Highlights in Semiconductor Device Development

The agricultural civilization in the cultural history of man was said to be the result of two genetic accidents which gave birth to a new species of bread wheat some 10,000 years ago, involving wild wheat and goat grass. Large-scale agricultural activity in man's society followed. Great inventions or discoveries could be considered to be such genetic accidents-mutations. New knowledge, arising from these inventions, often leads to a large-scale engineering effort which eventually has far-reaching consequences in our society. The invention of the transistor by three solid state physicists, Shockley, Bardeen, and Brattain, is one such example. The development of the transistor began in 1947 through interdisciplinary cooperation with chemists, metallurgists, and electronic engineers, at Bell Laboratories. A large-scale development effort for a variety of semiconductor devices followed in a number of institutes throughout the world. Semiconductor know-how, thus established, has revolutionized the whole world of electronics-communications, control, data processing, and consumer electronics. One of the major achievements of modern physics has been the success of solid-state physics in creating new technologies. Solid-state physics, which involves experimental investigation as well as theoretical understanding of the physical properties of solids, constitutes, by a substantial margin, the largest branch of physics; probably a quarter of the total number of physicists in the world belong to this branch. Semiconductor physics, one of the most important sub-fields of solid-state physics, covers electrica~ optica~ and thermal properties and interactions with all forms of radiation in semiconductors. Many of these have been of interest since the 19th century, partly because of their practical applications and partly because of the richness of intriguing phenomena that semiconductor materials present Point-contact rectifiers made of a variety of natural crystals found practical applications as detectors of highfrequency signals in radio telegraphy in the early part of this century. The natural crystals employed were lead sulphide (galena), ferrous sulphide, silicon carbide, etc. Plate rectifiers made of cuprous oxide or selenium were developed for handling large power [1).1 The selenium photocell was also found useful in the measurement of light intensity because of its photo-sensitivity. In the late 1920's and during the 1930's, the new technique of quantum mechanics was applied to develop electronic energy band structure [2] and a modern picture of the elementary excitations of semiconductors. Of course, this modern study has its roots in the discovery of x-ray diffraction by von Laue in 1912, which provided quantitative information on the arrangements of atoms in semiconductor crystals. Within this framework, attempts were made to obtain a better understanding of semiconductor materials and quantitative or semiquantitative interpretation of their transport and optical properties, such as rectification, photoconductivity, electrical breakdown, etc. During this course of investigation on semiconductors, it was recognized in the 1930's that the phenomena of semiconductors should be analyzed in terms of two separate parts: surface phenomena and bulk effects. Rectification and photo-voltage appeared to be surface or interface phenomena, while ohmic electrical resistance with a negative temperature coefficient and ohmic photocurrent appeared to belong to bulk effects in homogeneous semiconductor materials. The depletion of carriers near the surface primarily arises from the existence of surface states which trap electrons and, also from relatively long screening lengths in semiconductors because of much lower carrier concentra-