Silicon-germanium

Silicon germanium ( specialized claims; standard language silicon germanium ), short- SiGe, a IV - IV compound semiconductor consisting of the elements silicon ( Si) and germanium (Ge).

Production

Through the relationship with the silicon technology, many methods can be transferred. For the production of conventional silicon wafers are used, which are extended like a stretched silicon SiGe layer. The process technology is recognized by epitaxy. Here at temperatures around 600 ° C from silane ( SiH4) and German ( GeH 4 ) a fixed SiGe layer is deposited. With the gas flow can be the Ge content of the SiGe layer set (5 to 30 atomic percent). The single-crystal SiGe layer is braced by it. Only when exceeding a critical layer thickness relaxes ( relaxes ) the layer and there are undesired crystal dislocations.

In boron -doped base regions of bipolar transistors is preferably incorporated in addition carbon, therefore, the SiGe technology commonly known as SiGe: C is known. Wherein the diffusion rate of the dopant boron in the base region during subsequent temperature processes is significantly reduced, and prevents diffusion of the boron from the SiGe layer. It is possible to make the epitaxy such that a layer stack consisting of a Si seed layer, from the p-type SiGe base region and an n -type Si layer ( the emitter ) is deposited.

Application

The transistors are implemented as heterojunction bipolar transistors ( HBT). The main application is the high-frequency electronics and the area fast digital technology.

H.-M. research at the Ruhr- University Bochum in 2003 in the research group of Prof. Clean paved the way of SiGe for high- frequency range at 77 GHz by circuits have been developed in this process that could exploit the full potential of SiGe process. For this reason, SiGe suitable for example for use in the field of automotive radar at 77 GHz frequency generation or signal conversion. For use comes SiGe example, in a distance warning radar of Robert Bosch GmbH ( start of series production first quarter of 2009) as well as the used by him Infineon chipset ( RXN774x family). Next SiGe as the base material for high -frequency applications for 77 GHz is still gallium arsenide ( GaAs) mentioned, which, however, in the current state of technology (2006) does not reach the cut-off frequency of SiGe and is also much more expensive. Since power levels are possible with GaAs, in contrast to SiGe, it may be interesting to stay with GaAs, as long as this material has the required frequency still dominates.

Developments in the years 2008-2010 showed achievable transit frequencies from 250 GHz to 500 GHz.

Early 2000s found SiGe in conventional processors for desktop PC application (Intel, IBM / AMD). This exploits that the charge carrier mobility of electrons and holes can be increased by mechanical stress (so-called strained silicon ). This effect is dependent for electrons and holes on the type of strain and the crystal orientation, for example, compressive stress degrades the carrier mobility of electrons in <100> Si, but improves the Defekelektronen. The silicon germanium this case is therefore not used as a channel material. Instead, it is used for the strain of the channel. This after the production of the silicon gate Polysilicum the actual source - drain regions is etched in the channel area ( by reactive ion etching, and others), and then filled with a CVD epitaxial SiGe again. Due to the different volume expansion of Si and SiGe, the region between the source and the drain region (ie, the channel) clamped ( compressive stress ) during cooling. Principle (silicon dioxide and polysilicon), this technique may also be used in conjunction with conventional gate stacks constructed. Wherein the high- k metal gate technology, however larger strains may be achieved by first polysilicon gate is removed after the clamping; later this space is then filled again by metal.