European researchers drive semiconductor technology
Europeans continue to rise to the challenge of advancing communication, imaging and radar integrated circuits (IC) to work at high frequencies. A case in point is a team at Interuniversitair Micro-Electronica Centrum Vzw (imec) in Belgium; they recently presented the device 'fT/fMAX 245/450 GHz SiGe:C heterojunction bipolar transistor (HBT)'. This sophisticated device will help facilitate future high-volume millimetre-wave low-power circuits to be used in automotive radar applications. The study was funded in part by the DOTFIVE ('Towards 0.5 terahertz silicon/germanium hetero-junction bipolar technology') project, which received EUR 9.7 million under the 'Information and communication technologies' Theme of the EU's Seventh Framework Programme (FP7).
HBT devices are instrumental in helping silicon-based millimetre-wave circuits penetrate what is known as the terahertz (THz) gap. They enable enhanced imaging systems for security, medical and scientific applications, according to the researchers.
The team says the HBT devices are very fast and have a fully self-aligned architecture: self-alignment of the emitter, base and collector region. They can implement an optimised collector doping profile, they add. Where SiGe:C HBTs differ, in comparison with III-V-HBT devices, is that they combine high-density and low-cost integration. On account of this, they are better suited to consumer applications.
The researchers say these types of high-speed devices can also open up new application areas. They can work at very high frequencies with lower power dissipation, or with applications that require a reduced impact of process, and voltage and temperature variations at lower frequencies for better circuit reliability, the imec group said in a statement.
In order to secure the ultra-high speed requirements, sophisticated SiGe:C HBTs require additional upscaling of the device performance. For the most part, thin sub-collector doping profiles are considered a must for this upscaling. The collector dopants are typically introduced at the start of the processing and are therefore exposed to the complete thermal budget of the process flow. Because of this, the accurate positioning of the buried collector is harder to obtain.
In their statement, the imec researchers pointed out that performing in situ arsenic doping during the simultaneous growth of the sub-collector pedestal and the SiGe:C base allowed them to introduce both 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 led to a significant increase in the overall HBT device performance: peak fMAX values above 450 GHz are obtained on devices with a high early voltage, a BVCEO of 1.7 V and a sharp transition from the saturation to the active region in the IC-VCE output curve. According to the researchers, the collector-base capacitance values did not rise much even though they performed aggressive scaling of the sub-collector doping profile. They said the current gain is well defined, with an average around 400; the emitter-base tunnel current, visible at low VBE values, is limited as well.
The DOTFIVE project, which is headed by the STMicroelectronics SA group of France, brought together researchers and industry players from Belgium, Germany, France and Italy.