Establishing Reliable On-Wafer Characterization Techniques for Downscaled Millimeter-wave and sub-Terahertz Transistors

• Applicant: Professor Dr.-Ing. Wolfgang Heinrich, since 11/2024
• Subject Area: Communication Technology and Networks, High-Frequency Technology and Photonic Systems, Signal Processing and Machine Learning for Information Technology
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 493871066

The next generations of wireless systems operating in the 100 GHz to 1 THz frequency range will enable a multitude of applications such as wireless cognition, sensing, imaging, high-capacity communication and high-accuracy positioning. Such wireless systems are becoming a reality given the rapid increase in transistor speeds experienced over the past couple of years. A multitude of transistor technologies targets the upper millimeter-wave (mm-wave) and sub-Terahertz (sub-THz) frequency range. Indium-based compound semiconductor devices such as Indium-Phosphide (InP) high-electron-mobility-transistors (HEMTs) and heterojunction bipolar transistors (HBTs) have demonstrated the highest transistor speeds so far.When pushing the frequency limits of transistors, benchmarking plays a key role. Usually, the small-signal properties are used to compare their mm-wave and sub-THz performance. Amongst other figures of merit, the most popular ones are f_T and f_max representing the upper frequency limits at which short-circuit current gain and Mason’s unilateral power gain U become unity, respectively. While these figures of merit are commonly used, the way they are extracted involves quite some uncertainties. In fact, the extraction of f_max from Mason's gain is associated with problems. Typically, Mason’s gain is deduced from on-wafer S-parameter measurements far below f_max. The frequency at which U becomes unity (i.e. f_max) is determined by extrapolating Mason's gain from the measured results to higher frequencies by the assumption of a simple decay of -20 dB per decade. However, this fundamental assumption of the dependence of Mason's gain versus frequency is not observable in most cases for downscaled mm-wave and sub-THz transistors. This is attributed to parasitic effects present during the on-wafer transistor measurements.The objective of this proposal is to characterize, to understand and to minimize the uncertainties present in on-wafer measurements of mm-wave and sub-THz transistors. Of special interest is to reduce the influence of these uncertainties on the extraction of transistor equivalent circuit parameters and on prominent device figures of merit such as Mason’s gain or short-circuit current gain. Finally, guidelines to accurately characterize down-scaled mm-wave and THz transistors will be developed. For verification of these guidelines, test structures will be designed, fabricated and characterized.

• DFG Programme: Research Grants
• International Connection: Denmark
• Cooperation Partner: Professor Tom Keinicke Johansen, Ph.D.
• Ehemaliger Antragsteller: Dr.-Ing. Thomas Flisgen, until 10/2024