I. Introduction
The sub-THz and THz frequency range is gaining more and more interest for a wide range of applications e.g., communication system, imaging, radar, security, etc. [1]–[4] but many scientific and engineering challenges needs to be addressed in these frequency ranges. For the electronics system design, the challenges spans from high-speed semiconductor device design, electrical characterization of devices to system design. To design cost effective RF high speed integrated circuits for various applications, the transistors’ characteristics such as the maximum available power gain frequency () and the transconductance () play a very important role [4], [5]. The state-of-art silicon-germanium (SiGe) heterojunction bipolar transistor (HBT) shows a / of 505/720 GHz [6] and benefits from the silicon integration with an advantageous back-end-of-line environment compared to III-V technology based transistors [7]. Accurate, repeatable, and reliable S-parameter measurements of a transistor above 110 GHz are challenging but are urgently required to develop compact models for the high-speed RF circuit design [8]. In the sub-THz and THz ranges, the measured and calibrated S-parameters of a device can be influenced by the RF probe design, the RF pad design, the test structures design, the layout of the neighbouring structures, the substrate materials, etc., along with the used calibration and deembedding techniques [9]–[18]. A dedicated work focused on the improvement of the measurement's accuracy in sub-THz and THz ranges is necessary where an accurate and reliable simulation analysis can play a vital role.