Contains an introduction, report on one research project and a list of publications.Joint Services Electronics Program Contract DAAH04-95-1-0038Texas Instruments Agreement dated 11/23/9

del Alamo, Jesús A.

Ernst, Alexander N.

Somerville, Mark H.

Zeidenberg, Lisa B.

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Chapter 2. Heterostructure Field-Effect TransistorsChapter 2. Physics of Heterostructure Field-Effect TransistorsAcademic and Research StaffProfessor Jescs A. del AlamoGraduate StudentsMark H. SomervilleUndergraduate StudentsAlexander N. ErnstTechnical and Support StaffLisa Zeidenberg2.1 IntroductionSponsorsJoint Services Electronics ProgramContract DAAH04-95-1-0038Texas InstrumentsAgreement dated 11/23/94The goal of this project is to develop InAIAs/InGaAsheterostructure field-effect transistors suitable formillimeter-wave high-power applications. This is akey missing component for millimeter-wave radarand communication systems.Our team has been involved on research of high-power InAIAs/InGaAs heterostructure field-effecttransistors for several years. Two key contributionsin the past have been (1) the demonstration thatthe use of AlAs-rich InAIAs pseudoinsulators sub-stantially improves the breakdown voltage' and (2)the demonstration of selective recessed-mesasidewall isolation to reduce gate leakage current.2We also recently identified the detailed physicalmechanisms responsible for breakdown in InAIAs/-InGaAs HFETs.3In the last period of performance, we have studiedin detail the physical origin of the "kink effect" inInAIAs/InGaAs HFETs.4 This important anomaly inthe operation of these transistors deleteriouslyaffects their power performance. Our physicalunderstanding has culminated in the proposal of anew equivalent circuit model that successfully cap-tures the kink. This will enable first-pass success inthe design of future millimeter-wave systems basedon these devices.2.2 A New Physical Model for the KinkEffect on InAIAs/InGaAs HEMTsInAIAs/InGaAs high electron mobility transistors(HEMTs) show significant promise for low-noise andhigh-power millimeter-wave applications. A signif-icant anomaly in their behavior is the kink effect, asudden rise in the drain current at a certain drain-to-source voltage that results in high drain conduc-tance and reduced voltage gain. Conventionalwisdom suggests that traps are responsible for thekink. Most theories incorporating traps suggest thathigh fields and/or impact-ionization-generated holes1 S.R. Bahi, W.J. Azzam, and J.A. del Alamo, "Strained-Insulator InAl,_As/n-InoGao,,As Heterostructure Field-Effect Transistors,"IEEE Trans. Electr. Dev. 38: 1986 (1991).2 S.R. Bahl and J.A. del Alamo, "Elimination of Mesa-Sidewall Gate-Leakage in InAIAs/InGaAs Heterostructures by Selective SidewallRecessing," IEEE Electr. Dev. Lett. 13: 195 (1992).3 S.R. Bahl and J.A. del Alamo, "Physics of Breakdown in InAIAs/n--InGaAs Heterostructure Field-Effect Transistors," IEEE Trans.Electr. Dev. 41: 2268 (1994); S.R. Bahl, J.A. del Alamo, J. Dickmann, and S. Schildberg, "Off-State Breakdown in InAIAs/InGaAsMODFETs," IEEE Trans. Electr. Dev. 42:15 (1995).4 M. Somerville, J.A. del Alamo, and W. Hoke, "A New Physical Model for the Kink Effect on InAIAs/InGaAs HEMTs," InternationalElectron Devices Meeting, Washington, D.C., December 10-13, p. 201, 1995.Chapter 2. Heterostructure Field-Effect Transistorscharge traps either in the buffer or in the insulator,6leading to a shift in the threshold voltage. Such atheory, while plausible, is of little predictive valuebecause of the large number of variables involved.It is therefore important to search for other physicalorigins of the kink that might be amenable to simplemodeling in these devices.Recent experiments have provided indirect evi-dence linking the kink and impact ionization;7however, it remains unclear how the two phe-nomena are connected. Using a specially-designedsidegate structure, we have carried out extensivecharacterization of the kink effect in a double-heterostructure InAIAs/InGaAs HEMT. Our mea-surements provide direct evidence linking the kinkwith impact ionization, while at the same timeclearly showing that impact ionization current aloneis not responsible for the kink. Careful analysisleads us to postulate a new mechanism of barrier-induced hole pile-up at the source to explain thekink and to propose a simple equivalent circuitdescription of the phenomenon.A cross-section of the MBE-grown, double-heterostructure HEMT used in this study is pre-sented in figure 1. The channel sheet carrierconcentration is 3.5 x 101 cm- 2. Fabrication con-sists of device isolation via a mesa etch with asidewall recess, a PECVD Si3N4 layer for liftoffassistance, Au/Ge ohmic contacts, a selective gaterecess, and Pt/Ti/Au gates and interconnects.Devices with gate lengths between 0.6 pm and 2pm were characterized. The devices exhibit ID-, =520 mA/mm, gm.., = 440 mS/mm, and BVDS(off) 8 V.A relationship between the kink and impactionization has previously been postulated based onsimulation results8 as well as light emission andchannel-engineering experiments.7 However, theseexperiments only provided an indirect view ofimpact ionization. By using a specially designedsidegate structure,9 we have succeeded in directlytracking impact ionization in the device without per-turbing its behavior. The sidegate structure consistsof an ohmic contact on a 40 pm x 15 pm mesalocated 15 pm from the device under test. In themeasurement, the sidegate is held at a large nega-tive potential with respect to the source(VSG -S = - 20V). This allows the sidegate to collecta small fraction of the holes generated by impactionization, as sketched in the inset of figure 2.Thus, the sidegate current should approximatelytrack the impact ionization generation rate. Suchbehavior is observed in figure 2, where the ratio ofthe net sidegate current to the drain current isplotted as a function of 1/(VDs - VDS(sat)) for typicalvalues of VGS. Throughout the device's range ofoperation, ISG follows classical exponential impactionization behavior.Using ISG, we can now explore the relationshipbetween impact ionization and the kink effect. Infigure 3, we examine ID and ISG for VGS = 0 V. Thekink is clearly visible in ID starting at VDS - 1 V. Theonset of the kink coincides with the appearance ofISG. We have found that this is the case for othervalues of VGS. This is clearly seen in figure 4, whichshows ID, IG, and ISG as a function of VDG for dif-ferent VGS values. This figure illustrates a number ofkey characteristics of the kink: the kink in ID occursapproximately at constant VDG - 1.2 V; the size ofthe kink appears to increase with increasing VGS;and the onset of the kink coincides with the appear-ance of ISG and with a prominent rise in IG, presum-ably due to hole collection by the gate. These factsunequivocally establish the connection between thekink and impact ionization.5 A.S. Brown et al., "AllInAs-GalnAs HEMTs Utilizing Low-temperature AllInAs Buffers Grown by MBE," IEEE Electr. Dev. Lett. 10: 565(1989); T. Zimmer et al., "Kink Effect in HEMT Structures: a Trap-related Semi-quantitative Model and an Empirical Approach forSpice Simulation," Sol. State Electr. 35: 1543 (1992).6 y. Hori and M. Kuzuhara, "Improved Model for Kink Effect in AIGaAs/InGaAs Heterojunction FETs," IEEE Trans. Electr. Dev. 41:2262 (1994).7 G.G. Zhou, A.F. Fischer-Colbrie, and J.S. Harris, "I-V Kink in InAIAs/InGaAs MODFETs Due to Weak Impact Ionization in the InGaAsChannel," Sixth International Conference on InP and Related Materials, Santa Barbara, California, March 1994, pp.435.8 K. Kunihiro, H. Yano, N. Goto, and Y. Ohno, "Numerical Analysis of Kink Effect in HJFET with a Heterobuffer Layer," IEEE Trans.Electr. Dev. 40: 493 (1993).9 A.A. Moolji, S.R. Bahl, and J.A. del Alamo, "Impact Ionization in InAIAs/InGaAs HFETs," IEEE Electr. Dev. Lett. 15: 313 (1994).22 RLE Progress Report Number 138Chapter 2. Heterostructure Field-Effect TransistorsPt/Ti/Pt/Au70 A300 AAuGeIn.~Ga.4,,AsI n.52AI.48As.... s . ........... .................................................................... V 3 xl 0 1200 A In 5,3Ga.47Asi ........ '50 ......A .............................................................. 2 x102550 A I n. 2AI.48,AsSI InPFigure 1. Schematic cross-section of InAIAs/InGaAs double-heterostructure HEMT used in this work.10-510-610 -810-810"0.1(VDS - VDS-sat )1 (V 1 )0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6Figure 2. Semilog graph of I IsB - Isoo I/lo versus 1/(VDS - VDs(,,)). The approximately exponential behavior at small 1/(VDS - VD(satl)) confirms the onset of impact ionization. VSG_- = -20 V, L0 = 2 pm, T = 300 K.0CO(I(.(3m)-BISG D exp [ V -VDS - VDS-satI . I . I . I * II . I1 rI ' --" II I • •I " I "0 1.8Chapter 2. Heterostructure Field-Effect TransistorsFigure 3. Drain and sidegate current at Vas = 0. Notethe saturation of the kink, in contrast with the exponentialgrowth of the sidegate current. L = 2 pm, T = 280 K.Pure impact ionization has been proposed as anexplanation for excess output conductance inInAIAs/InGaAs HEMTs. 10 In this model, additionaldrain current originates from the impact ionizationgenerated holes and electrons. Such an explana-tion of the kink is not consistent with our exper-iments. If impact ionization alone were responsiblefor the kink, the shape of the kink would closelytrack the shape of the sidegate current. However,as seen in figure 3, the kink saturates while thesidegate current grows strongly with VDS. Clearlysome other effect must be at work.The kink effect in SOI MOSFETs is known to be aresult of impact ionization generated holes flowingthrough the p-type buffer into the n+ source." Thishole current forward biases the buffer-source p-njunction, thereby providing additional drive to thetransistor. While such an hypothesis may be appro-priate in some HEMT designs, 12 two facts make thisexplanation unlikely for current InAIAs/InGaAsHEMT designs. First, the presence of a significantvalence band discontinuity (0.2 eV) between thechannel and the buffer should confine most holes tothe narrow channel. In addition, the fact that the480V 400 0.6 VEE 320VE 240160800 2 4100Ves - 0.6 V"O'- AG -. 2VE10-2 ---- 10-31o. 4O 2 4302520-5AVOs = 0.2 V15v 10V= 0.6 V50 2 4VDG (V)Figure 4. Drain, gate, and sidegate currents for differentvalues of VGS. The onset of the kink directly correspondswith the onset of the sidegate current and a significantincrease in gate current. La = 2 pm, T = 300 K.channel and the buffer are undoped makes aparasitic bipolar effect less plausible.Simulation results have recently suggested anotherpossible explanation for the kink, source resistancereduction.13 In this model, holes drift into the lowfield source-gate region, where they diffuse andrecombine. To maintain quasi-neutrality, the elec-tron concentration must be increased, resulting inreduced source resistance. If this were the case,the excess current would be of the form10 M. Chertouk et al., "Metamorphic InAIAs/InGaAs HEMTs on GaAs Substrates with Composite Channels and fi,., of 350 GHz,"Seventh International Conference of InP and Related Materials, Sapporo, Japan, 1995, p. 737.11 K. Kato, T. Wada, and K. Taniguchi, "Analysis of Kink Characteristics in SOI MOSFETs Using Two Carrier Modeling," IEEE Trans.Electr. Dev. ED-32: 458 (1985).12 K. Kunihiro, H. Yano, N. Goto, and Y. Ohno, "Numerical Analysis of Kink Effect in HJFET with a Heterobuffer Layer," IEEE Trans.Electr. Dev. 40: 493 (1993); B. Brar and H. Kroemer, "Influence of Impact Ionization on the Drain Conductance of InAs/AISbQuantum Well HFETs," in press.13 T. Enoki, T. Kobayashi, and Y. Ishii, "Device Technologies for InP-based HEMTs and their Applications to ICs," IEEE GaAs ICSymposium, 337-339, 1994.24 RLE Progress Report Number 1388070so60E540Ea 30201000 50 100 150 200 250ID(pre-kink) (mA/mm)300 350Figure 5. Kink magnitude extracted for Vos - VDS(sat) = 3 Vat low temperature as a function of ID. The solid line the-oretical fit is discussed later in the text. La = 0.8 pm.AID = gm0IDAR (1)where gmo and ID are "pre-kink" values, and AR isthe drop in source resistance brought about by thehole accumulation. Since IARI should increasewith increasing ID, the kink current AID would besuperlinear in ID according to this hypothesis. Inorder to evaluate this hypothesis, we plot in figure 5the magnitude of the kink, extracted for constantVDS - VDS(sat) = 3 V, versus ID. This measurementclearly indicates that the kink has a sublinear IDdependence, which is inconsistent with sourceresistance reduction.Although simple source resistance reduction doesnot appear to explain our results, recent reports oflight emission from the extrinsic source' 4 and kinksuppression by means of a buried p-layer 5 moti-vate us to explore further the possible significanceof holes in the kink effect. As the source-resistancereduction model suggests, holes can drift throughthe channel and invade the extrinsic source. Partic-ularly effective hole pile-up might arise if there is apotential barrier at the source. Such a barrier canoccur between the ohmic contact's n+-region andthe channel, or at the transition between thecapped and uncapped portions. If this is the case,Chapter 2. Heterostructure Field-Effect Transistorsthe ohmic drop in conjunction with the barriercreates a triangular well where holes can accumu-late. Any pile-up of holes reduces the ohmic drop inthe region immediately adjoining the barrier (figure6). This provides an extra gate drive, Vkink, to thetransistor.A simple first-order analysis of this hypothesis pro-vides a number of key dependences in the behaviorof the kink that can be tested. An additional driveon the gate results in increased current:AID = gm0VkinkThe kink voltage is to first order determined by theexcess hole concentration at the barrier:kBT I no + PVkink q n0In the classical description of impact ionization, theionization rate is proportional to the exponential ofthe inverse of the field in the high field region. Theexcess hole concentration at the barrier will be pro-portional to the impact ionization generation rate, sopa ID exp -B ) (4)p limpact oD ( VDS - VDS(sat)where B is a constant. Plugging (4) into (3) and (2),we obtainAIDkBT In 1 + AID exp - B om0 1 +AD ex VDS - VDS(sat) ]where A is another constant.In examining (5), we note first that when the holeaccumulation is large with respect to the pre-kinkelectron concentration, the exponential term domi-nates, so that at large VDS values,-1AID IlargeVDS VDS - VDS(sat)14 N. Shigekawa, T. Enoki, T. Furuta, and H. Ito, "Electroluminescence of InAIAs/InGaAs HEMTs Lattice-matched to InP Substrates," inpress.15 B. Brar and H. Kroemer, "Influence of Impact Ionization on the Drain Conductance of InAs/AISb Quantum Well HFETs," in press; T.Suemitsu, T. Enoki, Y. Ishii, "Body Contacts in InP-based InAIAs/InGaAs HEMTs and Their Effects on Breakdown Voltage and KinkSuppression," Electr. Lett. 31: 758 (1995).Chapter 2. Heterostructure Field-Effect TransistorsFigure 6. Postulated kink mechanism. Holes generated by impact ionization drift into the extrinsic source and accumu-late in the well formed by the barrier and the ohmic drop. The resulting reduction in extrinsic voltage results in increaseddrive to the transistor.Such a dependence is observed in our exper-iments, as shown in figure 7.The direct relationship between the sidegate currentand impact ionization generation rate further impliesthat the kink should be predicted by the sidegatecurrent. In particular, from (4) and (5),kBTAID O gm 0  In [1 + CISGIqwith C another constant. We observe this in figureFinally, if VDS - VDS(sat) is held constant,should be a simple function of g, 0 and ID:kgTAID o m0 ~ In [1 + DID]AID0' MOthe kink(8)with D also a constant. Such a dependenceexplains our experimental observation of figure 5.The understanding provided by this physical modelallows us to build a simple equivalent circuit modeldescription of the kink. A new model element needsto be added in series with the intrinsic source of theFET (figure 9) that represents the additional driveprovided by the hole pile-up. This element is avoltage source that is controlled by VDS and ID.Only two parameters are required to fit completelythe characteristics of the transistor (figure 10).Figure 7. Kink magnitude vs. VDs - VDS(sat). LG = 0.8 pm.26 RLE Progress Report Number 138Chapter 2. Heterostructure Field-Effect Transistors1 2 3 4 5VDG (V)Figure 8. Comparison of the kink current, AID, with thekink predicted by the sidegate current. LG = 2 pm, T =280 K.Although the form of this model is very similar tothose used in SOI MOSFETs, the physics at playare significantly different.In conclusion, we have postulated a new physicalmodel for the kink effect in InAIAs/InGaAs HEMTs.The kink arises from hole pile up at a potentialbarrier in the source of the device that brings abouta reduction of the ohmic drop at the source. Thisresults in extra gate drive to the transistor. Ourfindings have allowed us to formulate a simpleequivalent model description of the kink effect inthese devices.DG + standardV.s :A modelkT -B+ VIk - T In [1 + A ID exp ( V )]SFigure 9. Proposed equivalent circuit model. A newelement with only two bias-independent parameters isrequired to model the kink completely.Figure 10. Comparison of model predictions for the kinkwith measured device characteristics. LG = 2 pm, T =280 K.2.3 PublicationsBahl, S.R., J.A. del Alamo, J. Dickmann, and S.Schildberg. "Off-State Breakdown in InAIAs/-InGaAs MODFETs." IEEE Trans. Electr. Dev.42: 15-22 (1995).Berthold, S., E. Zanoni, C. Canali, M. Pavesi, M.Pecchini, M. Manfredi, S.R. Bahl, and J.A. delAlamo. "Impact Ionization and Light Emission inInAIAs/InGaAs Heterostructure Field-Effect Tran-sistors." IEEE Trans. Electr. Dev. 42: 752-759(1995).Somerville, M.H., J.A. del Alamo, and W. Hoke. "ANew Physical Model for the Kink Effect onInAIAs/InGaAs HEMTs." Proceedings of theInternational Electron Devices Meeting.Washington, D.C., December 1995, p. 221.A cathodoluminescence micrograph from a 1 pm-thick ZnSe layeron a Zn-exposed, (2x4) reconstructed, 8 monolayer thick GaAslayer on a 4 pm graded InGaP layer. The surface was imaged atthe ZnSe wavelength at a magnification of 1700x using a probecurrent of 32 nA and an acceleration voltage of 20 kV. The samplewas grown in the Chemical Beam Epitaxy Laboratory which isunder the direction of Professor Leslie A. Kolodziejski.28 RLE Progress Report Number 138

Physics of Hemterostructure Field-Effect Transistors

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Physics of Hemterostructure Field-Effect Transistors

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