10 October 2011

Imec demos SiGe:C HBTs with 245GHz fT and 450GHz fMAX

Nanoelectronics research institute Imec of Leuven, Belgium says that it has realized a SiGe:C heterojunction bipolar transistor (HBT) device with a current-gain cut-off frequency (fT) of 245GHz and a power-gain cut-off frequency (fMAX) of 450GHz, a key enabler for future high-volume millimeter-wave low-power circuits to be used in automotive radar applications. IMEC recons that the 245GHz/450GHz HBT devices also pave the way to silicon-based millimetre-wave circuits penetrating the so-called THz (terahertz) gap, enabling enhanced imaging systems for security, medical and scientific applications.

The extremely high-speed devices have a fully self-aligned architecture through self-alignment of the emitter, base and collector regions, and implement an optimized collector doping profile. Imec says that, compared with III-V HBT devices, SiGe:C HBTs combine high-density and low-cost integration, making them suitable for consumer applications. Such high-speed devices can open up new application areas, working at very high frequencies with lower power dissipation, or applications that require a reduced impact of process, voltage and temperature variations at lower frequencies for better circuit reliability.

To achieve the ultra-high-speed requirements, 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, says Imec. Usually, the collector dopants are introduced in the beginning of the processing and are thus exposed to the complete thermal budget of the process flow. This complicates accurate positioning of the buried collector.

By in-situ arsenic doping during the simultaneous growth of the sub-collector pedestal and the SiGe:C base, Imec has introduced 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 has resulted in a considerable increase in 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. Moreover, the current gain is well defined, with an average around 400. Also, the emitter-base tunnel current, visible at low VBE values, is limited as well.

Tags: Imec SiGe:C HBTs

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