MHz load-line measurements for model parameter extraction
Fig. 1: 3D plot of drain current – drain voltage – gate voltage showing the coverage created by the measured load-line trajectories (red net-like pattern). The interpolation lines for the drain current – voltage extraction for 0.5 V steps of gate voltage can be seen also with black lines.
Fig. 2: Output characteristics derived from the trajectories presented in Fig. 1 and their simulations by the model. Gate voltage step is 0.5 V starting from -2 to 1 V.
Fig. 3: Load-pull contours for maximum output power tuning, simulated by the MHz-loadline-based model (left) and the pulsed-IV-based one (right). Both charts show the same measurement.
Accuracy is a necessity in circuit design, yet achieving it in GaN-based modules is challenging. In these systems, compact models must reliably capture the nonidealities intrinsic to GaN HEMTs. These arise from complex physical phenomena – most notably trapping and self-heating – whose impact in GaN HEMTs is particularly strong. For instance, trapping reduces drain current, output power, and overall device efficiency. Effective models must therefore account for these effects, while also ensuring that the required measurement techniques are both rapid and precise to maintain modeling efficiency and control over these phenomena.
A major challenge in current-voltage (I–V) characterization is the simultaneous control of device temperature and trap state. Accurate extraction of trap and thermal model parameters demands measurements under well-defined conditions, for which pulsed measurement techniques have long been the standard. However, these are often time-consuming, technically demanding, and require expensive equipment.
To address these limitations, we at FBH, in cooperation with BTU Cottbus-Senftenberg within the Joint Lab BTU–CS – FBH Microwave, have developed an innovative characterization technique that enables highly efficient I–V measurements for compact modeling of GaN HEMTs. This method utilizes a MHz-range active load-pull approach to generate load-line trajectories that comprehensively cover the entire I–V plane. From this extensive set of trajectories, it is possible to extract conventional output and transfer characteristics suitable for any device model.
Fig. 1 illustrates a set of trajectories, showing the drain current as a function of both drain and gate voltages. Using a single 10 μs measurement, the entire voltage plane is sampled, covering gate voltages from −4 V to 1 V and drain voltages from 0 V to 30 V. The figure also includes interpolated curves derived from the measured data, which can be directly used for device model extraction. This approach provides a highly efficient alternative to conventional pulsed measurement techniques. Specifically, it enables extremely dense voltage sampling, with voltage steps as small as 0.01 V achieved within only 10 μs, whereas accurate pulsed measurements typically require minutes to hours to obtain even lower resolution (for voltage steps ≥ 0.5 V).
To validate the technique, we incorporated MHz load-line measurements into the extraction procedure of our compact models [1], [2]. Fig. 2 compares the extracted output characteristics (red dots) with the corresponding model simulations (black lines). The measured I-V data are identical to the interpolated curves from Fig. 1. These curves enable straightforward extraction of trap model parameters without compromising accuracy. This is confirmed in Fig. 3, which compares models extracted from the I–V curves obtained via MHz load-line measurements and conventional pulsed measurements. The underlying model formulation remains unchanged; only the data used for parameter extraction differ. The comparison, performed using load-impedance contours under saturated output power conditions, reveals identical model performance for both extraction methods. These results demonstrate that MHz load-line measurements provide a fast, efficient, and accurate alternative to pulsed measurements for device modeling and parameter extraction in GaN HEMTs.
This work was financially supported by Deutsche Forschungsgemeinschaft (DFG) under grant no. RU 1203/16-1 and partially funded by the German BMFTR within the “Forschungsfabrik Mikroelektronik Deutschland (FMD)” under ref. 16FMD02.
Publications
[1] P. Beleniotis, C. Andrei, C. Zervos, U. L. Rohde, M. Rudolph, "GaN Trap Model Extraction Based on MHz Load-Line Measurements," 2025 IEEE/MTT-S International Microwave Symposium - IMS 2025.
[2] P. Beleniotis, C. Andrei, U. L. Rohde, M. Rudolph, "Determining GaN HEMT Trap Models from MHz Load-line Measurement – Synthesis and Evaluation," 16th German Microwave Conference (GeMiC) 2025.