Physics and Information Technology

Information technology, IT, which comprises electronic computer technology and telecommunications technology, has in a few decades changed our society radically. Behind this development lies a very advanced scientific and technical development originating largely from fundamental scientific inventions in physics.

The rapid development of electronic computer technology really started with the invention of the integrated circuit around 1960 and the microprocessor in the 1970s, when the number of components on a chip became sufficiently large to allow the creation of a complete micro computer. The rapid increase in the number of components was formulated as a prediction in "Moore's law": the number of components on a chip will double every eighteen months. This has happened since the 1960s and today there are chips with millions of separate components, at prices that are largely unchanged.

Chip development has been matched by equally dynamic and powerful developments in telecommunications technology. Just as the integrated circuit has been and is a prime mover for electronic computer technology, ultra-rapid transistors and semiconductor lasers based on heterostructures of semiconductors are playing a decisive part in modern telecommunications.

Heterostructures in mobile telephones, CD-players, bar-code readers, brake-lights etc

Electronic components are commonly made of semiconductors, i.e. material that is something between a conductor and an insulator. A measure of whether a semiconductor most resembles a conductor or an insulator is given in the band gap — the amount of energy needed to produce moving charge-bearers in the form of electrons and "holes".

Most semiconductor components are made of silicon, but composite semiconductors of type gallium arsenide are growing in importance. A semiconductor, consisting of several thin layers with differing band gaps is termed a heterostructured semiconductor. The layers can have a thickness varying from a few atom layers to micrometres and may consist of gallium arsenide (GaAs) and aluminium gallium arsenide (AlGaAs). The layers are generally selected so that their crystal structures fit one another and the charge-bearers can move almost freely at the interface. It is this property of heterostructures that can be exploited in a number of different ways.

Heterostructures are very important in technology. Low-noise high-frequency amplifiers using heterotransistors are used in satellite communications and for improving the signal-to-noise ratio in mobile telephony. Semiconductor lasers based on heterostructures are used in fibre-optical communication, in optical data storage, as reading heads in CD players, as bar-code readers and laser markers, etc. Heterostructure-based light-emitting diodes are used in car brake-lights and other warning signals and may one day replace electric bulbs.

Heterostructures have also been of great importance for scientific research. Properties of what is called a two-dimensional electron gas formed in the interface layer between semiconductors was the starting point for the study of the quantised Hall effects (Nobel Prize in Physics 1985 to Klaus von Klitzing and 1998 to Robert B. Laughlin, Horst L. Störmer and Daniel C.Tsui). Quantised conductance has also been studied in one-dimensional channels and point contacts, artificial atoms and molecules based on "quantum dots" with a limited number of free conduction electrons enclosed in very small spaces, one-electron components, etc.

The Heterotransistor

The first worked-out proposal for a heterostructure transistor was published in 1957 by Herbert Kroemer, then working at RCA (Radio Corporation of America) in Princeton, USA. His theoretical work showed that a heterotransistor can be superior to a conventional transistor, particularly for current amplification and high-frequency applications. A frequency as high as 600 GHz has been measured in a heterotransistor, i.e. about 100 times higher than the best ordinary transistors. In addition, the noise is low in amplifiers based on these components.

Fig. 1. In the fast transistors of the base stations of cell phones there are semiconductor heterostructures

The heterostructure laser

Heterostructures have been crucially important for the development of semiconductor lasers. Zhores I. Alferov of the Ioffe Institute of the Russian Academy of Sciences in what was then Leningrad and Herbert Kroemer then at Varian in Palo Alto proposed in 1963, independently of each other, the principle for the heterostructure laser, an invention that is probably as significant as that of the heterotransistor.

Alferov was the first to succeed in producing a lattice-adapted heterostructure (AlGaAs/GaAs, 1969) with clear borders between the layers. Alferov's research team succeeded in rapidly developing many types of components built up of heterostructures, including the injection laser which Alferov patented in 1963. A technological breakthrough occurred around 1970 when heterostructure lasers became able to work continuously at room temperatures. These properties have, for example, made fibre-optic communications practically possible.

Fig.2. The laser diode of the CD player contains a semiconductor heterostructure.