Semiconductor Electronics

the branch of electronics that investigates electronic processes in semiconductors and the use of such processes in, primarily, the conversion and transmission of information. The progress of semiconductor electronics has been the chief reason for the high rate of development of electronics from the 1950’s to the 1970’s and for the penetration of electronics into such fields as automation, communications, computer technology, control systems, astronomy, physics, medicine, and space research and into daily life.

Brief historical survey. The main landmarks in the development of semiconductor electronics follows. The British engineer W. Smith discovered the photoconductive effect in selenium in 1873. In 1874, K. F. Braun discovered the unidirectional conductivity of a metal-semiconductor contact. Crystal-line semiconductors such as galena (PbS) came into use as detectors for the demodulation of radiotelegraph and radiotelephone signals between 1900 and 1905. Copper oxide and selenium rectifiers and photoelectric cells were invented between 1920 and 1926. In 1922, O. V. Losev introduced the use of crystal detectors to amplify and generate oscillations. The transistor was invented by W. Shockley, W. Brattain, and J. Bardeen in 1948. Planar process technology was developed in 1959. The appearance of integrated electronics and the transition to the microminiaturization of electronic equipment occurred between 1959 and 1961. An important contribution to the development of semiconductor electronics was made by many Soviet physicists and engineers, including A. F. Ioffe, N. P. Sazhin, la. I. Frenkel’, B. M. Vul, V. M. Tuchkevich, G. B. Abdulaev, Zh. I. Alferov, K. A. Valiev, Iu. P. Dokuchaev, L. V. Keldysh, S. G. Kalashnikov, V. G. Kolesnikov, A. V. Krasilov, V. E. Lashkarev, and la. A. Fedotov.

Physical principles of semiconductor electronics. The development of semiconductor electronics became possible as a result of fundamental achievements in quantum mechanics, solid-state physics, and semiconductor physics.

The operation of semiconductor electronic devices is based on important semiconductor properties and electronic processes that occur in semiconductors. These properties and processes include (1) the simultaneous existence of charge carriers of two signs, conduction electrons being the negative carriers and holes the positive carriers; (2) the strong dependence of the magnitude and type of the electrical conductivity on the concentration and type of impurity atoms; (3) semiconductors’ high sensitivity to light and heat and sensitivity to the action of a magnetic field and mechanical stresses; (4) the effect of unidirectional conductivity for a current passing through the barrier layer of a p-n junction or Schottky barrier, the nonlinearity of the volt-ampere characteristics of such layers, the introduction (injection) of minority carriers, and the nonlinear capacitance of the p-n junction; (5) the tunneling of carriers through the potential barrier; (6) the avalanche multiplication of carriers in strong electric fields; and (7) the transition of carriers from one energy-band minimum to another with a change in the carriers’ effective mass and mobility.

One of the most widely used effects in semiconductor electronics is the formation of a p-n junction at the boundary between regions of a semiconductor that have different types of conductivity: electron conductivity in the n-region and hole conductivity in the p-region. The chief properties of a p-n junction are the strong dependence of the current on the polarity of the voltage applied to the junction (the current in one direction may exceed the current in the other direction by a factor of 10* or more) and the ability to inject holes into the n-region or electrons into the p-region when a voltage is applied in the direction of passage of current through the p-n junction. The Schottky barrier, which has rectifying properties (unidirectional conductivity) but lacks the capacity for injection, has properties close to those of a p-n junction. Both the p-n junction and the Schottky barrier have a capacitance that varies nonlinearly with changing voltage. When an applied reverse voltage exceeds a certain value, breakdown effects appear in these junctions. The combination of two p-n junctions close together in the same semiconductor crystal yields the transistor effect: the current of a closed junction can be controlled by means of the current of an open junction. A thyristor is formed by three p-n junctions in the same crystal that separate four regions of alternately n-type and p-type conductivity. The transistor effect is of crucial importance for semiconductor electronics. It is the basis of the operation of transistors, which are the principal type of semiconductor device. Transistors have brought about fundamental changes in electronic equipment and computers and the extensive use of automatic control systems in engineering.

The acousto-electric effect in dielectric and semiconductor materials is another of the physical phenomena that semiconductor electronics began to make use of in the early 1970’s. Devices based on this effect were constructed, including amplifiers of electrical oscillations, active electrical filters, and delay lines with signal gain. As a result, the new branch of semiconductor electronics called acousto-electronics appeared.

One of the most general characteristics of the development of semiconductor electronics has been the trend toward the integration of very different physical effects in one crystal. Semiconductor electronics is beginning to merge with the electronics of, for example, dielectric materials and magnetic materials and is gradually becoming solid-state electronics in the broadest sense of the word.

Semiconductor technology. The main engineering problems of semiconductor electronics are the production of semiconductor materials (for the most part, single-crystal materials) with the required properties, the realization of intricate semiconductor structures (primarily p-n junctions), and the development of techniques for the fabrication of semiconductor devices in which semiconductor films are combined with dielectric and metal films. The formation of p-n junctions reduces to the introduction of the requisite amounts of necessary impurities into strictly defined regions of a semiconductor. As of 1975, three techniques for obtaining p-n junctions were in use: alloying, diffusion, and ion implantation.

In the alloying technique, a piece of metal is placed on the surface of a semiconductor wafer possessing one type of conductivity—for example, n-type Ge, which is rich in donors—and the wafer is then heated. The metal must be such that the penetration of its atoms into the semiconductor is capable of imparting to the semiconductor the opposite type of conductivity. An example is a piece of In, whose atoms act as acceptors in Ge. Since the melting point of In is much lower than that of Ge, the In melts, while the Ge remains in the solid, crystalline state. The Ge dissolves to saturation in the droplet of molten In. Upon subsequent cooling, the dissolved Ge begins separating out from the melt and recrystallizing, thus restoring the dissolved part of the crystal. In the process of crystallization the Ge atoms engulf In atoms. The Ge layer formed is rich in In and has p-type conductivity. A p-n junction is thus formed at the boundary of this layer with the undissolved part of the Ge crystal.

In the diffusion technique—for example, diffusion from the gaseous phase—a semiconductor wafer possessing, say, n-type conductivity is placed in the vapor of a substance that imparts to the semiconductor p-type conductivity and whose temperature is 10–30 percent lower than the semiconductor’s melting point. The atoms of the diffusant substance are in random thermal motion and bombard the open surface of the semiconductor. They penetrate into the bulk of the semiconductor. The maximum concentration of these atoms is reached in the surface layer, which acquires p-type conductivity. The acceptor concentration decreases with increasing distance from the surface and, in some cross section, becomes equal to the donor concentration. This cross section corresponds to the position of the p-n junction. Donors predominate in deeper layers, and the semiconductor remains n-type. Other diffusion techniques are also in use, including diffusion from thin films of the diffusant applied directly to the surface of the semiconductor, diffusion from vitreous layers containing the diffusant, and diffusion in a stream of an inert gas mixed with vapors of the diffusant. Not only pure donors or acceptors but compounds thereof may be used as the diffusant. The diffusion technique is the chief method of forming p-n junctions.

Ion implantation is a method of producing p-n junctions that supplements and partially replaces diffusion.

The appearance and rapid spread of planar process technology played an exceptionally important role in the development of semiconductor electronics. Planar process technology is of great significance because it made possible (1) an extensive changeover to the batch fabrication of semiconductor devices, which means that several thousand devices can be fabricated simultaneously on a single semiconductor wafer; (2) a considerable improvement in the precision and reproducibility of the configuration of the elements of devices and an associated increase in the reproducibility of the electrical parameters; (3) a sharp reduction in the size and separation of elements to micron and submicron dimensions and the consequent development of microwave amplifying transistors and oscillistors; (4) the development of field-effect devices, including field-effect transistors; and (5) the construction of a complete electronic device on one semiconductor crystal—that is, the construction of a semiconductor integrated circuit that incorporates, the necessary number of individual semiconductor devices (such as diodes and transistors), resistors, capacitors, and interconnections. The main achievement of planar process technology is that it permitted the intensive development of integrated microelectronics and led to the disappearance of the distinction between the fabrication of electronic components and elements and the production of electronic equipment. The successive processes involved in producing semiconductor materials, then semiconductor devices, and, finally, equipment composed of such devices had previously been widely separated in time and space; they were now combined in a single manufacturing cycle.

Semiconductor fabrication. Semiconductor electronic products are very complex and have an extremely high sensitivity to microscopic levels of contamination. Moreover, it is impossible to correct flaws in them. For these reasons, exceptionally high requirements are imposed on the quality of materials, the precision of equipment operation, and production conditions. In many cases, these requirements are state-of-the-art, or at the present level of technology, and greatly exceed the requirements imposed by other branches of industry.

The materials of semiconductor electronics must have an exactly defined composition and structure and often must be of exceptionally high purity and structural perfection. For example, high purity Ge is characterized by a content of uncontrolled impurities of less than 10^-10.

An idea of the requirements for precision in equipment operation can be gained by examining optical-mechanical devices. Such techniques as the use of masks, by means of which several diffusion processes are carried out successively, and the application of metal films are employed to create hundreds of thousands of elements of different shapes and sizes on the surface of a wafer 30–80 mm in diameter. In fabricating photographic masks and matching a mask with the pattern previously applied to the semiconductor wafer, the precision of the operation of the optical-mechanical equipment must be a few tenths of a micron. The optical part of equipment specially developed for the requirements of semiconductor electronics is therefore characterized by extremely high resolution, which may exceed 1,000 lines/mm (it is still higher—1,500 lines/mm—in the photographic materials used) and has no analog in other fields of technology. If we reduce the dimensions of an element to 1 μ, or less, we encounter considerable difficulties due primarily to the phenomenon of diffraction. These difficulties can be overcome by changing from light rays to electron beams, which can be focused to a few tenths or hundredths of a micron. In this case, the minimum size of an element is determined by the diameter of the electron beam. In the Soviet system, the mechanical processing of semiconductor wafers must conform to grade 14 of surface finish, deviations from a plane (planarity) must not exceed 1 μ. Special requirements also have to be met with respect to thermal equipment: the precision of temperature setting and maintenance in the range 1000°-I300°C must be no worse than ±0.5°0.

Very strict requirements are imposed on the conditions of fabrication of semiconductor electronic products. The gaseous medium in which some of the most important fabrication processes occur must be subjected to thorough drying and dust removal. Its moisture content is measured in fractions of one percent and is estimated from the dew point, which is the gas temperature at which moisture condensation begins. If a comfortable humidity level—that is, a relative humidity of 50–60 percent—is maintained in the atmosphere of the work area, then air, nitrogen, or argon dried to a dew point between— 50° and —70° is fed into special controlled-atmosphere boxes in which, for example, the parts are assembled. Dust is one of the greatest enemies of semiconductor fabrication. An uncorrectable defect nearly always results when a dust particle a few microns in size strikes the surface of a wafer during photolithographic processes. Depending on the complexity of the product and certain other requirements, the dust content of the air in a workplace near a semiconductor wafer being processed should be no more than 4,000 dust particles per m. Such a low dust content is ensured by the provision within the factory of clean rooms, to which only a limited number of people are permitted access. The personnel who work in the clean rooms change into special clothing and proceed to their work stations through sealed antechambers, where the clothing is blown clean and the dust is removed. In the clean rooms, complete air circulation with passage through appropriate filters is carried out up to 300 times per hour. The observance of personal hygiene rules by employees is absolutely compulsory; for example, hands must be thoroughly washed at regular intervals, and special clothing, gloves, caps, and kerchiefs must be worn. Such measures are necessary to ensure high economic efficiency and output quality, including product reliability.

Improvement of electronic equipment on the basis of advances in semiconductor electronics. Equipment using electron tubes is considered to be the first generation of electronic equipment, equipment using discrete semiconductor devices the second, and equipment using integrated microcircuitry the third. The appearance of junction diodes and transistors permitted the replacement of electron-tube devices by semiconductor devices. This made it possible to reduce by a factor of many tens the weight, size, and power consumption of equipment and to improve operating reliability. The micromodular design became the practical limit for the miniaturization of electronic equipment by means of discrete elements. A further decrease in equipment size by reducing the size of discrete parts and elements would entail a considerable increase in difficulty of assembly and, what is particularly dangerous, a sharp lowering of equipment reliability owing to errors and to connections of insufficiently high quality. The changeover to integrated microelectronics was a qualitative leap that made possible further reductions in size and improvements in the reliability of electronic equipment. It became possible to incorporate in an integrated microcircuit, for example, various electric-conversion, optical-electronic, and acousto-electronic devices.

New principles of fabrication of electronic devices developed from printed-wiring assembly technology (hybrid integrated microcircuits) on the one hand and from the techniques of batch fabrication of many elements on one crystal (monolithic or semiconductor integrated microcircuits) on the other hand. These fabrication principles increased the equivalent packing density of such elements as transistors, diodes, and resistors to several thousand or tens of thousands of elements per cm³, Thus began the microminiaturization of electronic equipment. Integrated microcircuitry required the solution of problems of circuit techniques. Semiconductor electronics entered the microelectronic phase of its development.

The development of microelectronics has been characterized by a rapidly rising level of integration: from a few equivalent diodes and transistors in a single package to the fabrication of large-scale and ultralarge-scale integrated microcircuits. Such microcircuits may contain several thousand or even several tens of thousands of functional elements. Multichip ultralarge-scale integrated microcircuits may combine in one package several crystals of large-scale integrated microcircuits and discrete uncased diodes and transistors. The package may contain, for example, all the electronics of a computer, including the electronic memory. The designing of such complex electronic devices requires the solution not only of circuitry problems but also of problems of systems engineering. By increasing the degree of integration, various properties inherent in discrete devices have been realized within volumes of a crystal as small as a few tens or hundreds of interatomic distances in diameter. Examples of such properties are amplification, as in the transistor, and rectification, as in the diode. A transition to the distribution of properties throughout the bulk of a crystal is taking place. Instead of the integration of electronic devices whose functions are concentrated in a certain volume, this approach involves the integration of functions that are distributed throughout the entire bulk of the crystal. A fourth generation of electronic equipment is thus emerging.

Semiconductor electronic products. The variety of semiconductor devices in production is exceptionally wide. Tens of thousands of types—primarily silicon devices—are being manufactured. As of 1974, world industry was producing more than 10 billion discrete semiconductor devices and more than 1 billion integrated microcircuits a year. The development of microelectronics has not significantly affected the rate of growth of the production of discrete semiconductor devices; the demand for such devices will apparently remain for a long time to come. The variety of new semiconductor devices has permitted the designing of complex and, in part, fundamentally new electronic equipment and the creation of an independent branch of the electronics industry—namely, the industry that fabricates discrete semiconductor devices and integrated microcircuits.

The semiconductor electronic products manufactured by industry exhibit excellent performance characteristics. They can operate in the temperature range from — 60° to + 200°C. They can withstand considerable mechanical and climatic loads, such as vibration, shock, continual acceleration, cyclic variation of temperature, and exposure to moisture. Their failure rate is approximately 10^-6-10^–9 failures per hour under actual operating conditions.

Prospects for development. The development of semiconductor electronics is moving in the direction of a rapid increase in the level of integration. As of 1975, this level often reached 10,000–20,000 semiconductor devices per crystal. Another direction of development is an increase in the power and in the frequency of the electromagnetic oscillations converted in a single semiconductor device; a power of hundreds of watts and a frequency of tens of gigahertz are being attained. An important role here is played by the development of semiconductor generators and amplifiers operating in the extremely high frequency band. In addition to the integration of a large number of similar devices, the integration of devices making use of different physical principles in a single microcirucit is also being developed. Besides physical processes in semiconductors, processes in such materials as dielectrics, superconductors (for example, the Josephson effect) and magnetic films are being made use of here. Electron-tube electronics is now using such semiconductor elements as cold cathodes with semiconductor heterojunctions, p-n junction semiconductor anodes in which a current gain occurs, and the matrix targets of vidicons, which contain 0.5–1 million photodiodes. This development has made possible considerable improvements in some types of electron-tube devices.