Overview of Nanoelectronic Devices <s> D. Taxonomy of Nanoelectronic Devices 1) Resonant Tunneling Devices Further Explained: <s> We propose a new negative conductance device consisting of a heterojunction bipolar transistor with a quantum well and a symmetric double barrier or a superlattice in the base region. The key difference compared to previously studied structures is that resonant tunneling is achieved by high‐energy minority carrier injection into the quantum state rather than by application of an electric field. Thus this novel geometry maintains the crucial, structural symmetry of the double barrier, allowing unity transmission at all resonance peaks and higher peak‐to‐valley ratios and currents compared to conventional resonant tunneling structures. Both tunneling and ballistic injection in the base are considered. These new functional devices have significant potential for a variety of signal processing and multiple‐valued logic applications and for the study of the physics of transport in superlattices. <s> BIB001 </s> Overview of Nanoelectronic Devices <s> D. Taxonomy of Nanoelectronic Devices 1) Resonant Tunneling Devices Further Explained: <s> The authors formulate a new paradigm for computing with cellular automata (CAS) composed of arrays of quantum devices-quantum cellular automata. Computing in such a paradigm is edge driven. Input, output, and power are delivered at the edge of the CA array only; no direct flow of information or energy to internal cells is required. Computing in this paradigm is also computing with the ground state. The architecture is so designed that the ground-state configuration of the array, subject to boundary conditions determined by the input, yields the computational result. The authors propose a specific realization of these ideas using two-electron cells composed of quantum dots. The charge density in the cell is very highly polarized (aligned) along one of the two cell axes, suggestive of a two-state CA. The polarization of one cell induces a polarization in a neighboring cell through the Coulomb interaction in a very non-linear fashion. Quantum cellular automata can perform useful computing. The authors show that AND gates, OR gates, and inverters can be constructed and interconnected. <s> BIB002 </s> Overview of Nanoelectronic Devices <s> D. Taxonomy of Nanoelectronic Devices 1) Resonant Tunneling Devices Further Explained: <s> The discovery of periodic conductance oscillations as a function of charge density in very small transistors has led to a new understanding of the behavior of electrons in such small structures. It has been demonstrated that, whereas a conventional transistor turns on only once as electrons are added to it, submicronsize transistors, isolated from their leads by tunnel junctions, turn on and offagain every time an electron is added. This unusual behavior is primarily the result of the quantization of charge and the Coulomb in teraction between electrons on the small transistor. However, recent experiments demonstrate that the quantization of energy is important as well <s> BIB003 </s> Overview of Nanoelectronic Devices <s> D. Taxonomy of Nanoelectronic Devices 1) Resonant Tunneling Devices Further Explained: <s> The first experimental study of a new nanometer field‐effect transistor with a single barrier in its one‐dimensional channel is presented. At low temperatures and as charge density in the channel was varied, nine reproducible periodic oscillations of conductance, in addition to 2e2/h conductance plateaus, were observed before the onset of the first 2e2/h conductance plateau. It was found experimentally that each conductance oscillation corresponds to the Coulomb blockade of a single electron in the one‐dimensional channel. A model that describes the operation of the new single electron transistor is suggested. <s> BIB004 </s> Overview of Nanoelectronic Devices <s> D. Taxonomy of Nanoelectronic Devices 1) Resonant Tunneling Devices Further Explained: <s> Incremental single-electron charging of size-quantized states has been observed in the well in submicrometer double-barrier resonant tunneling devices. In order to distinguish between the effects of size quantization and the single-electron charging, the heterostructure material was grown asymmetrical so that one barrier is substantially less transparent than the other. In the voltage polarity such that the emitter barrier is more transparent than the collector barrier, electrons accumulate in the well; incremental electron occupation of the well is accompanied by Coulomb blockade, which leads to sharp steps of the tunneling current. In the opposite voltage polarity the emitter barrier is less transparent than the collector barrier and the tunneling current reflects resonant tunneling through size-quantized well states. <s> BIB005 </s> Overview of Nanoelectronic Devices <s> D. Taxonomy of Nanoelectronic Devices 1) Resonant Tunneling Devices Further Explained: <s> Abstract A coherent overview of the exciting field and challenging areas of research in mesoscopic physics and nanoelectronics is given. The unifying role of the S -matrix theory or “ input-quantum process-output ” language of quantum mechanics in the formulation of quantum transport in mesoscopic physics and nanoelectronics is clarified, with time-independent S -matrix theory applied to mesoscopic systems and double-time-axis time-dependent S -matrix thoory, coupled with the lattice Weyl-Wigner formulation of quantum dynamics of electrons in solids, applied to the highly nonlinear, far-from-equilibrium and high-speed nanoelectronic devices. At steady state and near equilibrium, the two independent formulations yield equivalent results. The technological impact of the many-body quantum distribution-function (QDF) (typified by the Wigner distribution function) approach is demonstrated by a complete transition from the level of basic nonlinear quantum transport physics to an engineering computer-aided design tool for integrated circuits (ICs). This is done in the derivation of the equivalent circuit model for resonant tunneling devices which incorporates the quantum inductance. In this review, emphasis is given to major developments in quantum transport, computational nanoelectronics, new device concepts, and novel transport physical phenomena found in small structures, with potential device and IC applications and deemed most likely to have a significant impact on the future developments of nanoelectronics. <s> BIB006 </s> Overview of Nanoelectronic Devices <s> D. Taxonomy of Nanoelectronic Devices 1) Resonant Tunneling Devices Further Explained: <s> We report on a novel method of fabricating 10-nm-scale GaAs dot structures using molecular beam epitaxy and in situ gas etching. Self-assembled nanometer-scale InAs dots are formed first and are used as masks for the subsequent etching of GaAs by Cl2 gas. In this approach, we also make use of HCl gas etching to remove selectively the InAs wetting layer as well as the InAs dots to yield clean and strain-free GaAs dot structures. The height and base diameter of resultant GaAs dots are studied by atomic force microscopy and found to be 10±2 nm and 30±5 nm, respectively. <s> BIB007 </s> Overview of Nanoelectronic Devices <s> D. Taxonomy of Nanoelectronic Devices 1) Resonant Tunneling Devices Further Explained: <s> A new type of ultradense logic based on single‐electron tunneling between conducting islands is proposed. The basic element is a short chain of islands which shows bistable polarization and affects the polarization of neighboring chains. The power needed for computation is delivered by ac external electric field. <s> BIB008 </s> Overview of Nanoelectronic Devices <s> D. Taxonomy of Nanoelectronic Devices 1) Resonant Tunneling Devices Further Explained: <s> Since first developed in the early sixties, silicon chip technology has made vast leaps forward. From a rudimentary circuit with a mere handful of transistors, the chip has evolved into a technological miracle, packing millions of bits of information on a surface no larger than a human thumbnail. And most experts predict that in the near future, we will see chips with over a billion bits. At the same time, this revolution in microelectronics has sparked a dramatic change in the way we live. An integral part of the computer industry, the microchip is found in everything from lasers, fax machines, and satellites to greeting cards and children's toys. And yet few people have any idea how chips work, or how so much information can be captured in such a miniscule space. Now, in The Quantum Dot, physicist Richard Turton provides a clear, informative look at the science that lies behind the modern revolution in microelectronics and offers an intriguing glimpse of the possible future of this rapidly evolving field. Turton illuminates the development of the microchip, in a discussion that ranges from a primer on atoms and electrons, to the properties of semiconductors (most notably, silicon), to the structure of the transistor. We learn how researchers have managed to pack the tiny silicon chip with more and more bits, and we get a state-of-the-art look at the microelectronic industry today, from the newest chip materials (such as gallium arsenide, a much faster material than silicon, used in the recently released Cray 3 supercomputer) to the exotic world of high-temperature superconductors. Perhaps most interesting, Turton offers a provocative glimpse of the future of microelectronics. Here readers enter the strange realm where quantum theory prevails and where physical events contradict our intuitive perceptions. Turton shows how researchers are leaving the transistor far behind as they struggle to exploit quantum effects to create incredibly small and fast devices, such as "designer atoms" and the quantum dot. He concludes that the range of future possibilities are immense, including devices in which electrons behave not as particles but as waves, and computers in which there are no electrical signals, only beams of light. Here then is an amazing scientific--and economic--success story, told with clarity and expertise. It will fascinate anyone curious about where modern technology is headed and what the world might look like when it gets there. <s> BIB009 </s> Overview of Nanoelectronic Devices <s> D. Taxonomy of Nanoelectronic Devices 1) Resonant Tunneling Devices Further Explained: <s> We study atomiclike properties of artificial atoms by measuring Coulomb oscillations in vertical quantum dots containing a tunable number of electrons starting from zero. At zero magnetic field the energy needed to add electrons to a dot reveals a shell structure for a two-dimensional harmonic potential. As a function of magnetic field the current peaks shift in pairs, due to the filling of electrons into spin-degenerate single-particle states. When the magnetic field is sufficiently small, however, the pairing is modified, as predicted by Hund’s rule, to favor the filling of parallel spins. [S00319007(96)01418-4] <s> BIB010 </s> Overview of Nanoelectronic Devices <s> D. Taxonomy of Nanoelectronic Devices 1) Resonant Tunneling Devices Further Explained: <s> We found a new self-organizing growth mode in the metalorganic vapor-phase epitaxy (MOVPE) on high-index GaAs (311)B substrates to form well-ordered arrays of quantum-dots. The spontaneous interaction of strained InGaAs layers with AlGaAs buffer layers results in high-density rows of disk-shaped InGaAs quantum dots buried within AlGaAs microcrystals due to lateral mass transport. The size and distance of the disks are controlled independently in the mesoscopic size range by the In composition and the InGaAs layer thickness, respectively, without affecting the uniformity and the shape. The photoluminescence spectra exhibit narrow linewidth and high efficiency at room temperature. Buried quantum disk structures are formed also on other GaAs (n11)B substrates and on InP (311)B substrates, indicating this growth mode to be universal in the strained layer growth on high-index semiconductor surfaces. <s> BIB011 </s> Overview of Nanoelectronic Devices <s> D. Taxonomy of Nanoelectronic Devices 1) Resonant Tunneling Devices Further Explained: <s> We describe a paradigm for computing with interacting quantum dots, quantum-dot cellular automata (QCA). We show how arrays of quantum-dot cells could be used to perform useful computations. A new adiabatic switching paradigm is developed which permits clocked control, eliminates metastability problems, and enables a pipelined architecture. <s> BIB012
A resonant-tunneling device BIB006 , BIB009 , BIB001 ,  usually has a long and narrow island (i.e., a "quantum wire" or "pancake") with shortest dimension 5-10 nm  . The island is made from a semiconductor containing many mobile electrons. The short dimension(s) make large, while the large dimension(s) keep small. (That is, .) This means that the spacings between allowed energies of collections of electrons on the island are determined solely by , because is a negligible term in calculating the total energy for adding an electron to the island. Thus, when a resonant-tunneling device is subject to a voltage bias between the source and the drain, it produces a current versus voltage plot like that sketched in Fig. 5(b) , in which the distance between current peaks is dominated by and there is little effect of the charging energy observable. Fig. 5 also contrasts the current versus voltage behavior of a resonant tunneling diode with the behaviors of a QD and an SET, both discussed in greater detail below. 2) Quantum Dots (QD's): QD's are constructed with islands that are short in all three dimensions, confining the electrons with zero classical degrees of freedom-electronic states are quantized in all three dimensions. The dot-like island may be made of either metal or semiconductor. It can consist of small deposited or lithographically defined regions BIB010 ; small, self-organized droplets BIB007 , BIB011 ; or nanocrystallites grown in situ or deposited in a film  ,  . Using the physical ideas outlined above, we observe that making an island short in all three dimensions leads to widely spaced quantum energy levels for an electron on the island-i.e., , and are all large. The charging energy is also large, because there is no way for a pair of electrons to get far from each other. As a result, both the interaction among the electrons on a QD and the energy levels for each individual electron influence the flow of current through the dot. A schematic plot of current versus voltage for a typical QD is shown in Fig. 5(b) . Because and are comparable in magnitude, a sequence of steps in current associated with each of the two energy scales is observed as the bias voltage is varied. The current jumps to a finite value when electrons can first travel through the island one at a time, and further large jumps herald the ability of electrons to go through two, then three at a time. This series of jumps in current is spaced by a voltage proportional to . The smaller and more frequent jumps occur when an electron can travel across the dot not just in the island's lowest-lying vacant quantum state but also in one or more excited states. The more paths available, the greater the current flow. In the category of QD's we include individual dots, also known as "artificial atoms" BIB009 ,  ,  , as well as coupled dots ("quantum-dot molecules")  , and a kind of composite device called a "QD cell," in which four or five QDs form a single two-state device. (QD cells BIB002 , the clever logic designs based upon them  , and related composite structures that might be built from QD's BIB008 are beyond the scope of this overview. However, they are discussed in detail elsewhere in this issue of the Proceedings BIB012 .) 3) Single-Electron Transistors: A single-electron transistor (SET) BIB003 ,  is always a three-terminal device, with gate, source, and drain, unlike QD's and RTD's, which may be two terminal devices without gates. An SET switches the source-to-drain current on and off in response to small changes in the charge on the gate amounting to a single electron or less (hence, the name). Unfortunately, the terms SET and QD are sometimes used interchangeably in the literature. But we will draw a clear distinction between the two based on the number of classical degrees of freedom retained by electrons on the island, zero for the QD and three for the SET, and the difference in behavior that results. SET's are based around an island, usually of metal, and usually containing a million or more mobile electrons. As opposed to a QD or RTD, an SET's island has no very short dimension and no very long one, either. Nonetheless, QD's may be physically just as large as SET's-what counts as "short" or "long" depends strongly on the materials used. An island with "short" dimensions will have wellseparated quantized energy levels for electrons, but in a semiconductor this may occur at lengths of 100 nm while for metals the lengths must be at least ten times smaller. Hence, making "small" metal particles requires heroic efforts [92] . Since is much less sensitive than to choice of material for the island, this choice may be used to tune the relative sizes of the two energies. Metal islands emphasize over , another defining characteristic of SET's. This limit is called "Coulomb blockade," since the Coulomb interactions among electrons (represented in ) block electrons from tunneling onto the island at low bias voltage. As depicted schematically in Fig. 5(d) , the current versus potential curve for a biased SET exhibits only thresholds associated with , not with , which is negligibly small for such a device. Increasing the gate voltage for an SET to a critical value suddenly allows current to flow from source to drain, but a further increase turns off the current just as suddenly. Additional increases in gate voltage repeat this on/off cycle BIB003 ,  ,  , BIB004 - BIB005 . Despite these similarities to an RTD, SET's operate according to a completely different physical principle. Electrons could, in principle, tunnel onto the island one at a time from the source, and then off onto the drain. This would produce a measurable flow of current. However, extra electrons generally cannot tunnel onto the island due to the electrostatic repulsion of the electrons already there, so no current flows. This Coulomb blockade is a classical effect, depending on the island being sufficiently isolated that an electron cannot quantum mechanically spread over both the island and the source or drain. Recall that in RTD's the opposition to current flow depends on quite different quantum mechanical effects, though the result appears quite similar. In order to control the number of electrons on the island, a metal gate electrode is placed nearby. A sufficient increase in the voltage of the gate electrode induces an additional electron to tunnel onto the island from the source. The extra electron soon tunnels off onto the drain. This double-tunneling process repeats millions of times a second, creating a measurable current through the island. Since the current between the source and drain is sensitive to the charge of single electrons on the gate, the amplification ratio, or "gain," can be extremely high. As the gate voltage is increased further, the number of electrons on the island stabilizes at a value one higher than before, and again no current flows. Yet further increases in gate voltage cause more electrons to migrate to the island, and each one-electron increase is heralded by a spike in current flow. Thus, in contrast to the RTD, which exhibits peaks in a plot of the current versus the bias voltage, an SET would exhibit current peaks similar to those in Fig. 5(b) in a plot of current versus its gate voltage. At high temperatures, however, the thermal energy of electrons in the source and drain may overcome the Coulomb blockade, allowing electrons to tunnel onto the island and current to flow under all gate voltage conditions. Thus far, the low temperatures needed to preserve the SET's ability to switch current on or off have been a major obstacle to their practical application. Sufficiently small SET's would work even at room temperature, though. A group at NTT in Japan has succeeded in making such an SET only 30 nm across. It exhibits clear periodic modulations in source-drain current due to Coulomb blockade at 150 K  ,  . This is well above the temperature of liquid nitrogen, a relatively cheap coolant which boils at 77 K. The NTT work eventually could lead to more routine use of SET's, and even allow their operation at room temperature (300 K).