Improving On-Wafer Characterization of Sub-THz Devices: A Probe Influence and Crosstalk Study

We evaluate the accuracy of small-signal on-wafer device characterization in the sub-THz frequency range when employing conventional and advanced calibration methods with state-of-the-art design of on-chip standards. We report that major discrepancies resulting from an interlaboratory comparison experiment are mainly attributed to the influence of microwave probes. When performing conventional multiline thru-reflect-line (mTRL) calibrations with six different models of probe from 140 to 325 GHz, we observe significant variations in the measured scattering-parameters (S-parameters) and show that probes made by various manufacturers induce disparate crosstalk responses that vary between -70 and -15 dB on open–open standards. After applying a crosstalk correction approach as a second-tier calibration, we obtain significantly better agreement between the RF performance of the same heterojunction bipolar transistor (HBT) measured with two different probe models. While the difference between the maximum stable gain (MSG) reaches 1.8 dB at 210 GHz after applying a conventional mTRL calibration, we reduce the error to ~0.5 dB after crosstalk correction. This study shows that crosstalk correction methods must be implemented in the sub-THz frequency range to accurately estimate the RF performance of active devices and circuits.