Imec demos faster HBTs in SiGe for wireless, imaging

October 10, 2011 – Imec says it has developed a fT/fMAX 245GHz/450GHz SiGe:C heterojunction bipolar transistor (HBT) device, useful for future high-volume millimeter-wave low-power circuits in auto applications as well as silicon-based millimeter-wave circuits used in imaging systems for security, medical, and scientific apps.

Working within a EU joint research project dubbed "DOTFIVE" seeking to keep Europe ahead of the ITRS in RF applications beyond compound semiconductors, these new SiGe:C HBT devices have a fully self-aligned architecture by self-alignment of the emitter, base and collector region, and implement an optimized collector doping profile, imec says. Their high density and low-cost integration vs. III-V HBT alternatives makes them suitable in consumer applications as well. Other potential areas of use include applications requiring very high frequencies with low power dissipation, or requiring better circuit reliability through lessened process/voltage/temperature variations.

Electrical parameters for a 0.15×1.0μm2 HBT device. (Source: IMEC)

To scale up new SiGe:C HBTs, thin sub-collector doping profiles are generally believed to be mandatory, usually introduced early in device processing and thus exposed to high thermal budgets, which complicates accurate positioning of the buried collector, imec explains. In-situ arsenic doping during the simultaneous growth of the sub-collector pedestal and the SiGe:C base creates a thin, well-controlled, lowly-doped collector region close to the base, as well as a sharp transition to the highly doped collector without further complicating the process. The result is "a considerable increase of the overall HBT device performance," imec says: peak fMAX values >450GHz (on devices with high early voltage), a 1.7V BVCEO, and "sharp transition from the saturation to the active region in the IC-VCE output curve." Even with the aggressively-scaled subcollector doping profile, collector-base capacitance "did not increase much," the group says, and current gain averaged around 400 with limited emitter-base tunnel current (visible at low VBE values).

Cross-section of bipolar HBT device in a B-E-B-C configuration after end-of-line processing. (Source: IMEC)

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