Valeria Vadala

Nonlinear Dispersive Modeling of Electron Devices Oriented to GaN Power Amplifier Design

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.

Characterization of GaN HEMT Low-Frequency Dispersion Through a Multiharmonic Measurement System

In this paper, the experimental characterization of low-frequency dispersion (i.e., long-term memory effects) affecting microwave GaN HEMTs is carried out by adopting a new nonlinear measurement system, which is based on low-frequency multiharmonic signal sources. The proposed setup, which has been fully automated by a control software procedure, enables given source/load device terminations at fundamental and harmonic frequencies to be synthesized. Different experimental results are provided to characterize well-known effects related to low-frequency dispersion (e.g., knee walkout and drain current collapse) and to demonstrate the validity of assumptions commonly adopted for electron device modeling.

Behavioral Modeling of GaN FETs: A Load-Line Approach

In this paper, a new model formulation is presented that correctly accounts for low-frequency dispersion (i.e., trapping and thermal phenomena) affecting field-effect transistors (FETs). In particular, for the first time a behavioral description is applied only to the intrinsic current generator, enabling the correct measurement-based evaluation of the intrinsic device operation. The model, which is by construction technology independent, has been extensively validated considering a GaN FET. This choice is justified by the large interest around this technology and by the presence of dispersion effects that must be accurately accounted for.

A Load–Pull Characterization Technique Accounting for Harmonic Tuning

A novel methodology for the characterization of the nonlinear dynamic behavior of electron devices (EDs) is presented. It is based on a complete and accurate ED characterization that is provided by large-signal low-frequency I/V measurements, performed by means of a low-cost setup, in conjunction with any model-based description of the nonlinear reactive effects related to ED capacitances. The unique feature of the proposed technique is that a fully harmonic control of waveforms at the current generator plane is achieved, and as a consequence, high-efficiency operation can be simply investigated. Different experimental data are presented on GaAs and GaN transistors, and to definitely verify the capability of the new approach, the design of a class-F GaN power amplifier is deeply investigated as a case study.

X-Band GaN Power Amplifier for Future Generation SAR Systems

A X-band GaN monolithic microwave integrated circuits (MMIC) High Power Amplifier (HPA) suitable for future generation Synthetic Aperture Radar systems is presented. The HPA delivers 14 W of output power, more than 38% of PAE in the frequency bandwidth from 8.8 to 10.4 GHz. Its linear gain is greater than 25 dB. For the first time an MMIC X-band HPA has been designed by directly measuring the transistor behavior at the current generator plane. In particular, optimum device load-line has been selected according to the chosen performance tradeoffs.

A new approach to Class-E power amplifier design

An innovative, recently introduced, methodology for microwave power amplifier design, is here extended to switching-mode Class-E amplifier operation. Such a technique is based on a complete and accurate electron device (ED) characterization, which is provided by both direct large-signal low-frequency I/V measurements, performed by means of a relatively simple low-cost setup, and a model-based description of nonlinear reactive effects related to ED capacitances. In order to verify the proposed design methodology, a Class-E power amplifier (PA) has been designed.

“Hybrid” approach to microwave power amplifier design

A new “hybrid” approach to microwave power amplifier design is presented which is based both on experimental large-signal low-frequency I/V load-line characterization and a model-based description of the device capacitances. Such a technique allows to get the same information obtained through nonlinear measurement setups operating at microwave frequencies. Several simulated and experimental data are proposed, based on GaN technology, in order to prove the effectiveness of the methodology.

A low-cost and accurate technique for the prediction of load-pull contours

Load-pull measurement systems are the most common and powerful instruments used for the design of power amplifiers. In fact, they allow to directly obtain output power, efficiency and gain contours which give a clear idea of the electron device optimum termination for the selected operation. Nevertheless, such measurement systems are also very expensive, especially if high frequencies and high power levels are addressed. In this paper, a new technique for drawing load-pull contours is presented which jointly exploits both large-signal low-frequency I/V device measurements and a nonlinear capacitance-based model, the latter one being obtained on the basis of bias-and frequency-dependent small-signal S-parameters. The proposed approach achieves the same level of accuracy of high-frequency measurement systems, by using general purpose instrumentation available in microwave laboratories. Different experimental examples, based on power GaN FETs, are provided to demonstrate the validity of the described technique.

Nonlinear model for 40-GHz cold-FET operation

We extract the nonlinear model of a 0.15 μm GaAs pHEMT for cold-FET mixer applications. The model parameters are extracted from experimental data obtained by simultaneously driving the device under test with low-frequency large signals and a tickle tone at the RF operating frequency. The advantage of this approach is twofold. Firstly, as a result of a single measurement one can get separately the nonlinear currents and charge. Secondly, one can perform nonlinear characterization, and subsequently modeling, even if the RF frequency is such that its harmonics cannot be measured by todays nonlinear network vector analyzers.

A new empirical model for the characterization of low-frequency dispersive effects in FET electron devices accounting for thermal influence on the trapping state

It is well known that low-frequency dispersive effects cause important deviations between static (dc) and dynamic electron device current\voltage (I\V) characteristics, which must be accurately accounted for in nonlinear device models for microwave circuit design. As a matter of fact, a very high level of accuracy has been obtained by exploiting modeling approaches based on bias-dependent model parameters (e.g. stored into look-up tables). However, experimental characterization of these parameters is usually limited by the device safe operating area. Moreover, their practical use can be limited in the circuit design phase due to simulation time and memory occupation problems. On the other hand, too much simple models based on easy-to-compute analytical expressions do not satisfy the accuracy requirements usually needed for first-run-success MMIC design. In this paper, a new analytical model for the characterization of low-frequency dispersive effects is presented, whose aim is essentially related to the request of very accurate prediction capabilities yet preserving the numerical efficiency.