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.