Contains an introduction and reports on experiments and device results.Joint Services Electronics Program Contract DAAL03-89-C-0001Charles S. Draper Laboratory Contract DL-H-40418

Azzam, Walid

Bahl, Sandeep R.

del Alamo, Jesus A.

Odoardi, Angela R.

DSpace@MIT

Chapter 7. High Frequency Semiconductor Field-Effect TransistorsChapter 7. High-Frequency InAIAs/InGaAsMetal-Insulator-Doped Semiconductor Field-EffectTransistors (MIDFETs) for TelecommunicationsAcademic and Research StaffProfessor Jesus A. del AlamoGraduate StudentSandeep BahlUndergraduate StudentWalid AzzamTechnical and Support StaffAngela R. Odoardi7.1 IntroductionSponsorsJoint Services Electronics ProgramContract DAAL03-89-C-0001Charles S. Draper LaboratoryContract DL-H-404180The goal of this project is to design, fabri-cate, test, and model submicronInAIAs/InGaAs Heterostructure Field-EffectTransistors (HFETs) on InP. These devicesare of great interest for applications in long-wavelength optical communications andultra-high frequency microwave telecommu-nications. We expect that extraordinarydevice performance will result from theunique combination of: (1) the large con-duction bandgap discontinuity betweenInAIAs and InGaAs, (2) the large F - L sep-aration in InGaAs, and (3) the highof electrons in InGaAs.velocityWe have recently pioneered HFETs in whichthe InGaAs channel is heavily doped but theInAIAs insulator is undoped (MIDFETs forMetal-Insulator-Doped semiconductor FET). 1We have found that these devices display aperformance comparable to InAIAs/InGaAsModulation-Doped FETs (MODFETs) ofsimilar gate length. In addition, thesedevices offer a number of unique benefits notfound in other device structures: reducedtransconductancel and fT collapse,2 higherbreakdown voltage,' and freedom to optimizegate insulator parameters.3  A newmodulation-doped channel MIDFET also dis-plays unprecedented microwave character-istics.4During 1989, we obtained the first workingInAIAs/InGaAs MIDFETs on InP fabricated1 J.A. del Alamo and T. Mizutani, "An In0. 52Al0.48As/nIn0.53Gao. 47As MISFET with a Heavily-Doped Channel," IEEEElectron Device Lett. EDL-8 (11): 534-536 (1987).2 J.A. del Alamo and T. Mizutani, "Bias Dependence of fT and fmax in an Ino.52Alo.48As/n -Ino.5 3 Gao.47As MISFET,"IEEE Electron Device Lett. EDL-9 (12): 654-656 (1988).3 J.A. del Alamo and T. Mizutani, "A Recessed-Gate Ino.52AIo.48As/n+-In0 .53Gao.47As MIS-type FET," IEEE Trans.Electron Devices ED-36 (4): 646-650 (1989).4 J.A. del Alamo and T. Mizutani, "An Ino.52AIo.48As/n+-Ino.53Gao. 47As MISFET with a Modulation-Doped Channel,"IEEE Electron Device Lett. EDL-10 (8):394-396 (1989).Chapter 7. High Frequency Semiconductor Field-Effect Transistorsat MIT.5 The performance and properties ofthese MIDFETs are similar to those fabricatedelsewhere.3 More relevant to the problem, wefabricated, for the first time, strained-insulator InAIAs/InGaAs MIDFETs. Thisreport gives a detailed description of ourresearch on these devices.We have exploited the inherent flexibility ofgate insulator design of a MIDFET byincreasing the AlAs content in the InAIAsinsulator from the lattice-matching composi-tion to InP (Ino.52Alo.48As), straining it towardslarger bandgaps. This rapidly effects anenhancement in the conduction band discon-tinuity (AEc) between the InAIAs and theInGaAs channel (figure 1). We expect, inthis manner, to obtain the following benefits:* reduced gate leakage* improved gate breakdown voltage* reduced real-space transfer of hot elec-trons from the channel to the gate.We have demonstrated all of these benefitsexperimentally while largely unaffecting therest of the device parameters (thresholdvoltage, transconductance, etc.).7.2 ExperimentsWe grew four wafers by molecular-beamepitaxy in MIT's Riber 2300 system. Fol-lowing standard procedures, we prepared thesurface of the InP wafer. Figure 2 shows thecross section of the grown heterostructures.The layer structure is similar to those utilizedpreviously,1 except for:1. The channel design is more aggressive:doping is higher, 4 x 1018 cm - 3, andthickness is lower, 100 A. This channelis expected to result in superior perform-ance.2. The Al composition in three of the wafersis increased over lattice matching condi-tions, i.e., wafers were grown in which xin the In(,-x)AIxAs layer is 0.48, 0.52, 0.60,Figure 1. Cross-sectional equilibrium band diagram ofdevice structure indicating the increase in AEg withincrease in Al composition.and 0.70 (all insulator layers are 300 Athick). This is the main purpose of thepresent experiment.We continued fabrication after growth withmesa isolation utilizing a selective chemicaletchant, AuGeNi ohmic contact formation,TiAu interconnect, and TiAu gate layer for-mation. Figure 2 shows the resulting devicecross section.We have integrated a number of devices ofvarying dimensions and a large set of teststructures into a 2.9 x 2.7 mm 2 chip. Wepresent here DC I-V measurements onMIDFETs with a nominal gate length of 1.5um and a gate width of 30 ym.7.3 Device ResultsFigure 3 shows a plot of the transconduc-tance versus gate to source voltage for fourrepresentative devices with varyingIn(,lx)AIxAs insulator compositions. Asexpected from simple theory, the thresholdvoltage is, to the first order, unaffected bythe introduction of strain in the insulator.The lattice-matched device has a maximumtransconductance of 220 mS/mm. This valueis higher than for any of the InAIAs/InGaAsMIDFETs previously reported in the literaturewith a similar gate length and overallepitaxial structure (the best was 1605 S. Bahl, W.J. Azzam, and J.A. del Alamo, "Fabrication of an InAIAs/n+-InGaAs MIDFET," unpublished paper pre-sented at the Fourth New England MBE Workshop, Cambridge, Massachusetts, May 4, 1989.54 RLE Progress Report Number 132- OChapter 7. High Frequency Semiconductor Field-Effect TransistorsTi/Au GATE/300A In(1-x)AIAs x=0.48, 0.52, 0.6, 0.7100 A n- InGAs (4 x o'Sc -C 3)100oo InGoAs1000 i InAIAsSI - InPFigure 2. Cross section of device structure.mS/mm). 3 We believe that is due to theenhanced channel doping in this new devicestructure.An increase of AlAs composition in theIn(lx)AIxAs gate insulator from the lattice-matched condition x = 0.48 to x = 0.6brings about a slight reduction of peak gmdue to an enhanced contact resistance. Firstorder theory predicts this result. A furtherincrease of x to 0.7 results in a catastrophicreduction in g, that cannot be explained by alarger source resistance. It appears that, froman electrical point of view, the critical layerthickness for the 300 A Inl_-x)AIxAs insulatorhas been exceeded in going from x = 0.6 tox = 0.7.While there is a small reduction of gm withAlAs composition, the gm curve broadens inV,s, as figure 3 indicates, because gatecurrent is being suppressed due to theenhanced conduction band discontinuity(discussed below). Thus, the maximumcurrent driving capability of the devices isnot degraded. In fact, this capabilityimproves with AlAs composition up to 0.6.The maximum drain current obtained is 300mA/mm for the x = 0.6 device. This is betterthan that which had been reported previouslyin similar devices and is a very competitivevalue for any 1.5 um gate length HFET.SAl'.0000 /--1.200\\I0 2.400VG .4000/div ( V)Figure 3. Transconductance versus gate to sourcevoltage for Vds = 4 V.Increasing the AlAs composition results in adrastic reduction of forward gate current.Figure 4 illustrates gate current as a functionof drain-source voltage for V,s = 1.2 V. Asthe AlAs composition increases, gate currentat all values of Vds is drastically suppressed.This results in broader gm versus Vgs charac-teristics (shown in figure 3) and higher IDmax,as mentioned above.A more significant result, shown in figure 5,is the impact of AlAs composition on thereal-space transfer of hot electrons from thechannel to the gate. Such a phenomenon isvery clearly observed in the devices withAlAs composition of 0.48 and 0.52. Itsunique signature is an increase in the gatecurrent as Vds increases for a given Vgs. Thisincrease is due to electrons that are heatedby the high electric field prevalent inside thechannel and then acquire sufficient energy toovercome the conduction band discontinuitybetween the channel and the gate insulator.The drain current saturates when the deviceitself enters saturation. An AlAs compositionof 0.6 results in a sufficient conduction banddiscontinuity to completely suppress thisphenomenon. This may be a major advan-tage of submicron strained-insulatorMIDFETs in which the electric field in thechannel can reach a very high value.Figure 5 shows that real-spaceelectrons to the gate causes adifferential output resistance.transfer of hotsmall negativeBecause elec-AuGeNiOHMIC CONTACTIGM(S )250.0 -E-0325. 00/divz-O.. 7Chapter 7. High Frequency Semiconductor Field-Effect Transistors(Vs,-t.2 V)(mA)10.00-I. 000/ d l v -. - .41 - 0-,.6.0000.0000-0.7 _ _ _ _VDS .4000/div ( V)ID (mA)10.001.000/dlivAnI I4.000 .0000(vgs-1.2 V)VDS .4000/div ( V)Figure 4. Gate current versus Vds at Vgs = 1.2 V, as afunction of AlAs composition.trons are lost to the gate, this phenomenonresults in drain current reduction. In addi-tion, it may produce device instabilities andoscillations. For the x = 0.6 device, real-space transfer is completely suppressed andthe drain current reaches higher values, asfigure 5 indicates. Overall, the x = 0.6device has ten times less gate current thanthe x = 0.48 device at practical operatingpoints.A final advantage of the strained-insulatorMIDFETs is the enhanced reverse-bias gatebreakdown voltage. Figure 6 shows the gateI-V characteristics measured with drain andsource shorted. We obtain a rather hardbreakdown for all devices. If we definebreakdown at a reverse current of 1 mA, thebreakdown voltage for the lattice matcheddevice is 17 V. As the AlAs composition isincreased in the insulator, the breakdownvoltage increases very quickly, up to 28 V forx = 0.7.The combination of the unique featuresinvestigated above makes strained-insulatorInAIAs/n+-InGaAs MIDFETs very suitableFigure 5. Drain current versus Vds at Vgs = 1.2 V, as afunction of AlAs composition.candidates for high-power microwave appli-cations. Figure 7, for example, shows thatIn. 40Al0.60oAs/n -In0.53Ga0.47As MIDFETs dis-played excellent I-V characteristics up toVds = 10 V.IG(mA)VG 3.000/div ( V)Figure 6. Reverse gate current characteristics of FETsas a function of AlAs composition Vds = 0 V.56 RLE Progress Report Number 132-0..1.48I7Z -. 7.vuv4 000.000I4Chapter 7. High Frequency Semiconductor Field-Effect TransistorsFigure 7. I-V characteristics of fabricatedIno.40AIo.soAs/n+-Ino.53Gao.47As MIDFET. Scales: ID:1 mA/div,Vds:l V-div,Vgs:0.2V/step, Vgs( max) = 0 V.7.4 PublicationsBennett, B.R., and del Alamo, J.A. "Index ofRefraction Anysotropy in MismatchedInGaAs/InP Heterostructures Measured byEllipsometry." Paper presented at theSymposium on Layered Heterostructures -Heteroepitaxy, Superlattices, Strain andMetastability of the 1989 Fall MeetingMaterials Research Society, Boston,Massachusetts, November 27-December2, 1989. Symposium Proceedings. Forth-coming.del Alamo, J.A., and Azzam, W.J. "AFloating-Gate Transmission-Line Model(FGTLM) Technique for Accurate Meas-urement of Source Resistance in HFETs."Paper presented at 1989 Workshop onCompound Semiconductor Materials andDevices, Hilton Head, South Carolina,February 20-22, 1989.del Alamo, J. A. and Mizutani, T. "ARecessed-GateIno.52AIo.48As/n+-I n0.53Gao.47As MIS-typeFET." IEEE Trans. Electron Devices ED-36(4): 646-650 (1989).del Alamo, J. A. and Mizutani T. "AnIno.52AIo.4As/n+-In.5 3Gao.47As MISFET witha Modulation-Doped Channel." IEEE Elec-tron Device Lett. EDL-10 (8): 394-396(1989).del Alamo, J.A., and Azzam, W.J. "AFloating-Gate Transmission-Line Model(FGTLM) Technique for MeasuringSource Resistance in HeterostructureField-Effect Transistors." IEEE Trans. Elec-tron Devices 36 (11): 2386-2393 (1989).Professor John M. Graybeal in his laboratory.58 RLE Progress Report Number 132

High-Frequency InAIAs/InGaAs Metal-Insulator-Doped Semiconductor Field-Effect Transistors (MIDFETs) for Telecommunications

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High-Frequency InAIAs/InGaAs Metal-Insulator-Doped Semiconductor Field-Effect Transistors (MIDFETs) for Telecommunications

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