehensive and accurate understanding. Additionally, there are varying perspectives on the classification of nanoelectronic devices within both international and domestic academic circles. This article seeks to act as a "facilitator," provoking academic discourse on the subject, which will undoubtedly be beneficial for future developments.
Nano-devices can be thought of as devices fabricated and prepared using nano-scale processing and preparation techniques, such as metal organic compound deposition technology (MOCVD), molecular beam epitaxy (MBE), electron beam technology (EB), scanning probe microscopy (SPM), and nano-material preparation methods (self-assembly growth, molecular synthesis). These devices are designed to possess nano-scale dimensions (1-100 nm) and specific functionalities.
When it comes to dividing nano-devices and nano-electronic devices, nano-devices encompass resonant tunneling devices, quantum dot devices, solid nanoelectronic devices, nanoelectronic devices, and "molecular electronic devices." However, the scope of nanoelectronic devices is confined to two key conditions: (1) the device's operational principle is based on quantum effects; and (2) it features a typical device structure—a tunnel barrier surrounding an "island" or "well" structure. Based on these conditions, while nano-CMOS devices, nano-magnetic devices, and nano-electromechanical systems (NEMS) are all fabricated using nano-processing technology and are indeed nanoscale, they do not fall under the general category of nanoelectronic devices. Instead, nanoelectronic devices can be categorized into two main groups: solid nanoelectronic devices, which include resonant tunneling devices (RTD and RTT), quantum dot (QD) devices, and single-electron devices (SED); and molecular electronic devices, which primarily consist of quantum-effect molecular electronic devices and electromechanical molecular electronic devices. This classification is illustrated in Table 1.
Table 1 Classification of Nano Devices
Nano Photonic Devices
Nano Magnetic Devices
Nano CMOS Devices
Nano Electromechanical Systems
Other Nano Devices (Quantum Interference Devices, etc.)
Single Electron Devices
Single Electron Devices
Quantum Effect Molecular Electronic Devices
Electromechanical Molecular Electronic Devices
Solid Nanoelectronic Devices
2.1 Classification of Solid Nanoelectronic Devices
Solid nanoelectronic devices represent a major branch of nanoelectronic devices. Their emergence is closely linked to advancements in nano-preparation, processing technology, mesoscopic physics, and microelectronics technology, particularly addressing the various "limits" faced globally. These devices possess a quantum effect and are structured with a tunnel barrier surrounding an "island" or potential well. In this structure, depending on the specific dimensions of the "island" or potential well, solid nanoelectronic devices can be classified into three types, as shown in Table 2.
Table 2 Classification Table of Solid Nanoelectronic Devices
Name | Working Mechanism | Number of Device Terminals | Characteristics
Resonant Tunneling Devices | Quantum Resonant Tunneling Effect | Two or Three | Electron energy levels in the potential well are discrete, and the current exhibits peaks at resonance.
Quantum Wire Devices | One-Dimensional or Two-Dimensional Quantization | Two or Three | Discrete energy levels, but no Coulomb blocking effect.
Quantum Dot Devices | Three-Dimensional Quantization | Three | Strong Coulomb blocking effect, discrete energy levels, and current steps at resonance.
In summary, for resonant tunneling devices or quantum wire devices, the energy distribution in the potential well forms a discrete energy level. When electrons resonate with the ground state energy level in the well, a current peak appears. As the voltage increases, resonance tunneling occurs with the first excited state energy level, creating a second current peak. Due to the larger energy gaps, the voltage shift between the two peaks is larger. For single-electron devices, the energy distribution in the potential well is quasi-continuous with small energy gaps. Ignoring the effect, electrons must be provided from the emitter to enter the potential well, resulting in a smooth large step in the characteristic curve. For quantum dot devices, the energy distribution in the potential well is based on significant energy gaps, leading to a characteristic curve showing large steps with smaller steps.
2.3 Resonant Tunneling Device
2.3.1 Working Principle of Resonant Tunneling Device
The resonant tunneling device includes two-terminal resonant diodes (RTD) and three-terminal resonant tunneling transistors (RTT). They are among the earliest, most studied, and relatively mature solid nanoelectronic devices. High-speed digital circuits containing up to 2000 RTDs have been developed abroad. The basic structure of RTD is a typical two-barrier single-potential well system. The potential barrier is typically composed of AlAs or AlGaAs, and the potential well is made of GaAs or InGaAs. The left-side emitter and right-side collector are composed of the same doped layers as the potential well. The barrier width is 1.5 to 3.0nm, and the potential well width is 3.0 to 5.0nm, prepared using MBE technology. A discrete energy level appears in the potential well due to quantization, and the ground state energy is "0". Without bias, "0" is higher than the Fermi level in the emitter. After biasing, "0" falls between the bottom of its conduction band "c", and when the Fermi level's electron energy coincides with "0", resonance tunneling occurs, and tunneling current appears. The conversion frequency between the peaks and valleys of the RTD is theoretically expected to reach 1.5 to 2.5THz, with actual RTDs reaching 650GHz and the shortest switching time being 1.5ps. Low operating voltage and low power consumption: the operating voltage of a typical RTD is 0.2 to 0.5V, and the general operating current is on the order of mA. By adding a pre-barrier layer during material growth, the current can be reduced to the order of A, achieving low power consumption. The power consumption of SRAM made by RTD is 50nW/cell.
Negative resistance, bistable and self-locking characteristics: Negative resistance is the fundamental characteristic of RTD and RTT. The inverter composed of RTD has bistable and self-locking characteristics.
To complete certain logic functions, only a few RTD devices are needed: due to its self-locking characteristics, a small number of devices can be used to perform the logic functions that would otherwise require multiple conventional devices. For example, to build an XOR gate, 33 devices are required for TTL, 16 for CMOS, 11 for ECL, and only 4 for RTD.
Application of RTD in microwave oscillation: RTD can be used to create microwave oscillators and mixers, but its power is limited.
Use RTD and other high-speed digital circuits: RTD and RTT constitute various logic units, such as RTD + HBT bistable logic unit, RTD + MODFET bistable logic unit, RTBT (bipolar RTT with RTD as emitter) + HBT XOR gate logic, monostable-bistable conversion logic unit (MOBILE), Schottky/RITD pipeline logic gate, etc. These basic logic units can be further combined into different basic logic gates, flip-flops, SRAM, frequency dividers, A/D and D/A converters, shift registers, adders, etc.
RTD photoelectric integrated circuit: UCT-PD and RTD form a high-speed photoelectric bistable logic unit, used in an 80Gb/s time division multiplexing (TDM) system. The RTD structure is also used to make a new photoelectric negative resistance RTD device.
2.4 Single-Electron Devices
Single-electron devices include single-electron transistors (SET) and single-electron memory (SEM). SET is more common. This section will only introduce SET.
2.4.1 Working Principle of Single-Electron Transistor
The upper part shows a schematic diagram of a single-electron transistor. The source, drain, and island are composed of semiconductors or metals. The tunnel junctions between the source and island, and between the island and drain form a barrier around the island, consisting of an insulating layer or a wide-bandgap semiconductor and a potential barrier formed by a negative voltage. Applying voltage to the gate (g) controls the potential energy of the electrons in the island. The corresponding energy band diagram is given below, where the island corresponds to the potential well between the two barriers. As mentioned earlier, for SET, the charging energy dominates, meaning that when an electron is moved from the source into the island, it must overcome a charging energy of "2 / 2C" (where e is the electron charge, and C is the total capacitance of the island to the surrounding area). Overcoming this charge on the island is only possible if the electron has sufficient energy. This phenomenon is called the Coulomb blocking effect. If the electron passes through the gate (g) or receives energy from the source, the Coulomb blocking effect disappears, and the conductive phenomenon resumes. Thus, as the gate voltage (g) or source-drain voltage (d) increases, a stepped d-I characteristic and oscillation waveform-dI characteristic curve appears, as shown. Since the electrons in the emitter (whose energy is f) obtain the energy of "2 / 2C" before entering the potential well, the energy of the electrons in the potential well must be higher than f + 2 / 2C. Because the electrons in the emitter and the holes in the well attract each other, the hole energy level in the well is lower than f + 2/2C. Therefore, in the potential well, the difference between the electron energy level and the hole energy level is 2 / C. Here, the electron energy in the well is higher than f, reflecting the Coulomb blocking effect.
2.4.2 Characteristics of Single-Electron Transistor
High-frequency high-speed operation: Since the tunneling mechanism is a high-speed process, and SET has a very small capacitance, the working speed is very fast.
Very low power consumption: Because the transport process involves a single electron, the current and power consumption are very low.
High integration: Because the SET device scale is very small, the degree of integration is high.
2.4.3 Applications of Single-Electron Transistor
Using SET to make the next-generation high-speed and high-density IC: Due to the features described above, SET is one of the best candidate devices for the next-generation high-speed and high-density IC.
Ultra-high sensitivity electrometer: Since SET can achieve single-electron conduction, it is suitable for ultra-high sensitivity electrometers. It is expected to increase the sensitivity by 1000 times compared to existing electrometers.
Realization of single-photon devices: Since SET can achieve single-electron transport, if a single-hole device (made with + type material) is used in conjunction with it, single-electron and single-hole recombination can be controlled to produce a single-photon generation device.
High-sensitivity infrared radiation detector: It has been found that the SET is near the Coulomb blocking threshold voltage, and the tunneling current is very sensitive to infrared radiation induction, which may also be called the "photoinduced tunneling" effect.
2.5 Quantum Dot Device
Here, a concept must be clarified. The quantum dot here refers to a few nanometers in each dimension in three-dimensional space, where significant quantization occurs. For dots and particles with a size of micrometers or even 102 nanometers, which have no significant quantization effect, they cannot be included in this QD category. In terms of energy, it must be very large. If the three dimensions are all in the order of micrometers, "4e", it belongs to the category of SET. Therefore, some of SET and D are indistinguishable. Real, D devices, there are few reports, please refer to.
3 Molecular Electronic Devices
3.1 Problems with Solid Nanoelectronic Devices
Although solid nanoelectronic devices currently dominate nanoelectronic devices, some devices (such as RTD) have entered the application stage, but in general, they face the following issues.
3.1.1 Problems with RTD
RTD is one of the fastest devices currently, but it is a two-terminal device, and the input and output cannot be isolated, and there is no current gain. If it is made into GRTD or parallel with HEMT, the working speed and frequency will be greatly reduced.
3.1.2 Low Temperature Operation
Currently, RTD can work at room temperature, but most devices of SET can only work at low temperatures. SET working at room temperature has very high requirements on the manufacturing process. It is only possible to reduce the capacitance of the island and the tunnel junction to a few aF (10^-18F), and the process is very difficult.
3.1.3 Material Issues
The performance of RTD and SET made with compounds is relatively better than those made with Si-based materials. Research on RTD and SET of Si-based materials should be vigorously carried out and combined with the mature Si process.
3.1.4 Background Charge Problems
Randomly distributed background charges often accumulate in the vicinity of quantum effect devices and single electron devices, affecting the normal operation of the device through electrostatic induction.
3.1.5 Control of the Accuracy and Consistency of the Manufacturing Process
The tunneling current is very sensitive to the barrier width, and controlling the accuracy and consistency of the barrier width and the "island" scale is a difficult problem in the process.
Due to the existence of the above problems, the research of solid nanoelectronic devices has entered a gentle stage in the recent period, while molecular electronics has entered a relatively rapid development period.
3.2 The Concept of Molecular Electronics
The concept of molecular electronics is different from the organic micro-transistors or organic devices produced in the "bulk" material and the "bulk" effect that appeared in the previous period. Molecular electronics, also known as "intramolecular electronics," is composed of covalently bonded molecular structures that are electrically isolated from the "bulk" substrate, or molecular wires and molecules that consist of discrete molecules and nanomolecular supramolecular structures. From the aspect of preparation technology, molecular electronics is easier to produce hundreds of millions of almost identical nanoscale structures at a lower cost than solid nanoelectronic devices. This is mainly due to the emergence of new methods of nano-processing and nano-manipulation, namely mechanical synthesis and chemical synthesis technology of nano-scale structure. Mechanical synthesis refers to the use of scanning tunneling microscope (STM), atomic force microscope (AFM) and a new micro-electromechanical system to control the micro-nanoelectronic technology to give the I-ness of the molecular RTD measured by Reed, its current peak the valley ratio is about 1.3: 1. The simulation results of a research team at the company are shown below. Two atomic wires are connected by a movable switching atom to form a relay. If the switching atoms are in situ, the entire device can conduct electricity; if the switching atoms are detached from the in situ, the resulting gap suddenly reduces the current flowing through the atomic wire, making the entire device into a control and operating molecule for synthesis. Chemical synthesis includes chemical self-assembly growth of nanostructures, methods borrowed from biochemistry and molecular genetics, etc. Chemical electronic methods can be used to synthesize molecular electronic devices in organic molecules.
3.3 Quantum Effect Molecular Electronic Device
The representative of quantum effect molecular electronic device is a molecular resonance tunneling diode, referred to as molecular RTD. It has a device structure similar to a solid RTD surrounding a potential well and the same working principle. The structure and working principle of the molecular RTDs recently synthesized by Tour and confirmed by Reed are given in. It can be seen from the figure that the molecular RTD consists of four parts: (1) The emitter and collector of the backbone molecular wire molecular RTD are composed of polyphenylene-based molecular chains. This aromatic organic molecule has a conjugated v electron orbital. More than one such long unfilled or partially filled Gan track can provide a channel. When a bias exists at both ends of the molecule, electrons can move from one end of the molecule to the other. It is estimated that 2-1011 electrons per second can pass through each molecule. This kind of molecular wire is usually called Tour molecular wire; (2) "Islands" or potential wells composed of a single fatty ring have lower energy, which The size is about 1 nanometer, which is smaller than the scale of the solid RTD potential well, namely! Bigger or bigger! > 4e; (3) Two potential barriers are formed by two fatty methylene molecules, that is, two methylene molecules with insulating properties (one CH, ―) are inserted on both sides of the "island" and form between the left and right molecular wires Two potential barriers for molecular RTD; (4) Terminal leads of molecular electronic devices. Both ends of molecular devices are often pasted on gold (Au) electrodes through thiol (-SH) as their leading ends. The one (SH) next to the metal is often called the "crocodile clip" of the molecular device. The working principle of a molecular RTD is basically the same as that of a solid RTD. When electrons are confined in a narrow potential well, their energy is quantized to form discrete energy levels. When the energy levels in the potential well and the emitter are not filled with electrons When the energy of the molecular orbital is misaligned, resonance tunneling does not occur and the device does not conduct. When the bias voltage is applied, the energy level in the well is aligned with the energy of the orbital filled with electrons, and the energy level in the well and the empty energy state of the collector are also aligned, the state of resonance tunneling occurs.
Bias voltage / V molecular RTD / -F characteristics Electromechanical molecular electronic devices There are many types of electromechanical molecular electronic devices, and two examples will now be given for explanation.
Atomic Relay
Atomic relay is similar to a molecular gate switch. In the original appliance, a movable atom is not fixedly attached to the liner, but moves forward or backward between the two electrodes. Hitachi is broken. The third atomic wire very close to the switch atom constitutes the gate of the atomic relay. A small negative charge is placed on the gate wire to move the switch atom away from its original position and turn off the device. To reset the gate, the switch atoms are pulled back to their original positions and the device is turned on again. In actuality of the atomic relay, the "switch" atom can be attached to a "rotor", which can make the "switch" atom fill the gap of the atomic wire by swinging, and make the atomic relay conduct (see (a)); Or make the "switch" atoms swing away from the atomic wires to turn off the current (see (b)). The direction of the rotor is controlled by adjusting the polarity of the charge on the gate molecules (located in the upper part of the figure). There are many types of molecular switching devices, limited to space, only to introduce these.
4 Suggestions for the Development of Nanoelectronic Devices
It is essential to focus on and vigorously carry out research on nanodevices, especially nanoelectronic devices. Academician Bai Chunli once proposed that "the level of nanodevice development and application is an important indicator of whether we are entering the nano era" and pointed out that "China must pay attention to the research work of nanodevice development and nanoscale detection and characterization." According to the current status of nanotechnology development in China, we must vigorously advocate the research, development, and application of nanodevices, especially nanoelectronic devices. Because the research of nanoelectronic devices is the fulcrum of nanotechnology and information technology, it plays a vital role in the economy and the entire science and technology.
In the research and development of nanoelectronic devices, in addition to strengthening the research of solid nanoelectronic devices such as RTD and SED, it is also necessary to vigorously carry out research work on molecular electronic devices in a timely manner. Internationally, the United States and Japan attach great importance to the research of molecular electronics.
The world's top ten scientific and technological progress reported the development of molecular transistors in the United States, saying that Bell Labs used a single organic molecule to make the world's smallest transistor, which is a molecular electronic device. This kind of chemical organic synthesis method for manufacturing electronic devices is much lower than EB, MBE and other technologies for manufacturing RTD and SED, and it is suitable for large-scale production. Chemists and electronics scientists should be called on to work closely together to conduct research on molecular electronic devices.
Effectively organize the domestic related departments of nanotechnology, especially nanodevice research units, concentrate technical force, key and key issues in cat quasi-nanodevices, avoid duplication of research content, and obtain the results of source innovation as soon as possible. It is hoped that the Nanotechnology Guidance and Coordination Committee can fully and specifically understand the actual situation of domestic nanodevice research units, mobilize the research enthusiasm of each unit, and contribute to the development of nanotechnology.
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