Process-induced variability is a growing concern in the design of analog circuits, and in particular for monolithic microwave integrated circuits (MMICs) targeting the 5G and 6G communication systems. The RF and microwave (MW) technologies developed for the deployment of these communication systems exploit devices whose dimension is now well below 100 nm, featuring an increasing variability due to the fabrication process tolerances and the inherent statistical behavior of matter at the nanoscale. In this scenario, variability analysis must be incorporated into circuit design and optimization, with ad hoc models retaining a direct link to the fabrication process and addressing typical MMIC nonlinear applications like power amplification and frequency mixing. This paper presents a flexible procedure to extract black-box models from accurate physics-based simulations, namely TCAD analysis of the active devices and EM simulations for the passive structures, incorporating the dependence on the most relevant fabrication process parameters. We discuss several approaches to extract these models and compare them to highlight their features, both in terms of accuracy and of ease of extraction. We detail how these models can be implemented into EDA tools typically used for RF and MMIC design, allowing for fast and accurate statistical and yield analysis. We demonstrate the proposed approaches extracting the black-box models for the building blocks of a power amplifier in a GaAs technology for X-band applications.

The Doherty Power Amplifier represents one of the most promising solutions for the design of high-efficiency power stages. In the widely adopted ABC scheme, the Doherty Amplifier design critically depends on the accuracy of the device model in different operating conditions, ranging from class AB to class C. For the class C case, library models are often inaccurate, while experimental characterization is difficult since it must be carried out in large signal conditions and with varying gate bias. In this paper, we propose an alternative approach, based on physics-based Technological CAD (TCAD) simulations of the complete Doherty amplifier along with the analysis of its individual MAIN (class AB) and AUXILIARY (class C) stages. TCAD simulations seamlessly provide an accurate modelling of the device behavior in all operation classes, including the device turn-on and the nonlinear capacitances, and easily account for the cross-loading effects of the MAIN and AUXILIARY devices through the output network and the effect of the device feedback (gate-drain) capacitance on the input matching. Analyzing a GaAs Doherty stage at 12 GHz, we show that the input phase of the auxiliary stage can be exploited for the Doherty power amplifier optimization in terms of gain, linearity and efficiency, showing a 9 dB gain with less than 1 dB gain variation from back-off to peak power with a power-added efficiency exceeding 45% over a Doherty region extending to a more than 6 dB output power back-off.

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