Advanced on-wafer calibration enables reliable sub-THz transistor characterization
Fig. 1: representation of fmax extracted using 8-term corrected S-parameters and 16-term corrected S-parameters for InP HBTs with emitter width of a) 850 nm and b) 500 nm.
One of the key performance metrics benchmarking RF transistors is the maximum frequency of oscillation, fmax. Typically, fmax is extracted from the so-called Mason’s gain U by conducting on-wafer measurements at frequencies well below fmax and then extrapolating this data. However, for down-scaled, high-speed transistors, on-wafer measurements are subject to systematic errors which compromise the accuracy and reliability of the extracted fmax values.
To identify the origin of these errors, we investigated indium phosphide (InP) heterojunction bipolar transistors (HBTs) operating beyond 100 GHz. We carried out a systematic analysis of realistic on-wafer environments considering neighboring structures and various probe types by means of full-wave 3D electromagnetic simulations. The results were verified by on-wafer measurements of transistors with down-scaled emitter widths. We found that conventional calibration procedures (e.g., 8-term calibration), which are intended to eliminate on-wafer parasitic effects, do not sufficiently compensate for these errors. Consequently, extracting fmax by extrapolating Mason’s gain can lead to either under- or overestimation of fmax [1].
To mitigate these unwanted effects, we employed an advanced on-wafer calibration procedure – a 16-term calibration – first validated by means of 3D EM simulations and then verified by on-wafer measurements of InP HBTs with emitter widths of 850 nm and 500 nm. A direct comparison between the conventional 8-term calibration and the advanced 16-term calibration shows that the 16-term approach provides more accurate estimations of fmax values. The resulting curves lead to a stable value for fmax as soon as data up to 10 GHz and 20 GHz are used as a basis [2] (see Fig. 1).
Our current 23IND10 OnMicro project aims to extend these investigations by fully characterizing the sensitivity of the 16-term calibration against different asymmetric calibration standards, and by assessing its validity at higher frequencies and at various bias conditions. This work will significantly enhance our capabilities in on-wafer calibration and measurement methodologies for reliable sub-THz device characterization.
This work was supported in part by the Deutsche Forschungsgemeinschaft (DFG) under the grants FL 1201/1-1 and HE 1676/25-1. It was also supported in part by the European Partnership on Metrology (EPM) project under the grant 23IND10 OnMicro. The 23IND10 OnMicro project is co-financed by the European Union’s Horizon Europe Research and Innovation Program and by Participating States.
Publications
[1] A. Kanitkar, R. Doerner, T. K. Johansen, W. Heinrich, T. Flisgen, "Influence of On-Wafer Parasitic Effects on Mason’s Gain of Down-Scaled InP HBTs," 54th European Microwave Conference (EuMC), France, 2024.
[2] A. Kanitkar, R. Doerner, T. K. Johansen, W. Heinrich, T. Flisgen, "On-Wafer 16-Term Calibration for Characterization of InP HBTs Featuring Sub-THz fmax," 55th European Microwave Conference (EuMC), Utrecht, Netherlands, 2025 (accepted for publication).