Beach, MA

Fornetti, F

Morris, KA

English

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Evaluation of commercial GaN HEMTS for pulsed power applications

The present study investigates the behaviour and performance of commercially available GaN HEMTs provided by Cree Inc. The output power variation at pulse repetition frequencies (PRFs) in the range 100–450 kHz is presented. Rise and fall times are also investigated at different PRFs and power levels. The pulsed RF waveforms are obtained by switching the gate bias of the transistor on and completely off to ensure that the device goes through full transients for every pulse. The RF frequency at which the study is conducted is 3.5GHz. The aim of the study is to assess the suitability of commercial GaN HEMTs to pulsed RF applications such as Radar.The present study investigates the behaviour and performance of commercially available GaN HEMTs provided by Cree Inc. The output power variation at pulse repetition frequencies (PRFs) in the range 100-450 kHz is presented. Rise and fall times are also investigated at different PRFs and power levels. The pulsed RF waveforms are obtained by switching the gate bias of the transistor on and completely off to ensure that the device goes through full transients for every pulse. The RF frequency at which the study is conducted is 3.5GHz. The aim of the study is to assess the suitability of commercial GaN HEMTs to pulsed RF applications such as Radar

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                          Fornetti, F., Morris, K. A., & Beach, M. A. (2009). Evaluation of commercialGaN HEMTs for pulsed power applications. In IET European Pulsed PowerConference, 2009, Geneva, Switzerland. (pp. 1 - 4). Institution ofEngineering and Technology (IET).Link to publication record in Explore Bristol ResearchPDF-documentUniversity of Bristol - Explore Bristol ResearchGeneral rightsThis document is made available in accordance with publisher policies. Please cite only the publishedversion using the reference above. Full terms of use are available:http://www.bristol.ac.uk/pure/about/ebr-terms.htmlTake down policyExplore Bristol Research is a digital archive and the intention is that deposited content should not beremoved. However, if you believe that this version of the work breaches copyright law please contactopen-access@bristol.ac.uk and include the following information in your message:• Your contact details• Bibliographic details for the item, including a URL• An outline of the nature of the complaintOn receipt of your message the Open Access Team will immediately investigate your claim, make aninitial judgement of the validity of the claim and, where appropriate, withdraw the item in questionfrom public view.IntroductionThe present study investigates the behaviour and performance of  commercially availableGaN HEMTs when used for the amplification of  pulsed waveforms.GaN technology is not as well understood and established as its GaAs counterpart and a num-ber of  studies have reported potential problems, namely:These effects could potentially be exacerbated by operating the device in pulsed mode and theexperiments conducted aimed to study such fenomena and assess the suitability of  commercialGaN HEMTs to pulsed RF applications such as Radar.The frequency at which the study was conducted was 3.5GHz• Current reduction due to surface states & buffer traps• Virtual gate formation• Gate and drain lag transients• Non-linearities in the source resistanceEvaluation of  Commercial GaN HEMTs for Pulsed Power ApplicationsF. Fornetti, K.A. Morris, M.A. BeachCentre for Communications Research, University of  Bristol, UKTest SetupMatLab was used to control the instruments through GPIB and LAN interfaces and alsofor data acquisition and processing. The pulsed RF was generated by a purpose builtinterface board which safely turned the transitor fully on and fully off  to ensure that full tran-sients would be undergone by the device. Control Signals RF SignalsWaveform DataPulse GeneratorHigh Frequency ScopeD.U.T.Pre-driver stageSignal GeneratorThe board was also used totransform the TTL voltagelevels generated by the Agi-lent 81110A into the bias lev-els required to switch thetransistor on and off.ResultsIn this section we present the changes in pulse power, rise times and fall times at differentinput powers as the PRF is increased. Transients on output waveforms are also presentedand analysed.The pulse power was foundto remain largely constantacross the PRF range. This would suggest that thereis no current reduction due totrapping effects or self  heat-ing at the power levels used.It is apparent that the rela-tionship between the pulseoutput power and the inputpower is a linear one albeitsome slight variations as thetransistor approaches satura-tion.Rise and fall times of  the RFpulses do increase as theinput power is increased.This is likely to be due to agreater capacitive load deter-mined by higher RF voltagesbeing present. The maximum variations ob-served on the rise and falltimes were 10 and 8 ns re-spectively.The accuracy of  such figureswas however affected by thedifference in the number ofsample points available at dif-ferent PRFs  and by the enve-lope averaging preformed during data analysis.The rise and fall times maytherefore be considered large-ly constant across the PRFrange for a given power level .At input powers higher than22dBm, corresponding tooutput powers greater than36dBm, the fall time wasseen to increase signifi-cantly in an exponentialfashion as can be seen in thegraph.In the time domain this wasseen to be due to an addi-tional transient on the pulsefalling edge at transistorturn-off.At higher power levels therise time does not change sig-nificantly. The Fall time how-ever suffers a considerableincrease due to the presenceof  an exponential decay onthe falling edge. This demonstrates that thedevice output capacitanceplays a much more signifi-cant role when operatingnear the 1dB compressionpoint of  the amplifier.ConclusionsI n this study the suitability of  commercially available GaN samples to pulsed operation hasbeen demonstrated. The devices presented a consistent and reproducible performance atspecific power levels across a broad range of  Pulse Repetition Frequencies (PRFs). These de-vices could find application in S-band radars which utilise much lower PRFs than thoseanalysed in this study.The reason for pushing the devices to higher PRFs was to analyse the response that their struc-ture would have to repetition frequencies which are used at higher RF frequencies.Seeing that higher frequency devices will maintain some commonalities with their lower fre-quency counterparts, this gives some degree of  confidence that future devices could be utilisedfor pulsed RF applications which operate in higher frequency bands.Further investigations are needed to gain a better understanding of  the effects that operatingthe devices near their compression point will have on the pulse shape.Future WorkF uture work will focus on repeating the experiments presented in this paper at much higherpower levels and on devices with higher power ratings.Bare die HEMTs will also be subjected to similar tests at frequencies ranging from 6 to 10 GHz. Additional tests will also be performed to study the spectral content of  pulsed waveforms am-plified with GaN devices.AcknowledgementsThe authors wish to thank Cree Inc and Novacom Microwave for supplying free samplesand providing great assistance and support.This work has been partially supported by MBDA.Interface Board

Evaluation of commercial GaN HEMTs for pulsed power applications

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Evaluation of commercial GaN HEMTs for pulsed power applications

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