Gian Piero Gibiino

A Double-Pulse Technique for the Dynamic I/V Characterization of GaN FETs

Standard dynamic characterization methods based on periodic narrow-pulse low duty-cycle excitation waveforms provide suboptimal I/V curves when used along with GaN field effect transistors (FETs), due to complex nonlinear charge trapping effects. Thus, a double-pulse technique for the dynamic characterization of GaN FETs is here presented. The double-pulsed I/V characteristics are shown to be not only isothermal but also corresponding to a fixed charge trapping state.

GaN FET Nonlinear Modeling Based on Double Pulse

A state-space empirical nonlinear model for GaN-based field-effect transistors (FETs) is defined, along with the associated identification procedures based on a recently published double pulse measurement technique. Charge trapping phenomena are dealt with in terms of a nonlinear state equation, which describes the rate of change of the trap state as a function of its actual distance from the corresponding steady state. Model experimental validation is carried out, after on-wafer characterization of a 1-mm AlGaN-GaN on SiC FET, both under strong and mild nonlinear operation.

{ I}/{ V}

This paper presents the response surface methodology in modeling of nonlinear microwave devices. First, different combinations of sampling techniques and types of radial basis functions are evaluated in simulations of the drain current of a 0.15- μm GaAs HEMT transistor described by the Chalmers model. It allows to determine the best settings of the response surface methodology for the modeling of active microwave devices. It is shown that the best sampling strategy is a combination of space-exploration (Voronoi), problem-exploitation (LOLA), and model-error-driven sample rankers. From the various radial basis function models, the fastest convergence is achieved with exponential functions. This knowledge is then used in behavioral modeling of a low-power amplifier AG303 measured in the load-pull setup. It is shown that the response surface methodology outperforms commonly used factorial design of experiments. Moreover, it gives accurate models within just a few tens of samples. However, attention has to be paid at the noisy regions, which might be oversampled by the sampling techniques.

Characteristics

We propose a three-port nonlinear dynamic behavioral model for supply-modulated power amplifiers (PAs). The proposed model not only accounts for the radio frequency (RF) input-output relationship, but also for the interaction between a modulated voltage supply, the RF output power and the supply current. The model is based on a modified Volterra formulation which accounts for the dynamic deviations with respect to a quasi-static model. The frequency-domain kernels of the proposed model are directly extracted from measurements performed with a low-frequency-extended large-signal network analyzer on an RF hand-set PA. The model is validated under random multitone modulated RF input and supply. The presented technique allows for the independent control of the RF and the supply ports. As such, it allows a separate description of both the dynamic contribution of the RF modulated input and of the dynamic supply voltage. The proposed model shows an improvement with respect to a quasi-static approach in predicting the RF output, the supply current, as well as the power-added efficiency.

Compact Behavioral Models of Nonlinear Active Devices Using Response Surface Methodology

In this paper, the response surface methodology is proposed to model nonlinear microwave devices using different sampling techniques. Each of the methods represents a distinct approach: exploration-oriented (Voronoi tessellation), nonlinearity-exploitation-oriented (LOcal Linear Approximation) and model-error-minimization-oriented. This allows to build accurate and compact global behavioral models of drain voltage at different harmonics of a 0.15 μm GaAs HEMT transistor with only few hundreds of samples. After choosing the best sampling technique, two types of global models are compared: Radial Basis Function and Kriging. It is shown that the modeling convergence depends on the model type, and better results are obtained using the Kriging model.

A Three-Port Nonlinear Dynamic Behavioral Model for Supply-Modulated RF PAs

We present an approach for the dynamic characterization of the non-idealities arising in envelope tracking (ET) systems when either the radio-frequency (RF) power amplifier (PA) or the supply modulator (SM) are driven into nonlinear operation. Impedance-like nonlinear functions, derived through a modified Volterra formulation, are here introduced and measured at the supply terminal over 40 MHz bandwidth (BW), allowing better predictions of the dynamic voltage and current at the SM-PA interface.

Nonlinear behavioral models of HEMTs using response surface methodology

In this paper, the centroidal Voronoi tessellation (CVT) is proposed as a design of experiments (DoE) for the nonlinear modeling of active devices. Different method’s flavors are being described, allowing to maximize the total amount of information gathered during measurements. As a case study, the CVT designs have been tested for both simulation-based experiments of nonlinear test functions, as well as for the extraction of nonlinear transfer functions of a handset radio-frequency power amplifier. The use of CVT allowed to achieve lower interpolation error, and contrary to the most popular design in microwaves, i.e., factorial DoE, the proposed tessellation can handle any arbitrary number of samples.

Supply-terminal 40 MHz BW characterization of impedance-like nonlinear functions for envelope tracking PAs

In this paper, we perform a simultaneous low- and high-frequency characterization of a microwave power amplifier (PA) circuit under dynamic biasing conditions. Particular attention is paid to the drain bias port characterization and the effects of the dynamic biasing on the radio-frequency (RF) behavior of the PA under test. Experimental results are obtained in order to evaluate the long-term memory effects and the LF-to-RF conversion function, providing an innovative approach for characterizing supply modulated PAs.

Design of Experiments Using Centroidal Voronoi Tessellation

A fast and simple method for the direct characterization of nonlinear charge functions of electron devices is presented. The input and output transistor ports are simultaneously excited through single-tone sources at different frequencies and calibrated large signal waveforms are measured by means of an advanced NVNA-based setup. Proper choice of the two frequencies guarantees an almost complete coverage of the voltages domain in a single and very fast measurement set and allows the extraction of the charge functions by direct integration of currents in the frequency domain, since, contrary to other methods, the measured waveforms are both iso-thermal and iso-dynamic (i.e. at fixed charge trapping status). The method is validated by characterizing the gate charge function of a 5W 8×125μm GaN FET and implementing a simple table-based model of the transistor input port. Very good results are achieved by comparison with large-signal measurements under conditions different than the ones used for the characterization.

Nonlinear characterization of microwave power amplifiers with supply modulation

We propose an efficient procedure for the extraction of a charge-controlled nonlinear model of a 1-mm gallium nitride on silicon carbide field-effect transistor (L = 0.25 μm) from nonlinear vector network analyzer acquisitions. A fast, single-shot measurement technique is described, in which the two device-under-test (DUT) ports are excited by single-tone sources at carefully selected tone frequencies, acquiring calibrated waveforms at the on-wafer DUT ports with an almost complete coverage of the voltages domain. The gate and drain charge functions identification is executed by the integration of the displacement currents in the frequency domain. A suitable approach for separating the conductive and displacement drain current components is provided. The presence of thermal self-heating and charge trapping phenomena is empirically evaluated, and accounted through an equivalent voltage approach. Experimental validation is provided at 2.5 and 5 GHz for a continuous-wave excitation, and at 2.5 GHz for a two-tone excitation.