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I've heard that SiGe chips can be faster than ordinary silicon chips.

What is SiGe and why is it faster than ordinary silicon?

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    \$\begingroup\$ I'm aware this information is available on Wikipedia. I'm asking the question to help make EE.SE into a comprehensive reference site in its own right. \$\endgroup\$ – The Photon Sep 25 '13 at 17:15
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    \$\begingroup\$ Seed questions are relatively common, as long as someone only does it irregularly I consider it fine, if you disagree please post on meta. \$\endgroup\$ – Kortuk Sep 25 '13 at 19:54
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    \$\begingroup\$ @GustavoLitovsky, The point is to build up EE.SE as a reference site for people to learn about electronics. I will answer after a day or two if I have something to add after seeing other answers. But first I'll give others a chance to earn some +1's. \$\endgroup\$ – The Photon Sep 25 '13 at 20:21
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    \$\begingroup\$ I think that the question needs much elaboration: what do you mean by "faster chips" and "faster than ordinary silicon", what topology are you asking about and what degree of details do you expect to see. Otherwise it is way too broad because there are tons of academic papers on SiGe and it is not practical to post all this information as an answer. \$\endgroup\$ – Vasiliy Sep 25 '13 at 21:36
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    \$\begingroup\$ The question is intentionally written from a somewhat naive point of view. A good answer will give a broad overview. Drilling down into detail can be left for more specific questions that might be asked in the future. \$\endgroup\$ – The Photon Sep 25 '13 at 21:43
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SiGe is a semiconductor alloy, meaning a mixture of two elements, silicon and germanium. Since 2000 or so, SiGe has become widely used to enhance the performance of ICs of various types. SiGe can be processed on equipment nearly the same as used for ordinary silicon. SiGe doesn't have some of the drawbacks of III-V compound semiconductors like gallium arsenide (GaAs), for example it doesn't lack a native oxide (important for forming MOS structures) and doesn't suffer from mechanical fragility that limits the wafer size of GaAs. This results in costs that are only a small multiple of ordinary silicon, and so much lower than competing technologies like GaAs.

SiGe allows two main improvements compared to ordinary silicon:

First, adding germanium increases the lattice constant of the alloy. If a layer of Si is grown on top of SiGe, there will be mechanical strain induced by the lattice constant mismatch. The strained layer will have higher carrier mobility than unstrained Si. This can be used, for example, to balance the performance of PMOS and NMOS transistors, reducing the area needed for a given CMOS circuit.

Second, the SiGe alloy can be used selectively in the base region of a BJT to form a heterojunction bipolar transistor (HBT). SiGe HBT's have been demonstrated with speeds (fT) to 500 GHz, and are commercially available with fT up to 240 GHz. The SiGe HBT also has lower noise than a standard silicon BJT.

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In addition to the answer by The Photon (which concern embedding small portions of SiGe into otherwise canonical Si ICs), there are also potential benefits in contaminating Si with Ge atoms during ingots fabrication.

There are reports that SiGe structure is more mechanically strong and is less prone to various defects introduced as part of manufacturing process.

The reduction in fabrication defects achieved with Ge contamination is beneficial not only to VLSI, but also to Photovoltaics.

The above technique is yet to be employed, but the results of ongoing research suggest that it won't take long time for it to become a major vector in semiconductor industry.

For completeness and impartialness we must not forget also the disadvantages of this technology:

  • Higher cost associated with more processing steps
  • Difficulties in growing an oxide on SiGe
  • Ge has lower thermal conductivity than Si
  • Surely much more
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protected by Kortuk Sep 25 '13 at 19:53

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