Rafael Cignani

Scalable Nonlinear FET Model Based on a Distributed Parasitic Network Description

Electron device modeling requires accurate descriptions of parasitic passive structures connecting the intrinsic electron device to the external world. In conventional approaches, the parasitic phenomena are described by a network of lumped elements. As an alternative, a distributed description can be conveniently adopted. This choice has been proven very appropriate when dealing with device scaling and very high operating frequencies. In this paper, a novel approach to distributed parasitic modeling is adopted for the very first time in association with a nonlinear electron device model. In particular, it is shown how an equivalent intrinsic device and a suitably defined distributed parasitic network can be accurately defined and modeled on the basis of standard measurements and easy electromagnetic simulations. Wide experimental validation based on GaAs pseudomorphic HEMTs is provided, showing accurate prediction capabilities both under small- and large-signal conditions. The proposed model is shown to perform optimally even after periphery scaling.

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.

New pulsed measurement setup for GaN and GaAs FETs characterization

A new setup is proposed for the measurement of current–voltage pulsed characteristics of electron devices. The main advantages of the system consist in: shorter pulse widths through generation in a 50-Ω environment, simple average current monitoring through separation of the direct and alternate current paths, setting of average voltage values independently of pulse amplitudes and duty cycle, and stability of the setup guaranteed by wide-band dissipative terminations. The system is used for the characterization of dispersive effects due to carrier energy traps and thermal phenomena in GaAs and GaN on SiC field effect transistors. The basic differences between the two technologies are highlighted in the paper.

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}

A laboratory setup, along with a set of measurement and identification procedures, have been developed expressly for the characterization of the thermal behavior of AlGaN/GaN HEMTs, suitable for microwave high power amplifier (HPA) design. The setup allows the measurement of the drain current time-domain dynamic response to positive drain bias pulses, performed at different temperatures and different dissipated power densities. The proposed measurement conditions discriminate thermal phenomena from electrical dispersive effects for this particular technology. Both the thermal resistance and the “transient thermal resistance” are identified for a single-cell 1-mm device and for a 4-mm power-bar composed of four devices, designed to be used as the final stage of a monolithic

Characteristics

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Characterization of the Nonlinear Thermal Resistance and Pulsed Thermal Dynamic Behavior of AlGaN–GaN HEMTs on SiC

-band HPA for pulsed radar application. Transient data allow to compute the device operative channel temperature as a function of the pulsewidth and duty cycle, which is a crucial feature for pulsed HPA applications, typical for the GaN technology. The measured thermal data point out the nonlinearity of the thermal resistance versus dissipated power and base-plate temperature and the consequent critical thermal issue inherent in physically packing together such devices.

Accurate EM-Based Modeling of Cascode FETs

Cascode field-effect transistors (FETs) are widely used in the design of monolithic microwave integrated circuits (MMICs), owing to their almost unilateral and broadband behavior. However, since a dedicated model of the cell is rarely provided by foundries, a suboptimal description built by replicating the standard foundry model for both the common source and common gate device is often adopted. This might limit the success of the MMIC design at the first foundry run. This paper describes an electromagnetic-based empirical model of cascode cells, covering topics from the formulation and identification procedures to the corresponding validation described in an exhaustive experimental section. A MMIC low-noise distributed amplifier case is then presented and the proposed model is used for circuit analysis and instability detection. Clear indication is provided about the improvement in the prediction of critical behaviors with respect to conventional modeling approaches. A cascode cell with a symmetric layout is also successfully modeled.

A C-Band AlGaN-GaN MMIC HPA for SAR

A C-Band MMIC high power amplifier (HPA) has been designed exploiting a 0.25 μm HEMT GaN process on SiC substrate. The HPA is designed for future synthetic aperture radar (SAR) antenna applications. The HPA delivers 16 W output power with PAE over 38% at 6 dB gain compression within a 900 MHz bandwidth around 5.75 GHz. Up to 20 W output power and 40% PAE are obtained at higher gain compression. A comparison with another amplifier, differing only for the layout of the devices in the final stage, points out that the transistor thermal conditions represent the main limitation for this high power density technology.

10 Watt High Efficiency GaAs MMIC Power Amplifier for Space Applications

This paper describes the design of a GaAs monolithic high power amplifier at Ku band. The chip delivers about 40 dBm of saturated output power, in CW operating conditions, at 11.7 GHz central frequency, with 17% of bandwidth. The saturated power gain is 12.4 dB with 2 dB gain flatness across the application bandwidth while the chip power added efficiency is estimated between 33% to 47%. The amplifier is designed to be used as final stage of a downlink satellite transmitter for Tracking Telemetry & Command system. A commercial power p-HEMT process capable of handling a power density higher than 1 W/mm has been selected for the MMIC design. Due to the space application, special attention must be put on the process and MMIC reliability: to this aim performances must be guaranteed in de-rated conditions respect to the process maximum ratings and, in addition, the channel temperature of the active devices must be kept within the value established by Space Requirements and carefully controlled. This makes the design objective very tight. The MMIC power amplifier design and some measurement results are presented in the paper.