19 Septembre – Thesis defense - Soumya Panda

17 h ESB-244, IIT Madras (Inde)

Investigating high-frequency sige hbts: assessment of characterization and new architecture exploration

Silicon germanium heterojunction bipolar transistors (SiGe HBTs) are rapidly evolving due to current communication systems' (4G, 5G & 6G) increased functionality and speed. Since the evaluation of SiGe BiCMOS technology, it has been serving the continuous demand for higher functionality and front-end performance quite well at low cost and medium to low volumes. As the operating frequency of SiGe HBTs exceeds 300 GHz, thus it allows critical circuits to operate beyond 100 GHz, which is called the lower end of the THz gap. The upper limit of the THz gap extends up to 30 THz. Within this THz gap range, a lot of applications are envisioned like (i) THz imaging and sensing (ii) Radar applications (iii) in measurement equipment like in ultra-high bandwidth analog to digital converters. With the emerging mm-wave and THz market in sight, the precise characterization and modeling of the devices in the sub-THz frequency range is compulsory to optimize the circuit performance and to minimize the number of design to fabrication loops. However, as we continue to develop devices with increased frequency performance, one of the major problems is accurate characterization at high frequencies (> 100 GHz).
In this seminar, firstly, a systematic method for verifying high-frequency measurement (up to 500 GHz) of SiGe HBT is proposed. The procedure entails a precise calculation of the passive environment's effect on the entire measurement via a comprehensive electromagnetic (EM) simulation. This ensures that the entire measuring environment is precisely incorporated into the framework for EM modeling. In order to additionally include the active SiGe HBT device, a technology computer aided design (TCAD) tool is used to simulate the device S-parameters. TCAD simulation results are fed into an EM-plus-SPICE simulation framework to emulate a complete on-wafer measurement environment. The final simulation results show an appreciable correlation with the on-wafer measurement data up to 500 GHz. Further, the need for proper calibration and de-embedding in high-frequency characterization is emphasized by investigating the S-parameters corresponding to a narrow-band amplifier at 170 GHz suitable for G-band radar applications.
Alongside, to bridge the THz gap further research on BiCMOS compatible SiGe devices with increased speed and breakdown voltage is being continued by various research groups across the globe. To accomplish this, two different SiGe HBT device architectures have been proposed in this work: one based on a nanowire device architecture with less lateral parasitic, which predicts an fMAX above 900 GHz, and the other one is an SOI-based lateral SiGe HBT device that demonstrates an fMAX of above 2.7 GHz.
The asymmetric lateral SiGe HBT has a lightly doped collector that can be electro-statically adjusted by tuning the substrate bias (Vsub). The light collector doping of the device is very sensitive to substrate bias and allows one to switch from a high-speed device to a high voltage device. The novelty of this device is, that it achieves an fMAX of 2.7 THz at Vsub=2V with a BVCEO=2.2V and can be switched to an fMAX of 0.8 THz with a BVCEO=3.6 V at a Vsub=-2V. Indeed, this lateral SiGe HBT device provides additional leverage in switching between high-speed and high-power modes in response to the applied bias at the substrate contact, which will be very much helpful for RF circuit design engineers.

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