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Ultrahigh power-bandwidth-product performance oflow-temperature-grown-GaAs based metal-semiconductor-metaltraveling-wave photodetectorsKian-Giap GanDepartment of Electrical and Computer Engineering, University of California, Santa Barbara,California 93106-5050Jin-Wei Shi and Yen-Hung ChenGraduate Institute of Electro-Optical Engineering, National Taiwan University, Taipei 10617, TaiwanChi-Kuang Suna)Graduate Institute of Electro-Optical Engineering and Department of Electrical Engineering, NationalTaiwan University, Taipei 10617, TaiwanYi-Jen Chiu and John E. BowersDepartment of Electrical and Computer Engineering, University of California, Santa Barbara,California 93106-5050共Received 12 December 2001; accepted for publication 1 April 2002兲High-output-power and high-bandwidth performances are usually two tradeoff parameters in thedesign of high-speed photodetectors. In this letter, we report high peak-output-voltage 共⬃20 V兲 andpeak-output-current 共⬃400 mA, 50 ⍀ load兲 together with ultrahigh-speed performances 共1.5 ps, 220GHz兲, observed in low-temperature-grown-GaAs 共LTG-GaAs兲 based metal-semiconductor-metal共MSM兲 traveling-wave photodetectors 共TWPDs兲 at a wavelength of 800 nm.Ultrahigh-peak-output-power and ultrahigh-electrical-bandwidth performances were achieved dueto the superior MSM microwave guiding structure and a short carrier trapping time in theLTG-GaAs layer, which reduced the space-charge screening effect and increased thephotoabsorption volume without sacrificing electrical bandwidth significantly. We also observeddifferent bias-dependent nonlinear behaviors in MSM TWPDs under high and low illuminatedoptical power excitations, which are possibly dominated by the space-charge screening and thelifetime increasing effects, respectively. © 2002 American Institute of Physics.关DOI: 10.1063/1.1482139兴Ultrahigh speed photodetectors 共PDs兲 attract much atten-tion due to their applications in fiber communication andoptical measurement systems.1Recently, the development ofoptical amplifier has created a new demand of high input-optical power durability in these high-speed PDs. Some re-ceiver circuits use fiber amplifiers as preamplifiers, whichrelieve or even eliminate the necessity of electricalamplifiers.1By utilizing photomixing techniques, microwavephotonic systems are expected to generate tunable and high-power microwaves or submillimeter waves under intense op-tical excitation.2However, ultrahigh bandwidth PDs areknown to suffer bandwidth degradation problems under in-tense optical illumination.3,4Maximum output current/powerand electrical bandwidth performances are usually twotradeoff parameters in the design of high-speed PDs.1Byproperly scaling down the size and photoabsorption volumesof PDs, ultrahigh speed performances can be achieved due tothe reduction of parasitic capacitance and resistance in PDs.5However, the small photoabsorption volume 共⬃1␮m3兲would cause high densities of photogenerated free carriersinducing strong space-charge fields that screen the externalapplied bias field. The device electrical bandwidth wouldthus degrade seriously due to the reduction in drift velocitiesof photogenerated carriers.3,4On the other hand, by reducingthe optical modal absorption constant and increasing thephotoabsorption volume, the output power can be increasedsignificantly at the expense of the electrical bandwidth lim-ited by R–C time constant and high microwave loss in thelarge-area and long-absorption-length devices.6,7There aretwo major ways to increase the output-saturation-current共power兲-electrical-bandwidth-product performances.One is to distribute the photocurrents along edge-coupledPDs, such as the velocity match distributed photodetectors共VMDPs兲,8the other is to increase the carrier velocity usingunitraveling carrier PDs 共UTC-PDs兲.1In this letter, we dem-onstrate the high power performance in an ultrahigh speedlow-temperature-grown GaAs 共LTG-GaAs兲 based metal-semiconductor-metal 共MSM兲 traveling-wave photodetector共TWPD兲.9By utilizing the MSM microwave guiding struc-ture, a large photoabsorption volume can be achieved with-out serious electrical bandwidth degradation.7The space-charge screening effect in most high power PDs could alsobe reduced with the LTG-GaAs photoabsorption layer due toits short carrier trapping time and due to the fact that most ofthe collected photogenerated carriers are from the regionnear the metal contacts, where least space-charge screeningeffects could occur.1,4Performance with record-high peak-output-voltage 共current兲 bandwidth product 关20 V 共400 mApeak output current at 50 ⍀ load兲, 220 GHz, 4.4 THz-V 共88GHz-A兲兴 is thus observed in the demonstrated MSM TWPD.Distinct bias-dependent nonlinear behavior has also been ob-a兲Author to whom correspondence should be addressed; electronic mail:[email protected] PHYSICS LETTERS VOLUME 80, NUMBER 21 27 MAY 200240540003-6951/2002/80(21)/4054/3/$19.00 © 2002 American Institute of PhysicsDownloaded 13 Dec 2002 to 128.111.29.118. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jspserved under different excitation power levels, which aredominated by the lifetime increasing effect10or a combina-tion of the defect saturation and space-charge screeningeffects11,12under low and high optical radiation powers, re-spectively.The structure of the measured MSM-TWPDs is similarto the structure shown in Ref. 9 except for the AlGaAs opti-cal cladding layers, which were modified to reduce substratemode for better optical guiding. The cross sectional diagramis shown in Fig. 1. The thickness of each layer from surfaceto the bottom semi-insulating GaAs substrate is 500 nm ofLTG-GaAs, 100 Å of AlAs, 400 nm of Al0.2Ga0.8As, 1␮mofAl0.3Ga0.7As, and 3␮mofAl0.7Ga0.3As. All samples wereannealed in molecular beam epitaxy chamber at 600 °C withdetailed fabrication processes given in Ref. 9. By measuringdevices with different absorption lengths, the waveguidelength for complete absorption was determined to be 20␮m.Compared with the performance of traditional p–i–nTWPDs with a ‘‘slow-wave’’ microwave mode,13MSMTWPD structure ensures higher velocity-mismatchbandwidth14not only due to lower microwave propagationloss but also due to higher propagation velocity with the‘‘quasi-TEM’’ mode.7,13Because of superior microwaveguiding structure, MSM TWPDs can increase


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