A. Nanni

GaN-Based Robust Low-Noise Amplifiers

In this paper, an overview of recently reported low-noise amplifiers (LNAs), designed, and fabricated in GaN technology is provided, highlighting their noise performance together with high-linearity and high-robustness capabilities. Several SELEX-ES GaN monolithic technologies are detailed, providing the results of the noise characterization and modeling on sample devices. An in-depth review of three LNAs based on the 0.25- μm GaN HEMT process, marginally described in previous publications, is then presented. In particular, two robust and broadband 2-18-GHz monolithic microwave integrated circuit (MMIC) LNAs are designed, fabricated, and tested, exhibiting robustness to over 40-dBm input power levels; an X-band MMIC LNA, suitable for synthetic aperture radar systems, is also designed and realized, for which measurement results show a noise figure ~ 2.2 dB with an associated gain and robustness up to 41-dBm input power level.

Buffer Traps in Fe-Doped AlGaN/GaN HEMTs: Investigation of the Physical Properties Based on Pulsed and Transient Measurements

This paper presents an extensive investigation of the properties of the trap with activation energy equal to 0.6 eV, which has been demonstrated to be responsible for current collapse (CC) in AlGaN/GaN HEMTs. The study was carried out on AlGaN/GaN HEMTs with increasing concentration of iron doping in the buffer. Based on pulsed characterization and drain current transient measurements, we demonstrate that for the samples under investigation: 1) increasing concentrations of Fe-doping in the buffer may induce a strong CC, which is related to the existence of a trap level located 0.63 eV below the conduction band energy and 2) this trap is physically located in the buffer layer, and is not related to the iron atoms but-more likely-to an intrinsic defect whose concentration depends on buffer doping. Moreover, we demonstrate that this level can be filled both under OFF-state conditions (by gate-leakage current) and under ON-state operation (when hot electrons can be injected to the buffer): for these reasons, it can significantly affect the switching properties of AlGaN/GaN HEMTs.

An ultra-broadband robust LNA for defence applications in AlGaN/GaN technology

The design, fabrication and test of a 2–18 GHz monolithic Low Noise Amplifier utilizing 0.25 µm AlGaN/GaN HEMT technology is reported. The measured noise figure of the amplifier is less than 4.7dB over the 2 – 18 GHz frequency range, exhibiting a minimum of 3.3 dB at 3 GHz. The LNA gain is 23dB. Even being a low-noise amplifier, the MMIC can withstand 10W input CW RF power, demonstrating no apparent degradation: to the authors knowledge this is the best RF LNA survivability reported to date in this frequency range using GaN technology.

Hot-Electron Degradation of AlGaN/GaN High-Electron Mobility Transistors During RF Operation: Correlation With GaN Buffer Design

Comprehensive RF stress-test campaign has been performed over AlGaN/GaN high-electron mobility transistor employing different GaN buffer designs, including unintentional doping, carbon doping and iron doping. No signature of gate-edge degradation has been found, and good correlation emerges between the buffer composition, subthreshold leakage current, and permanent degradation of the RF performance. The degradation mechanism, more pronounced in devices with parasitic buffer conductivity, involves the generation of additional deep trap states, the worsening of the dynamic current collapse, and the subsequent degradation of RF output power.

Power Amplifier Design Strategy to null IMD asymmetry

In this contribution the minimisation of the asymmetry between lower and upper sideband intermodulation products is discussed, using a Volterra series approach. From the inferred relationships new conditions to minimise IMD asymmetry are obtained and verified through the design of a C-band 2nd harmonic tuned hybrid power amplifier using a GaN PHEMT device. The proposed approach, without linearization schemes, achieves the minimisation of AM-PM conversion and asymmetry up to 400 MHz tone spacing, with a minimum 32.5 dBm output power and over 60 % drain efficiency @ 5.5 GHz

Field plate related reliability improvements in GaN-on-Si HEMTs

Abstract State of the art GaN on Silicon HEMTs fabricated with and without a field-plate structure have been tested by means of DC and RF reliability tests. The introduction of the field-plate structure greatly improves device reliability both during DC as well as RF testing. Results are thus suggesting that reliability in NOFP and FP devices is mainly limited by the high electric fields within the device structure causing an increase in traps concentration.

MMIC Chipset for wideband multifunction T/R Module

This paper reports on the design, fabrication and test of key MMIC components developed for wideband Transmit/Receive Module applications in the framework of a bilateral Research Programme at System level between Italy and Sweden MoDs (M-AESA) with Technological Studies performed in order to guarantee the feasibility of main hardware critical aspects. The Chipset comprises a multi-function phase-amplitude control chip (“core-chip”) and two wideband HPA MMICs. In the 2–18GHz bandwidth the True Time Delay (TTD) core-chip enables a time delay up to 124ps with a LSB of 4ps and an unwrapped phase deviation from linear behaviour of ± 7deg. The two-octave bandwidth HPA operates in the 4.5–18GHz in CW and provides an output power of 33±1dBm. The other wideband HPA is focused on C and X band radar frequencies (5–12GHz) and operates in pulsed mode with an output power of 36±1dBm.

Large-Signal modeling of power GaN HEMTs including thermal effects

In this paper a procedure to extract temperature dependent equivalent circuits for modeling the small and large signal behavior of GaN HEMTs is presented. The technique explained in this work uses pulsed I-V measurements to obtain the temperature dependence of the parameters describing the nonlinear drain current source behavior. The equivalent circuits extracted are capable of correctly modeling the DC, small signal and large signal characteristics of GaN HEMTs devices. Simulations and measurements carried out on three transistors developed by SELEX-SI are compared over a wide range of frequencies, bias and load conditions.

Characterization and modeling of low-cost, high-performance GaN-Si technology

A complete characterization of HEMT devices fabricated on a GaN-on-Silicon process developed by SELEX Sistemi Integrati is presented, together with the characterization and modeling of passive elements fabricated on the same substrate. Experimental results demonstrate that the power management capability of 1-μm GaN-Si HEMTs is suitable for most high power applications; moreover, the noise performance of such devices are in line with those of 0.5-μm GaN-SiC HEMTs fabricated by the same foundry. Finally, from the passive elements characterization the suitable operating bandwidth for the MMICs based on this technology can be assessed at least up to C-band.

Design and Realization of GaAs Digital Circuit for Mixed Signal MMIC Implementation in AESA Applications

A complete design flow starting from the technological process development up to the fabrication of digital circuits is presented. The aim of this work is to demonstrate the GaAs Enhancement/Depletion (E/D) double stop-etch technology implementation feasibility for digital applications, aimed at mixed signal circuit integration. On the basis of the characterization of small E/D devices with different Gate peripheries, developed by the SELEX-SI foundry, and the analysis of several GaAs-based logical families, the most suitable logic for the available technology has been selected. Then, simple test vehicles (level shifters, NOR logic gates and D Flip-Flops) have been designed, realized, and measured to validate the design strategy applied to the GaAs E/D process. These logical circuits are preliminary to the design of a more complex serial-to-parallel converter, to be implemented onto the same chip together with RF analog blocks, such as stepped attenuators and phase shifters.