AMD ‘super’ SOI to boost chip speeds by 30%

Silicon-on-insulator to get even better

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AMD boffins have extended silicon-on-insulator (SOI) chip technology to create a transistor that can operate 30 per faster than the best of today's metal-oxide semiconductor transistors.

The transistor uses what AMD calls 'fully depleted silicon-on-insulator' technology. SOI works by placing an insulating oxide between a silicon substrate and the silicon that makes up transistor switch. It improves over traditional MOS transistors by reducing the switch's capacity to hold charge - its 'capacitance' - and thus reducing the time it takes for the switch to flip on and off.

When a transistor is switched on, it has to charge up before the current will flow through the switch. The greater the switch's capacitance, the longer that takes. Equally, when the transistor is switched off, current continues to flow until the switch has been emptied of charge. SOI reduces the switch's capacitance significantly, so the transistor operates much more quickly.

SOI transistors are harder to make than traditional MOS transistors because the different physical structures of the silicon and the insulator can lead to electrical imperfections where the two materials meet, slowing the switch down and allowing current to leak. Essentially, the silicon ceases to have a pure crystalline structure, degrading its performance as a semi-conductor.

SOI implementations typically use a thick, 'partially depleted' silicon films to overcome these defects - a sort of half-way house between the traditional transistor and the perfect SOI component.

The goal has been the application of thin, 'fully depleted' silicon films, closer to the ideal, and thus offering better performance. This is presumably what AMD's scientists have achieved, and have figured out broadly how it can be used in commercial products - chips cheap enough to sell.

We'll find out in June when AMD officially details the results of its research at the VLSI Symposium, to be held in Kyoto. AMD says the technique will benefit chip production in the latter half of the decade, so don't expect it boosting processor performance any time soon. Commercialisation is clearly going to take some time.

AMD researchers have also demonstrated a strained silicon transistor that uses metal gates to improve performance over conventional strained silicon devices by 20-25 per cent.

Straining silicon - in effect adding a lattice of silicon and germanium to the transistor's silicon layer in order to improve its ability to conduct an electrical current - improves chip performance by up to 35 per cent, according to IBM, which developed the technique. ®


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