The present paper focuses on the system-level optimization of GaN technology for high voltage applications. We will show that a key requirement for the future success of the GaN technology is the full system-optimization achieved by a simultaneous optimization of technology, packaging and applications. We will also show that Virtual Prototyping (VP) becomes, in GaN technology, a fundamental tool that allows not only to have a fundamental understanding of the device properties but more importantly it allows to strongly link device optimization, technology and system-level performance. In the present paper we will describe our view on the system-level optimization of high voltage GaN technology and present detailed simulations and comparison with experiments for both normally on isolated GaN transistors and cascoded GaN devices in real switching applications.

Threshold voltage instabilities observed in GaN HEMTs designed for power switching applications when submitted to either DC or pulsed testing are here presented and interpreted. Main results can be summarized as follows: i) two acceptor trap levels, characterized by two well distinct time constants, are present in the UID GaN channel and C-doped GaN buffer respectively and behave as electron and hole traps respectively; ii) the trapped charge is modulated by the high voltage biasing of the gate and drain terminals; iii) when empty, channel electron traps induce a negative threshold-voltage shift, while buffer hole traps induce a positive threshold-voltage shift; iv) when the device is pulsed from off- to on-state conditions, trap charge/discharge dynamics induces negative and positive threshold-voltage instabilities over distinct time scales.

The development of a single bias S-band MMIC HPA designed for radar T/R module applications is reported. The chip was fabricated with a low-cost 0.5µm GaAs PHEMT process and is composed of two stages, with a final stage of 29 mm gate-width periphery. The MMIC power amplifier is designed by using a zero gate bias configuration and therefore is attractive due to the size and cost reduction. In the frequency bandwidth 2.4–3.6GHz, the HPA biased at Vd=10V delivers an output power of 20W @ 4dB of gain compression, with an associated PAE of circa 28%.

High Voltage Breakdown (HVB) HPA MMIC based on GaAs pHEMTs technology represents a useful way for high power RF application. To increase the breakdown voltage respect to conventional device, a field-plate (FP) gate structure is implemented in our standard 0.5 µm process. With this solution the off-state breakdown voltage of the device has been improved from 18V to 28V, while keeping constant the drain current. As expected, FP devices showed smaller drain current collapse than standard devices under pulsed DC measurement conditions and a significant increase in power density, i.e. 1.4 W/mm. To guaranty a full advantage for real HPA applications we have carried out numerical simulations, to optimise device thermal behaviour as a function of GaAs thickness and gate pitch. On the basis of this technology development, reliable and reproducible HPA have been fabricated with an output power of circa 30 W and power added efficiency (PAE) of circa 35% in the 4.9–6.1GHz frequency range. Therefore this proposed technological solution enables the realization of very high power T/R modules with reliable GaAs technology, waiting for more disruptive and less mature GaN HEMT one.

RF-MEMS represent a feasible solution to obtain very low power dissipation and insertion loss, very high isolation and linearity switch respect to "solid state" technologies. In this paper we demonstrate the possibility to fully integrate the process fabrication of RF-MEMS switches in the GaN-HEMT manufacturing steps to develop a RF-MEMS/MMIC prototype. MEMS RF performance reveals an insertion loss and an isolation respectively better than 0.6 dB and 25dB in the frequency range 5-50 GHz. Moreover the coexisting HEMT devices show a f(max)=40 GHz and 6.5 W/mm density power, demonstrating the integration achievability.

DC and RF performance of submicron gate-length metal-semiconductor field effect transistors (MESFETs) fabricated on hydrogen-terminated polycrystalline diamond is investigated in detail for different material electronic quality (grain size in the range 100-200 mu m) and device geometry (drain-source channel length in the range 1-3 mu m). DC characteristics appear almost independent of both properties, giving maximum drain-source current values in the range 120-140 mA/mm in MESFETs having same gate length (0.2 mu m) and gate width (25 mu m). The layer properties underneath the hydrogenated surface seem then to affect the DC behaviour to a lesser extent when the same hydrogenation procedure is used. At variance, the electronic quality of diamond layers employed for MESFETs realization largely affects the RF performance, resulting into a low oscillation frequency f(max) for a MESFET realized by a self-aligned process (1 mu m drain-source channel length) onto low quality diamond polycrystalline film. Such a performance improves to f(max) = 35 GHz for devices realized onto large grain polycrystalline diamond, although fabricated without self-aligned gate procedure (3 mu m drain-source channel length). These findings are discussed in terms of different roles played by surface hydrogenation, device geometry detail and electronic quality of the polycrystalline diamond substrate for MESFET realization. (C) 2009 Elsevier B.V. All rights reserved.

The design and the experimental results of some prototypes of SiC X-ray detectors are presented. The devices have been manufactured on a 2’’ 4H-SiC wafer with 115 m thick undoped high purity epitaxial layer, which constitutes the detection’s active volume. Pad and pixel detectors based on Ni-Schottky junctions have been tested. The residual doping of the epi-layer was found to be extremely low, 3.7 x 1013 cm-3, allowing to achieve the highest detection efficiency and the lower specific capacitance of the detectors. At +22°C and in operating bias condition, the reverse current densities of the detector’s Schottky junctions have been measured to be between J=0.3 pA/cm2 and J=4 pA/cm2; these values are more than two orders of magnitude lower than those of state of the art silicon detectors. With such low leakage currents, the equivalent electronic noise of SiC pixel detectors is as low as 0.5 electrons r.m.s at room temperature, which represents a new state of the art in the scenario of semiconductor radiation detectors.

This paper presents the results obtained both by experimental measurements and numerical simulations carried out on state-of-the-art Field-Plated GaAs-based pHEMTs. The effect of field-plate length on DC and RF operation of pHEMTs will be discussed showing that the adoption of an optimal field-plate structure can significantly boost the device RF power performance, resulting in power density up to 2 W/mm measured under continuous wave RF signals at 2 GHz. The physical origin of the DC-to-RF dispersion in the fabricated devices has been associated with a hole-trap located at 0.65 eV from the valence band as obtained from current-DLTS measurements. The experimental results will also be supported and validated by numerical simulations. It will be shown that the beneficial effects arising from the adoption of the field-plate structure lie in its control on the trapped charge population responsible for the DC-to-RF dispersion mechanism.

In this paper a first iteration design, fabrication and test of a two-stage X-Band MMIC HPA in micro-strip AlGaN/GaN technology is reported. With 20 V drain voltage operating bias point, at 3 dB compression point, the HPA delivers a pulsed output power ranging from 21 to 28.5 W, an associated gain from 12.9 to 16.5 dB and an associated PAE from circa 30% to 40%, over the 8-10.5 GHz frequency bandwidth. In the best performance frequency points (8.5 and 9 GHz) the HPA exhibits a saturated output power of 30 W with an associated PAE of 40%.

Cr- and Al-gate MESFETs have been fabricated on deuterium-implanted polycrystalline diamond and characterized both in DC and RF regime. Their performances are compared with those of similar devices fabricated on plasma hydrogenated polycrystalline diamond, which suffer for instabilities related to the large sensitivity of the channel to surface adsorbates. It is shown that similar characteristics can be achieved both in MESFETs with deuterium-implanted and plasma hydrogenated diamond.

High breakdown voltage pHEMTs have been successfully developed by implementing a field-plate (FP) structure. Devices with and without FP have been fabricated on the same wafer in order to compare the improvements induced by adopting the FP. Both kinds of devices showed little or no current dispersion under pulse measurement conditions. Moreover the off-state breakdown voltage improved from 23V, for the devices without FP, to 38V for the field-plated devices. At 4GHz an output power as high as 1.6W/mm was measured for a FP device, resulting in a 60% improvement with respect to the device without FP. The fabricated structures were also evaluated by carrying out 2D numerical simulations. Experimental results on MIS pHEMTs have been explained by means of a donor trap at the SiN/GaAs interface located at 0.18eV from the GaAs conduction band. Finally, a good agreement between experimental and simulated device characteristics was obtained

complexity. Abstract- Inthis paper thedesign ofatwo-stage Class Fhigh Inthis workanX-BandPAispresented, based onreliable poweramplifier madeupbytenactive devices ispresented. The GaAsPHEMT device with0.6ptm gatelength, andadopting design isbased onanewapproach forcombining thedevices and anhighefficiency Class Fdesign solution (3)-(4), integrating designing thematching networks. The amplifier hasbeen t., designed forSynthetic Aperture Radarapplications byusingthehtandarc tprmat tontexombini Cstrs.Intfac SELEX0.6,mGaAsPHEMTtechnology. Thedesign approach the standard approachto exploit ClassF potential will bediscussed andsimulation results will bepresented. In improvements inPA design implies theoptimization ofeach particular, 38.5dBmoutput powerwith40% poweraddedsingle device output network, according tosuitable criteria, efficiency hasbeenobtained at9.6GHzinan operating whiletheinput isconjugately matched to50Q.SinceinSAR bandwidth widerthan10%.*** applications theoutput powerlevels achievable byasingle IndexTerms-ClassF,X-BandApplications, Synthetic active device isnotsufficient toassuretherequired levels, the Aperture Radar, high efficiency. adoption ofcombining structures ismandatory. Thesimplest approach isbased onthecombination oftwosingle-device powerstages matched to50Q.Nevertheless inthis waythe I.INTRODUCTION resulting overall amplifier will befeatured byalower output YNTHETICAperture Radars (SAR) aremodemsensorspower andefficiency, ascompared tothesum ofthesingle )concurring toachieve ameaningful progress indistance stages, together withawider areaoccupation. Toalleviate observation systems, sincethey canoperate indiversified suchdrawbacks inthepast we haveproposed a different water conditions. Inparticular, aSARsystem iscomposed by approach, inwhicha couple of active devices are a hugenumberofTransmit/Receive (T/R) modules, eachpreliminarily combined bymeansofa verysimple and electronically controlled andusedtosynthesize therequired compact structure, andsubsequently theunique input and beamshaping. DuetothehugenumberofT/Rmodulesoutput networks aredesigned tofulfill Class Frequirements employed torealize thephased array, thesize andweight of acrosseachdevice(5). Thesuggested approach hasbeen theequipment mustbeminimized, while alowcostbuthighly applied tothedesign ofa X-BandClass FPA,employing in reproducible andreliable solid-state technology becomesthelast stage thecombination ofeight PHEMTdevices. mandatory.

In this work, we present the characterization results for several HEMT GaN-based devices developed by SELEX Sistemi Integrati. Due to the wide band-gap properties of this material, these devices are very well-suited for high-power applications, and must be characterized under strongly nonlinear and high-power conditions. An extensive power characterization of devices fabricated on GaN grown either on SiC or sapphire substrates is carried out, including pulsed I-V, power sweeps, and load-pull measurements in different bias conditions from class A to class B. An active load-pull bench optimized for high-voltage and high-power measurements allows the load-pull characterization to be extended to the whole Smith chart and the optimum loads to be localized, even for devices with almost reactive optimum terminations. The characterization procedure is performed on HEMT devices fabricated with different technologies and different layouts, in order to improve and refine the fabrication methodology and verify the scaling rules and the effects of defects and thermal degradation. SELEX SI devices show a growing maturity, with performances comparable to state-of-the-art technology. © 2005 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2006.