Overview of Nanoelectronic Devices <s> B. Resonant Tunneling Devices <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> B. Resonant Tunneling Devices <s> A bipolar tunneling transistor in which ohmic contact is made to the strained p/sup +/ InGaAs quantum well of a double-barrier resonant-tunneling structure is discussed. The heterojunction transistor consists of an n-GaAs emitter and collector, undoped AlGaAs tunnel barriers, and a pseudomorphic p/sup +/ InGaAs quantum-well base. By making ohmic contact to the p-type quantum well, the hole density in the quantum-well base is used to modulate the base potential relative to the emitter and collector terminals. With control of the quantum-well potential, the tunneling current can be modulated by application of a base-to-emitter potential. The authors detail the physical and electrical characteristics of the device. It is found that the base-emitter voltages required to bias the transistor into resonance are well predicted by a self-consistent calculation of the electrostatic potential. > <s> BIB002 </s> Overview of Nanoelectronic Devices <s> B. Resonant Tunneling Devices <s> Much of our understanding of electron transport in solids is based on the semiclassical picture that views electrons as particles obeying Newtonian mechanics in an external field and occasionally scattered by phonons and impurities. Quantum effects such as size quantization and tunneling, however, cannot be understood within this conceptual framework. <s> BIB003 </s> Overview of Nanoelectronic Devices <s> B. Resonant Tunneling Devices <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> BIB004 </s> Overview of Nanoelectronic Devices <s> B. Resonant Tunneling Devices <s> A vertical field-effect resonant tunneling transistor is demonstrated consisting of a triple-barrier, double-well resonant tunneling diode (3bRTD) that can be depleted by the action of side gates. The 3bRTD features a double peak current-voltage characteristic in which the second valley current is less than the first valley current. Combination of the resonant tunneling transistor and a constant current load is shown to yield both binary and ternary logic and memory functions. > <s> BIB005 </s> Overview of Nanoelectronic Devices <s> B. Resonant Tunneling Devices <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> BIB006 </s> Overview of Nanoelectronic Devices <s> B. Resonant Tunneling Devices <s> We discuss the opportunities for Nanoelectronic devices based on resonant tunneling to impact integrated circuit (IC) technology. We believe that resonant tunneling diodes (RTDs) integrated with conventional compound semiconductor transistors will be the first opportunity for products. The development of silicon heterostructure technology should permit RTDs to augment CMOS ICs by permitting novel circuit applications that will have speed and functional density advantages. In our view, the most promising avenue for downscaling to exist beyond CMOS will be Quantum Dot IC technology. Numerous challenges for nanotechnology exist along each step of these paths. <s> BIB007
Although resonant tunneling devices BIB004 , BIB006 , BIB001 ,  were not the first category of solid-state nanoelectronic device listed above, we explain their operation in detail first, because it illustrates energy quantization and tunneling in their simplest form. It is crucial to the operation of resonant tunneling devices (and the other categories of nanoelectronic devices) that the energy of the quantum states in the potential well on the island can be adjusted relative to the energy of the bands in the source and drain. An example of this is diagramed in Fig. 3 for the two-terminal nanoelectronic device shown in Fig. 3(a) . Increasing the applied voltage bias across the device progressively lowers the energy of all the states in the well relative to the energies the electrons in the source. This is shown in Fig. 3(b) and (c). When the bias potential is sufficient to lower the energy of an unoccupied one-electron quantum state inside the well to be within the range of energies for the source conduction band, the quantum well is said to be "in resonance" or "on," and current can flow onto the island and out to the drain. This is shown schematically in Fig. 3(c) . Otherwise, current through the device is blocked-the device is "out of resonance" or switched "off," as in Fig. 3(b) . This use of a variable applied bias to switch a tunneling current on and off characterizes the operation of a two terminal resonant-tunneling device called a resonant-tunneling diode or RTD. Similar adjustment of the energy levels in the potential well relative to those in the source also can be achieved by varying the voltage on a third (gate) terminal, rather than the voltage on the source. This is illustrated in Fig. 4 . In this three-terminal configuration, shown in Fig. 4 (a) and termed a resonant tunneling transistor (RTT), a small gate voltage can control a large current across the device [ Fig. 4 (b) and (c)]. Thus, an RTT can perform as both switch and amplifier, just like the conventional MOSFET described above. Actually, nanometer-scale, quantum-effect devices such as these can have switching properties that are superior in some ways to those of MOSFET's. RTD's and RTT's can have multiple on and off states associated with multiple discrete quantum levels inside the potential well on a very small or very narrow island. If these levels are spaced widely enough in energy (that is, if is greater than the energetic difference between the band edge and the Fermi level for the source), then each of the different levels in the well can be brought successively into and out of resonance with the source conduction band in succession, as the bias voltage (or gate voltage) is increased. These multiple on and off states are illustrated for an RTD in Fig. 5 . The peaks in the plot correspond to the alignment of the energy levels in the well with the occupied part of the conduction band for the source. The current falls off between the peaks on the curve as the voltage is varied to make the energy of a quantum level in the well pass below the energy of the conduction band edge. Two current peaks are shown in Fig. 5 (b), corresponding to resonance with each of the two energy states in the potential well shown in Fig. 5 (a). Comparable multistate behavior can be obtained by varying the gate voltage in an RTT  ,  , BIB003 , BIB005 . Fewer devices are required to implement a given logic function when using such multistate devices rather than two-state MOSFET's. This generates circuitry with a higher density of logic functions per switching device  ,  ,  . (Fewer devices per logic function could also imply less heat dissipation per function, which might help these nanoelectronic devices  . (b) Barriers in device create potential well around the island, and usually prevent charge from flowing through the device, even when it is under a voltage bias (downward slope in energy from source to drain), as shown. (c) Potential well has configuration similar to that for the RTD shown in Fig. 3 until the gate electrode is charged, lowering the energy for all the states in the well and bringing them into resonance with the mobile electrons in the occupied conduction band in the source, so that an electron current can be transmitted through the device. circumvent one of the FET scaling problems enumerated above.) This advantage for logic that includes quantumeffect devices has led investigators to build hybrid microelectronic-nanoelectronic devices, in which tiny quantum-effect RTD's are built into the drain (or source) of a micron-scale MOSFET. A schematic of such a device is shown in Fig. 6 . The hybrid RTT also exhibits multistate behavior-the drain current can be switched on and off several times for various values of the bias voltage. Thus, the logic density of circuitry containing such hybrid devices can be made much higher without a significant decrease in the feature sizes on the chip  ,  , BIB005 . Fabricating circuits with this relatively large, hybrid type of three-terminal RTT is easier than fabricating circuits with the much tinier, complex structures for purely nanoelectronic RTT's, such as the one shown in Fig. 4 . For this reason a number of groups are experimenting with such devices. Seabaugh et al. have constructed such devices, used them in circuitry  , BIB007 , BIB002 -  and even 6 . Schematic of hybrid RTD-FET. Nanometer-scale RTD's are built into the drain of a micron-scale FET, creating a "hybrid" microelectronic-nanoelectronic device which has more logic states than a regular FET and is easier to build into circuitry than the tiny RTD. demonstrated their operation at room temperature  . Hybrid logic can be viewed as a practical engineering step on an evolutionary path toward nanoelectronics. It could accelerate the availability of quantum-effect devices in integrated circuits with very dense functionality. This progress on resonant tunneling devices is based upon research dating back to the early 1970's, when Esaki and his collaborators first reported the observation and use of the resonant tunneling effect in a device. However, for nearly a decade, such devices were thought to be limited by problems such as low current density on-resonance, until work on RTD's by Sollner and his collaborators [96] at the MIT Lincoln Laboratory demonstrated otherwise. That breakthrough and the results of simultaneous investigations by Reed [97] led Capasso and Kiehl BIB001 to develop an early RTT. Work on resonant tunneling devices is being carried on by a number of groups BIB004 , BIB003 ,  , with particular progress on the hybrid RTT's and circuitry by Seabaugh and his collaborators at Texas Instruments  , BIB007 . Also notable is MIT Lincoln Laboratory's construction of a fabrication facility that is producing very large scale integration (VLSI) wafers containing large numbers of RTD's in high-speed circuits intended for digital signal processing applications  , [93] .