Imec demos speedy HBTs using complex process
Imec used a silicon-germanium: carbon heterojunction bipolar transistor (SiGe:C HBT) process technology which improves device performance by adding small amounts of germanium and carbon to the silicon transistor base with a process that forms electron paths that allow high-speed transport at high density in the semiconductor using heterojunctions.
These HBT devices also pave the way to silicon-based millimeter wave circuits penetrating the so-called THz gap, enabling enhanced imaging systems for security, medical and scientific applications.The devices have a fully self-aligned architecture of the emitter, base and collector region, and implement an optimized collector doping profile.
Compared to III-V HBT devices, SiGe:C HBTs combine high-density and low-cost integration, making them suitable for consumer applications. To achieve the ultra high-speed requirements, Imec researchers contend that state-of-the-art SiGe:C HBTs need further up-scaling of the device performance.
Thin sub-collector doping profiles are generally believed to be mandatory for this up-scaling. Usually, the collector dopants are introduced in the beginning of the processing and thus exposed to the complete thermal budget of the process flow. This complicates the accurate positioning of the buried collector. Imec researches used in-situ arsenic doping during the simultaneous growth of the sub-collector pedestal and the SiGe:C base.
This created a thin, well-controlled, lowly-doped collector region close to the base and a sharp transition to the highly doped collector without further complicating the process. This resulted in a considerable increase of the overall HBT device performance: Peak fMAX values above 450GHz are obtained on devices with a high early voltage, a BVCEO of 1.7V and a sharp transition from the saturation to the active region in the IC-VCE output curve.
Despite the aggressive scaling of the sub-collector doping profile, the collector-base capacitance values did not increase much, report the researchers. Moreover, the current gain is well defined, with an average around 400 and the emitter-base tunnel current, visible at low VBE values, is limited as well.
The HBTs were developed within the framework of the European joint research project DOTFIVE which aims at developing SiGe:C HBT devices that operate at 500 GHz at room temperature.Meanwhile, earlier last month, Renesas Electronics Corp. announced the availability of a new SiGe:C heterojunction bipolar transistor, the NESG7030M04.
For its part, Renesas is applying its HBT for use as a low-noise amplifier transistor for wireless LAN systems, satellite radios, and similar applications. Renesas’ SiGe:C HBT achieves a noise figure of 0.75 decibel (dB), which the company claims is the industry’s top level for the 5.8 gigahertz (GHz) band used by wireless LANs and other applications.
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