This paper presents a new modeling approach accounting for the nonlinear description of low-frequency dispersive effects (due to thermal phenomena and traps) affecting electron devices. The theoretical formulation is quite general and includes as particular cases different models proposed in the literature. A large set of experimental results, oriented to microwave GaN power amplifier design, is provided to give an exhaustive validation under realistic device operation

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The purpose of this invited paper is to give readers a comprehensive and critical overview on how to extract equivalent-circuit models for GaN HEMTs, which are the preferred devices for high-power high-frequency applications. This overview is meant to provide a practical modeling know-how for this advanced type of transistor, in order to support its development for improving device technology and circuit design. With the aim to broaden knowledge to empower models, experimental results are presented as illustrative examples of the most crucial challenges faced by the microwave engineers in modeling high-power GaN HEMTs. All the relevant aspects are covered, going from linear (also noise) to nonlinear models. The analysis is mainly focused on the modeling of distinctive peculiarities of GaN HEMTs. Particular attention is paid to study the importance of accurately modeling the kink effect in the output reflection coefficient, because of the relatively high transconductance, the peak in the magnitude of the short circuit current-gain, because of the relatively large intrinsic capacitances, and the low-frequency dispersion, because of trapping and thermal effects. Furthermore, to emphasize the key role of accurate device models for a successful circuit design, a practical example of power amplifier is discussed. Copyright © 2015 John Wiley & Sons, Ltd

In the paper, the nonlinear model of a microwave transistor is extracted from large-signal measurements acquired under “dynamic-bias” operation. Specifically, the transistor is driven by low-frequency large signals while a high-frequency tickle is applied on top of them. The low-frequency large signals, along with the dc bias voltages, set the large-signal operating point which represents a dynamic-bias condition for the device under test. Thanks to this technique, one can get at once and separately the nonlinear currents and charges of the transistor as a result of a very few nonlinear measurements. Additionally, the proposed technique allows one to accurately reconstruct the time-domain waveforms at the device-under-test terminals while the frequency of the tickle can be set as high as the bandwidth of today’s vector calibrated nonlinear measurement systems (i.e., 67 GHz). The approach, which is general and independent of device technology, is applied on a 0.15- m GaAs pHEMT specifically designed for resistive cold-FET mixer applications

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This letter is aimed at discovering and analyzing anomalous phenomena affecting millimeter-wave FETs, focusing on a GaN HEMT as a case study. For the first time, we show that the real parts of the impedance parameters can increase and then decrease with frequency, due to the resonance of the extrinsic reactive elements. This resonance may be detected as a peak in the magnitude of the short-circuit current-gain. Such a peak is found to be substantially bias and temperature insensitive and to manifest at frequencies higher than the other current-gain peak, due to the resonance between intrinsic capacitances and extrinsic inductances, giving origin to the double current-gain peak.

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The purpose of this study is to present an advanced technique for accurately modeling the behavior of a GaN HEMT under realistic working conditions. Since this semiconductor transistor technology has demonstrated to be very well suited for high-frequency (HF) high-power applications, an equivalent circuit model is developed to account for the device nonlinearities at microwave frequencies. In\ud particular, the proposed model includes bias dependence of both low-frequency (LF) dispersive effects affecting GaN devices and HF nonquasi-static effects, since these two types of frequency dependent\ud phenomena play a crucial role under microwave large-signal condition. The extraction procedure consists of two main steps. First, an accurate multibias small-signal nonquasi-static equivalent circuit is analytically extracted from scattering parameters measured under a wide range of bias points. Thereafter, this linear model is used as a cornerstone for building a nonlinear nonquasi-static equivalent circuit, which is expanded to account for the LF dispersive phenomena by using an empirical formulation directly identified from the HF large-signal measurements. The accuracy of the proposed modeling approach is completely and successfully verified by comparing model simulations with LF and HF large-signal measurements