Biodegradable mems. Vocabbox. A lot of machines made by physicists are used in medicine now, страница 9

One material accounts for over 98% of sales in the global semiconductor market: silicon. There are many reasons why silicon devices completely dominate the microelectronics market, but the overriding factor is their low manufacturing cost. Billions of transistors with virtually identical properties can be fabricated across increasingly larger silicon wafers, driving down the cost per transistor.

The fabrication processes and the performance of the devices rely heavily on a number of inherent properties of silicon. However, the properties of silicon dioxide and silicon nitride are even more important. These two materials form the all-important insulating layer in transistors and reduce leakage currents. Indeed, it is these insulating materials that have allowed silicon to dominate over other faster materials, such as gallium arsenide and other so-called III- V semiconductor compounds. Moreover, it is more expensive to fabricate devices from III-V materials than from silicon because different processing techniques must be used and these have not yet reached the phenomenal yields of silicon chips.

Processors like the PowerPC and Pentium chips contain a total of around 11 million transistors, each transistor costing just 0.003 cents. The Semiconductor Industry Association's 1997 Roadmap predicts that by 2012 the number of transistors on a chip will rise to 1,4 billion - that is an incredible 180 million transistors per square centimeter  - while the cost will drop to just 50 microcents per transistor. The storage capacity of dynamic random access memory (DRAM) is set to increase an even more impressive rate. No other technology gets even close to these values. This relentless exponential growth is known as Moore's law, which predicts that the number of transistors that can be fitted onto a chip doubles every 18 months. This allows computers to double in speed or half in price during that time.

However, there are a number of areas where silicon devices cannot compete with other semiconductor materials. In radio-frequency devices, such as the transmitters and receivers in mobile phones and global positioning systems GPS, silicon field-effect transistors suffer from a greater level of undesirable electronic noise compared with most III-V semiconductors. Gallium arsenide is better for optoelectronic devices, such as light-emitting diodes and lasers. This is because the electrons in the "conduction band" can combine with holes in the "valence band" much more easily in gallium arsenide than in silicon.

By adding another semiconductor material to silicon, however, we can improve the performance of transistors and circuits, thus opening up a number of new applications. Silicon germanium (SiGe) is one such material.

Moreover, it can be grown onto silicon wafers such that its lattice constant is matched to that of the silicon. If we replace some of the silicon atoms with germanium atoms, we can "engineer" the band gap of the material (that is the difference in energy between the conduction and valence bands and change the mobilities of the charge carriers and numerous other properties. In other words, we have greater flexibility for designing devices. At the same time, we can fabricate these circuits using the techniques and tools that are used to manufacture conventional silicon chips. SiGe therefore combines the cost benefits of silicon with the speed of more expensive technologies such as gallium arsenide.

Many SiGe products are already available, including low-noise amplifiers for mobile phones and high-data-rate transmission systems for wired networks. Major players in the market include Daimler-Chrysler, IBM, NEC, Siemens and Temic, while a number of smaller companies, such as SiGe Microsystems and Amberwave, provide the SiGe material or technical advice.