GaN Technologies and Developments: Status and

GaN Technologies and Developments: Status and Trends M. Buchta1, K. Beilenhoff1, H. Blanck1, J. Thorpe1, R. Behtash1, S. Heckmann2, H. Jung1, Z. Ouarch2, M. Camiade2 1

United Monolithic Semiconductors GmbH, Ulm, Germany 2 United Monolithic Semiconductors S.A.S, Orsay, France Email: [email protected]

Abstract-Today the European GaN interest is slightly shifting away from academic research to development and industrialization of technology, devices and applications. The recent public funding was started to accelerate the transition from academica to industry on all levels like EC, national and by Space Agencies. In the paper the today’s running projects and the upcoming trends will be described.

I.

INTRODUCTION

Based on a long history in the III-V semiconductor technologies for RF applications Europe stimulated the research on GaN for high power RF devices. Although in the past USA and Japan only have been seen to be at the forefront of this technology, Europe take its capabilities to find its own way. A large community was built including Academia, Industry and National research institutes and is involved in the development of GaN devices for several applications. This work will try to summarize these activities and give the current status of European GaN technology and will take a look on the upcoming trends. Since a couple of years the GaN activity in Europe has increased dramatically. The space community, in Europe focused in the European Space Agency (ESA), has put this technology on their roadmap for use in microwave payload [1]. But the interest is not limited to space applications, also the European Commission decided to stimulate research in this field and several projects were hosted during the Sixth Framework programme for Research (FP6) and continued in the actual FP7 programme. Additionally most National Research Programmes within Europe supported GaN activities in a broad way and pushed the results from academic research towards commercial use.

crystalline silicon SiC and ULTRAGAN [4,5] into InAlN/GaN epitaxy. Both were focusing on reaching high power and high power density devices for telecom applications. In the frame of actual FP7, the running project MORGaN [6] looks for GaN substrates for harsh environments like temperatures higher than 500°C to be used in sensors or for telecommunication. The high power density of GaN causes a high heat flow, which is addressed by the project AGAPAC [7]. New packaging materials are investigated to enable high power density if only passive cooling is desired, like in space applications but not limited to. Diamond composite materials and nano tubes are being developed and tested finally with large GaN devices. First promising results will be presented [8]. For space application it is very important to ensure high reliability, resulting in a minimum life time under space condition. This is addressed by the ESA funded project GREAT2 [9], which aim to establish a suitable European supply chain for GaN devices. There are several other projects like “GaN Technology for Robust Communication Receivers” [10] targeting space applications, which often use European industrial GaN foundry services. Additionally the European Defense Agency (EDA) is running KORRIGAN [11,12], where the defense applications are addressed, starting from substrate and epitaxy up to technology and circuit and end at system level. Fig. 1 shows the partners of KORRIGAN and their role in the project.

II. PROGRAMS A. European Projects The European projects are divided into the ones funded by EC through the FP7, related on space and terrestric applications, and the ones by ESA, which targets the space applications. During FP6, the project HYPHEN [2,3] investigates into alternative substrates for GaN like poly-

Fig. 1: KORRIGAN members and their role within this project.

There are other GaN related projects runnning focusing on frequency ranges below 10MHz as requested by automotive industry for electric propulsion or DC/DC coonverters for use in photovoltaic systems, which is not addresseed by this work. B. National projects The national projects are thematically oriented to the European wide road maps, but are more conncentrated on the available resources in each European countryy. Thus not only one source in Europe is funded which has m major benefits for the ongoing industrialization and leads to specialization of each competitor. In Germany the Federal Government fosterr several projects in the frame of “GaN-Elektronik”, which aim med the research of epitaxy and technology for GaN transistors for communication and the related non lineaar measurement technique. The outcome should be transsferred later to industrial oriented user. The GANIND projecct, as an example, was focused on the GaN technology for higgh power devices suitable for cellular base transceiver stations (BTS) [13]. In Italy several projects targeting space appplications in xband are running, funded by Italian Ministry of Research and Technology (MiUR) and the Italian Nationnal Physics Labs (NPL). In France GaN related projects are mainlly funded by the French General Delegation for Ordnance (DG GA). Great Britain, Sweden and Spain have seveeral institutes and companies working in the GaN related domaain, but the most of them are also involved into the European prrojects. In general the research for GaN has left thee domain of basic technologies like substrates, epitaxy and proocess and is now more and more shifting to device-relatedd questions like reliability or improvement of efficiency or seeking for new applications like robust LNAs. III. MATERIALS A. Substrates For the use of GaN two major substrate maaterials have been favored: Si and SiC. While Si promises low ccost at very large diameter, which is still under investigation beecause of a quite high lattice mismatch of 17% and the large thhermal expansion coefficient mismatch of 54% [14], for RF poower applications mainly the semi insulation SiC (s.i. SiC) is used for further developments. The smaller lattice mismatch of 3.5% to GaN and the very high thermal conductivity promiises reliable high power GaN devices. The SiC material is widely used in optoelectronic like LED, but this is the 6H SiC C, which slightly differ in the substrate growth from the s.i. SiiC [15]. Sapphire and Diamond are also good candidates to act as a substrate for GaN, but compared to the other materialss there are less advantages to Si and SiC, esp. then takinng the cost into account. The lattice mismatch forced the inntroduction of a nucleation buffer layer, consisting of AlN fo for SiC e.g. [16]. Today the US company Cree is the leading oone in delivering

high quality 3” and 4” s.i. SiC sub bstrates, but there are very promising European competitors ah head. B. Epitaxy Various research insitutes could d cover the need of even exploratory structures like AlInN//GaN [17]. For the work horse AlGaN/GaN epitaxy for HEMT several European sources offer 100mm and 150mm m diameter material. Even research institutes offer epi wafeer grown in a multiwafer MOCVD with reasonable homogen ny sheet resistance, see Fig. 2.

Fig. 2: Sheet resistance homogeneity of a 100mm AlGaN/GaN HEMT epilayer, grown in a 12x 3” multiwafer MOCVD. M Courtesty of IAF [18].

The sheet resistance shows a variance of 0.9% of 500 Ohm/sq over the surface [18]. Sev veral research projects are covering the use of larger diam meter of GaN on SiC at reasonable quality to ensure a Euro opean supply chain for low and for high volume needs.

LOGY IV. TECHNOL

First technologies were develo oped by several research institutes within Europe. Germany, France, Italy, Sweden and Great Britain played a great role during this time, several universities and research instituttes from these countries published their contribution to th he GaN community. Now companies in Germany and France (UMS e.g.), Italy (SELEX e.g.) and Great Britain (QinetiQ e.g.) are short before to introduce their GaN technology or products p to the market. The most offer two different gate lenghts, 0,5µm for 1 and around 6GHz [19, 20] and 0,25µm mainly for X band (8 to 12 GHz) and up to Ku band [21]. All processes are dedicated to high RF power and offer devicces with high breakdown voltages and high efficiency. Quitee all known processes offer the use of coplanar or microstrip lin nes [11], in the last case this means that the backside processs module contain wafer thinning and via hole etching. Espeecially the via hole etching is a time consuming process step because b of the high binding energy of SiC [22]. The UMS processes GH50 (0,5µ µm gate length) and GH25 (0,25µm) are developed in paraallel to enable maximum synergy between them in order to use u the same machines and tools as much as possible [23].

V. DEVICES AND MMICS

cover applications in the frequency range from 4 up to 20 GHz

Following the certain technologies deeveloped for a dedicated application and frequency domain, the devices also show a broad field of interest by researcherrs and engineers. For high power application Uniteed Monolithic Semiconductors (UMS) developed a GaN HEMT process named GH50, which offer 0.5µm gate lengthh power bars and large transistors for up to 7GHz. For BTS aapplication UMS teamed together with NXP to have a directt feedback and a very experienced partner within this field. As an example first results are shown in Fig. 3. This power bar cconsists of a total

Fig. 4: First demonstrator of GH25, a thrree-stage wideband HPA, measurements done in pulsed mode.

[23]. A first MMIC GaN demon nstrator showed promising results, Fig. 4. The onwafer measurements weree carried out at Vd of 25V and a pulse condition of 25µs / 10% %. The three stage HPA has a gain of greater than 20dB over 6 to 18GHz with a output power greater than 6W. The differeent colors show the results of several MMICs measured. Fig. 3: A UMS power bar with 36mm gate periphery, measured at 2GHz fficiency 52%. and VDS=50V in cw. Gain is 17dB, Pout 128W and eff

gate periphery of 36mm and was mounted in a standard package. The GH50 process mentioned is dedicatedd for BTS, Radar in L band up to C band and SSPA for Satccom applications [23]. This process will be available end oof 2010 and use 100mm and 150mm GaN on SiC wafers tto ensure proper thermal management of the device. This is im mportant since the power bar shown in Fig. 3 has an output pow wer of 128W at an efficiency of 52%. This means that 118W oof thermal power has to be managed so that the junction temperrature stay below the limit, which is foreseen to be at 175°C. Foor this a new base plate approach has to be considered and is ddiscussed later in this paper. Several transistor geometries havee been developed to ensure suitable solutions for the applicaations mentioned above. The bigger the device the lower the powerr density will be. This is due to the thermal management on the device, only small devices could be able to handle densities higher than 5W/mm. Also the Drain-Source voltage has to be taken into account, for certain applications a high voltage means additional advantages, because the output im mpedance is very close to that of the following antenna and thhus the matching network could be designed smaller and more bbroadband. For higher frequencies UMS developed tthe 0.25µm gate length process GH25. It offers full MMIC cappabilities and will

The development will move to o higher power and more complex MMICs using GaN. Unfortunately, U the today’s available packaging solutions are not n designed to handle high power density. This limits the GaN N development and is the reason why the possible power density of more than 10W/mm is not used and quite all commerciaal available devices show a density of less than 3W/mm. This iss targeted by the EC funded project AGAPAC [7], which aim ms to improve the thermal conductivity of base plate material from 300W/mK to more than 500W/mK. The approach is to o use a diamond composite material for the base plate. There arre first measurements done with the use of RAMAN equipmen nt [24, 8]. Fig. 5 shows one

Fig. 5: Line temperature profile between source and drain of a GaN m Courtesy of HEMT, as recorded by IR and Raman measurement. University of Bristol [25].

example of a Raman and an IR temperature measurement done at the University of Bristol [25]. This kind of detailed line temperature profile allows being more close to the real peak temperature compared to IR. Together with new base plate approaches the thermal management will be more relaxed even at high power density devices. VI. CONCLUSION The today’s status of the European GaN technology was described with emphasis on high power RF devices. As an example two processes were introduced and some demonstrator results shown. There are other remarkable results within Europe available; the achievements will lead to enable GaN into a lot of applications. ACKNOWLEDGMENT The autors would like to thank all the European GaN community for their support and contribution to this work. The authors would also like to acknowledge the financial and technical support from: EDA, EC, ESA, BMBF, DLR, BWB, DGA, CNES, WTD81, FMV, MDE, Difesa, dstl, Defensie, ANR, MiUR, EPSRC. REFERENCES [1] [2] [3]

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