The desire to expand the frontiers of computer science has prompted consideration of the factors that contribute to the limitations of current computers. Silicon is at the heart of today's computer. The advances in computing power and speed over the years have largely been a consequence of better understanding the fundamental properties of silicon and harnessing those properties for practical effect. Initial progress was predicated on building basic electronic components such as transistors and diodes out of silicon and later progress followed from the development of integrated circuits. More recent advances represent a continuation of these trends and currently emphasize miniaturization and the integration of an ever larger number of microelectronic devices on a single chip. Smaller devices lead to higher memory storage densities, more highly integrated circuits and reduced interaction times between devices on the same chip.
Since future improvements in computing power and functionality are currently predicated on further improvements in silicon technology, there has been much recent discussion about the prognosis for continued miniaturization of silicon-based electronic devices. A growing consensus is emerging that believes that the computer industry is rapidly approaching the performance limits of silicon. The feature size in today's manufacturing technologies is approximately 0.10 micron and it is expected that this can be reduced to about 0.02 micron in the future. Further decreases in feature size, however, are deemed problematic because sizes below about 0.02 micron lead to a change in the fundamental behavior of silicon. More specifically, as the dimensions of silicon devices decrease to tens of nanometers and below, silicon enters the quantum regime of behavior and no longer functions according to the classical physics that governs macroscopic objects. In the quantum regime, phenomena such as tunneling lead to delocalization of electrons across many devices. Consequences of tunneling include leakage current as electrons escape from one device to neighboring devices and a loss of independence of devices as the state of one device influences the state of neighboring devices. In addition to fundamental changes in the behavior of silicon, further decreases in the dimensions of silicon devices also pose formidable technological challenges. New and costly innovations in fabrication methods such as photolithography will be needed to achieve smaller feature sizes.
One strategy for advancing the capabilities of computers is to identify materials other than silicon that can be used as the active medium in data processing and/or storage applications. Such alternative computing media could be used independent of or in combination with silicon to form the basis of a new computing industry that seeks to offer better performance and more convenient manufacturing than is possible with silicon.
Chalcogenide materials are an emerging class of alternative materials for the storage and processing of information. Chalcogenide materials have been previously utilized in optical and electrical memory and switching applications and some representative compositions and properties have been discussed in U.S. Pat. Nos. 5,543,737; 5,694,146; 5,757,446; 5,166,758; 5,296,716; 5,534,711; 5,536,947; 5,596,522; and 6.087,674; the disclosures of which are hereby incorporated by reference herein, as well as in several journal articles including “Reversible Electrical Switching Phenomena in Disordered Structures”, Physical Review Letters, vol. 21, p. 1450-1453 (1969) by S. R. Ovshinsky; “Amorphous Semiconductors for Switching, Memory, and Imaging Applications”, IEEE Transactions on Electron Devices, vol. ED-20, p. 91-105 (1973) by S. R. Ovshinsky and H. Fritzsche; the disclosures of which are hereby incorporated by reference herein.
Chalcogenide phase-change materials form the basis of OUM (Ovonic Universal Memory) technology. OUM is a non-volatile form of memory that is viewed in the near term as a viable alternative to flash memory and DRAM and in the longer term as a viable alternative to SRAM. The functional characteristic of chalcogenide phase-change materials that underlies memory operation is the ability of phase-change materials to undergo a reversible transformation between two or more structural states. The chalcogenide phase-change materials have structural states that include a crystalline state, one or more partially crystalline states and an amorphous state. The crystalline state may be a single crystalline state or a polycrystalline state. A partially crystalline state is a structural state of a phase-change material that includes an amorphous portion and a crystalline portion. Chalcogenide phase-change materials include a plurality of partially crystalline states that differ in the relative proportion of the amorphous and crystalline portions included within a volume of the material. The various structural states of a phase-change material may be distinguished on the basis of electrical resistance. Memory functionality can be achieved by associating different memory states with different structural states and using electrical resistance as the means to read the memory device and discriminate among the different memory states. In a binary memory device having memory states “0” and “1”, for example, state “0” may be associated with a substantially crystalline state and the state “1” may be associated with a substantially amorphous state. Since the resistance of a substantially crystalline state is at least an order of magnitude lower than the resistance of a substantially amorphous state, the two states are readily distinguished through a simple resistance measurement. The operation of storing information (writing or programming) occurs by providing energy (most commonly in the form of electrical current pulses) to the phase-change material to induce the structural transformations needed to establish the desired proportions of crystalline and amorphous phase domains within a volume of the phase-change material. Controlled applications of energy can be used to reversibly and continuously vary the relative proportions of crystalline and amorphous phase domains to establish the structural state corresponding to the information that the programmer wishes to store. Once established, a memory state is stable until further energy having a magnitude sufficient to reprogram the material is applied. The current used to determine the resistance of the device (and thus to read the device) is too low to alter the structural state of the phase-change material.
Chalcogenide switching materials form the basis of the Ovonic Threshold Switch (OTS) technology. Chalcogenide switching materials are substantially amorphous materials that exhibit little or no tendency to undergo a structural transformation to a crystalline or partially crystalline state, but which instead undergo rapid switching from a resistive state to a conductive state upon application of a threshold voltage, Vth. According to a leading model of the switching event, application of the threshold voltage causes the formation of a conductive channel or filament within the chalcogenide material. At the threshold voltage, the electric field experienced by the material is sufficiently, high to induce a breakdown or avalanche-like effect whereby electrons are removed from atoms to form a highly conductive, plasma-like filament of charge carriers. Rather than being bound to atoms, some electrons become unbound and highly mobile. As a result, a conductive channel or filament forms. The conductive filament constitutes a conductive volume within the otherwise resistive chalcogenide material. The conductive filament extends through the chalcogenide material between the device terminals and provides a low resistance pathway for electrical current. Creation of a conductive state upon switching enables the device to support high currents.
In order to advance a new chalcogenide-based computing paradigm, it is necessary to develop devices and circuits for performing data storage and processing operations. Chalcogenide OUM technology provides a versatile and robust memory platform for storing data. Representative examples of the application of chalcogenide phase change materials to data processing include mathematical operations (U.S. Pat. No. 6,671,710 (“Methods of Computing with Digital Multistate Phase Change Materials”)) factoring algorithms (U.S. Pat. No. 6,714,954 (“Methods of Factoring and Modular Arithmetic”), modular arithmetic (U.S. Pat. No. 6,963,893 (“Methods of Factoring and Modular Arithmetic”)), and neural network processing (U.S. Pat. No. 6,999,953 (“Analog Neurons and Neurosynaptic Networks”). Applications of chalcogenide switching materials to data processing include U.S. Pat. No. 5,543,737 (“Logical Operation Circuit Employing Two-Terminal Chalcogenide Switches”).
Recent work in the area of chalcogenide switching devices has demonstrated the operability of a three-terminal chalcogenide switching device. In these devices, a third terminal is added to the standard two-terminal chalcogenide switching device to enable control over the operating characteristics of the device. Application of a voltage signal or electric field to the third terminal, for example, provides a mechanism for controlling the magnitude of the threshold voltage needed to effect the switching transition between the other two terminals of the device. (U.S. Pat. Nos. 6,967,344 (“Multi-Terminal Chalcogenide Switching Devices”) and 6,969,867 (“Field Effect Chalcogenide Devices”, the disclosures of which are incorporated by reference herein).
With the advent of new chalcogenide devices having increased functionality, it is desirable to consider their potential to further expand the capabilities of chalcogenide-materials in the realm of computation. In particular, it is desirable to consider the suitability of three-terminal chalcogenide devices for applications in data storage or data processing and to devise device structures and circuits that exploit the capabilities of three-terminal devices. In U.S. Pat. Nos. 6,967,344 ('344 patent) and 6,969,867 ('867 patent), the disclosures of which are hereby incorporated by reference herein, Ovshinsky et al. further develop the notion of phase change computing by presenting additional computing and storage devices. The '344 patent discusses a multi-terminal phase change device where a control signal provided at one electrical terminal modulates the current, threshold voltage or signal transmitted between other electrical terminals through the injection of charge carriers. The '867 patent describes a related multi-terminal device that utilizes a field effect terminal to modulate the current, threshold voltage or signal transmitted between other terminals. The devices described in the '344 and '867 patents may be configured to provide a functionality related to that of a transistor.
In addition to new storage and processing devices, progress in the field of chalcogenide electronics would further benefit from the introduction of logic circuits based on chalcogenide materials that are capable of performing one or more logic functions. In particular, it is desirable to develop logic circuits based on chalcogenide memory and/or switching devices. The utilization of two-terminal chalcogenide switching devices in logic circuits has been discussed in U.S. Pat. Nos. 5,543,737 ('737 patent) and 5,694,054 ('054 patent); the disclosures of which are incorporated by reference herein. The potential for chalcogenide electronics further expand through the development of logic circuits that utilize the beneficial properties of the three-terminal family of chalcogenide devices.