Asenov, A.

Babiker, S.

Barker, J.R.

Beaumont, S.P.

Roy, S.

English

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      Roy, S. and Asenov, A. and Babiker, S. and Barker, J.R. and Beaumont, S.P. (1997) RF performance of strained Si MODFETs and MOSFETs on "virtual" SiGe substrates: A Monte Carlo study. In, Grünbacher, H., Eds. European Solid-State Device Research Conference, 22-24 September 1997, pages pp. 192-195, Stuttgart, Germany.        http://eprints.gla.ac.uk/3035/     Glasgow ePrints Service http://eprints.gla.ac.uk RF performance of strained Si MODFETs andMOSFETs on virtual SiGe substrates: A MonteCarlo studyS.Roy, A. Asenov*, S. Babiker, J.R. Barker, and S. P. BeaumontNanoelectronics Research CentreDepartment of Electronics and Electrical EngineeringUniversity of Glasgow, Glasgow G12 8LT, Scotland, UK*Tel: ++44 141 330 5233, Fax: ++44 141 330 4907,E-mail: A. Asenov@elec.gla.ac.ukWe employ a comprehensive RF analysis technique based on transientensemble Monte Carlo simulations to study and compare the highfrequency potential of strained Si channel MODFETs and MOSFETs -correctly accounting for realistic device geometries and parasitics.Both devices show well pronounced overshoot effects. The MOSFETchannel is more closely confined by the self aligned source and draincontacts, resulting in a shorter effective channel length and betterdevice performance. The cut-off frequency fT is robust in the presenceof realistic device contact and gate resistances, unlike the maximumfrequency of oscillation fmax, which is strongly effected by the deviceparasitics.1 INTRODUCTIONIn Si layers grown pseudomorphically on SiGe substrates, induced strain breaks thesix-fold degeneracy of the Si conduction band ∆ valleys. Two valleys shift downwardsresulting in a type II heterojunction. Electrons in a 2DEG formed at this interface havetransverse effective mass in the plane of the heterojunction. These effects combine to givereduced intervalley scattering, higher in-plane mobility and enhanced velocity overshoot inthe strained layer. MODFETs with pseudomorphic Si channels have been demonstrated andshow clear potential for RF applications [1]. Even greater RF performance has beenpredicted for strained Si channel MOSFETs, although these conclusions are based onsimplified hydrodynamic simulations [2]. Both types of device rely on relaxed SiGe ’virtualsubstrates’ with graded Ge concentration, grown on Si. Compatibility with Si VLSIprocessing technology makes the devices important candidates for Si MMICs andmicrowave signal processing applications integrated on conventional Si chips.We employ, for the first time, a comprehensive RF analysis technique to study andcompare the high frequency potential of strained Si channel MODFETs and MOSFETsgrown on SiGe ’virtual substrates’. RF analysis requires a realistic simulation geometry andcorrect handling of device parasitics. Our technique is based on transient ensemble MonteCarlo simulations, using the Monte Carlo module of the Heterojunction 2D Finite Elementsimulator H2F [3].2 MONTE CARLO SIMULATION OF THE DEVICESFigure 1a describes the structure of a 120nm gate length pseudomorphic MODFET.It has a δ-doping layer separated by a 2.5nm spacer from the channel, a T shape recess gatewith 50nm recess offset and heavily doped (n-type 1018 cm-3) cap layer. An effectiveδ-doping of 5×1012 cm-2 and p-type background substrate doping of 1014 cm-3 is assumed.The structure of this device corresponds to that of a state of the art InGaAs pseudomorphicHEMT fabricated at the Glasgow Nanoelectronics Research Centre and used for validationof the RF analysis technique described below [4]. We note that advances in processingtechnology will be required to fabricate the described SiGe MODFET. (e.g. the ability toform a well defined As δ-doping supply layer.) The MOSFET structure in figure 1b isdesigned to have an equivalent effective gate-to-channel separation and channel length.Fig.1a,b Vertical layer structure of 120nm channel lengthstrained Si, Si:SiGe pseudomorphic MODFET (a) andMOSFET (b). Assume negligible Si layer above spacer.Fig.2 Drift velocity in bulk Si on appli-cation of field in (100) direction at t=0, toinitially thermal electron distribution.Inclusion of strained Si material parameters into the Monte Carlo model followsYamada [5]. Conduction band ∆ valleys split by ∆Ec = 0.67x (eV) and there is a modifiedband gap of Eg = 1.11 - 0.74x (eV) where x is the fractional Ge content of the relaxed SiGesubstrate. For simplicity of modelling, a uniform SiGe substrate is assumed, instead of a’virtual substrate’ with graded Ge concentration. Parameters for Si1-xGex are obtained from[6]. We implement the six phonon scattering model of Jacoboni [7], including acoustic andionised impurity scattering modes. Modelled bulk velocity-field characteristics are inagreement with experimental unstrained Si data and previous results for strained Si [5].Calculations of velocity overshoot in strained and unstrained Si are shown in figure 2.x (cm)Fig.4 Distribution of superparticles withinMonte Carlo model of a pseudomorphicMODFET at VD=1.5V, VG=—0.25V, showingdeconfinement at drain end of gate.Fig.3 Velocity profile in the channel ofpseudomorphic MODFET and MOSFET atVD =1.5V, VG =—0.25V, below onset ofsignificant parallel conductanceThe distribution of superparticles within the device is shown in figure 3. Due to themodest band offset of the pseudomorphic Si channel, real space transfer and parallel8.8nmSi channel10.0nmSiGe bufferSiGe spacer2.5nmAs 5×10   cm-212SiGe spacerSi Oxide3.0nmgate drainn+ SiGe cap30.0nmSiGe Schottky17.5nmSi channel10.0nmSiGe bufferSiGe spacer2.5nmAs 5×10   cm-21250.0nm(a) (b)00.511.522.53Drift Velocity (×107  cm/s)0 0.25 0.5 0.75 1time (ps)Field = 60kV/cmStrained Si(∆E=0.2eV)Unstrained Si00.511.52Velocity×107  (cm/s)-0.4 -0.2 0 0.2x (µm)MODFETMOSFETconductance significantly affect the device performance. The gate biasing conditions chosenfor figure 3 are below the onset of parallel conduction in the δ-doping region. Howeverdeconfinement of high energy carriers at the drain end of the device is still obvious.3 R.F. ANALYSISThe transient Monte Carlo simulations begin by following 5×104 superparticles for 2pssettling time at d.c. bias, then a further 2.0→2.6ps during which device statistics arerecorded at 1fs intervals. A step change ∆VG = 0.2V, or ∆VD= 0.3V is then consecutivelyapplied, and the transient response measured for a further 2ps. According to the Ramo-Shockley theorem [8] the instantaneous transient current at each device electrode i due to astep perturbation of a terminal voltage ∆Vj on electrode j, can be separated into two parts.Firstly a term describing the current due to the step voltage perturbation on electrode j, andcalculated from the capacitance matrix coefficients Cij associated with the device electrodes.Secondly, a term summing the current contribution due to each of N charged particleswithin the device. This result allows efficient calculation of terminal transient currents.It is found that structure and doping dependant THz oscillations in device drain current(possibly due to plasma oscillations in the channel, or in the heavily doped cap layer) maymask the detailed form of the transients, and so multiple traces are averaged to define theresponse. Complex y-parameters are derived by Fourier transforming the terminal currents,and used to extract the small signal equivalent circuit. The cut-off frequency of thesimulated device fT is extracted by solving log[ ]Gc(logf)  = 0 where G c=d˚Id/dIg is thecurrent gain expressed as a function of y-parameters. In order to extract the maximumfrequency of oscillation of the simulated device fmax, the y-parameters are transformed intothe s-parameters. fmax is then extracted by solving log [˚ ]MAG(logf)  = 0 where MAG is themaximum available gain. Finally, the small signal equivalent circuit is augmented by theaddition of external gate and contact resistances, and thus the effect of contact and gateresistance on the ’real’ device operation estimated.4 RESULTSThe average velocity profiles in the Si channel of the MODFET and MOSFET atVG˚= -˚0.25V and VD˚=˚ 1.5Vare illustrated in figure 4. Both exhibit a well establishedspatial overshoot region. In the MOSFET this overshoot region is more closely constrainedunder the gate, and its magnitude is somewhat improved along the channel. This is due tothe self aligned doping of the source and drain contact regions. Device transfercharacteristics are presented in figure 5, which also graphs the proportion of current flowingoutside the channel of each device. Parallel conduction for typical biasing conditions isequivalent for the two devices. The MOSFET, however, shows a somewhat improvedoutput current and transconductance, as a result of the constrained overshoot regioneffectively shortening the gate length in comparison with the MODFET.Complex y-parameters are derived by Fourier transforming the terminal current transientsassociated with small changes in the gate and drain voltages, from which the small signalequivalent circuit and figures of merit fT and fmax are extracted. Results are given in Table1. fT is only modestly effected by the inclusion of realistic device parasitics. However fmaxis strongly eroded by the inclusion of the contact and gate resistances. The variation of fmaxas a function of these resistances is shown in figure 6. This underlines the need for parasiticsto be included in the realistic modelling of ultrasmall devices. Equivalent simulations forthe complimentary MOSFET are in progress and will be presented.Fig.5 V G - ID characteristic (and parallelconduction fraction) for a pseudomorphicMODFET and MOSFET. VD=1.5V.Fig.6 Variation of f max in a pseudomorphicMODFET as a function of gate and contactresistances.RF figure of merit(GHz)with negligiblecontact resistancegate, source and drainresistance all 5ΩfT 77.4 63.9fmax 161 68.0Table1 calculated RF figures of merit for pseudomorphic MODFETat bias VD=1.5V, VG=-0.25V. Device width of 100µm is assumed.5 CONCLUSIONSAn RF analysis technique based on transient ensemble Monte Carlo simulations has beenused to comprehensively study strained Si channel MODFETs and MOSFETs. Both the d.c.characteristics and microwave potential of realistic device structures have been investigated,correctly accounting for device parasitics. It was noted the velocity overshoot region in thechannel of the pseudomorphic MOSFET is better constrained over that of the MODFET,due to its self aligned source and drain contacts. This shortens its effective channel lengthand improves device performance. fT was shown to be robust in the presence of realisticdevice contact and gate resistances. However fmax is strongly effected by these deviceparasitics.REFERENCES1. K. Ismail, S. Rishton, J. Chu, K. Chan and B. Meyerson, "High-performance Si/SiGe n-type modulation-doped transistors." IEEE Electron Device Letters 14(7): p. 348-350, 1993J. Welser, J.L. Hoyt and J.F. Gibbons, "Electron Mobility Enhancement in Strained-Si N-Type Metal-Oxide-Semiconductor Field-Effect Transistors", IEEE Electron Device Letters 15(3): p. 100, 1994.2. A O’Neill and D.A. Antoniadis, "Investigation of Si/SiGe-Based FET Geometries for High FrequencyPerformance by Computer Simulation", IEEE Trans. Electron Devices 44(1) pp. 80-88, 1997.3. S.Babiker, A. Asenov, et al., "Complete RF analysis of compound FETs based on transient Monte Carlosimulation", accepted for IWCE’97.4. N.I. Cameron, M.R.S. Taylor, et al., "A high performance, high yield, dry-etched, pseudomorphicHEMT for W-band use" IEEE Microwave Theory and Techniques Symposium, Orlando, FL, 435, 19955. T. Yamada, J. Zhou, H. Miyata and D.K. Ferry, "In-Plane Transport Properties of Si/Si1-xGex Structureand its FET Performance by Computer Simulation", IEEE Trans. Elec.Devices, 41(9) 1513, 1994.6. M.V. Fischetti and S.E. Laux, "Band structure, deformation potentials, and carrier mobility in strainedSi, Ge and SiGe alloys", J.Appl.Phys. 80(4) p. 2234, 1996.7. C. Jacoboni and P. Lugli, ’The Monte Carlo Method for Semiconductor Device Simulation’, Wein,Springer-Verlag, 1989.8. H. Kim, H. Min, T. Tang and Y. Park, "An Extended Proof of the Ramo-Shockley Theorem", Solid StateElectronics 34(11) pp. 1251-1253, 1992.0255075100% Parallel Conduction0123456Current (A/cm)-1.5 -1 -0.5 0 0.5Gate Voltage (V)MOD/MOS transferMOSFETMODFETMOSFETMODFETCurrent|| Conduction5075100125150175f max (GHz)0 2 4 6 8 10Gate Resistance (Ω)Source, DrainContact Resistance5 Ω4 Ω3 Ω2 Ω1 Ω0 Ω

A high performance, high yield, dry-etched, pseudomorphic HEMT for W-band use&quot;

An Extended Proof of the Ramo-Shockley Theorem&quot;,

Band structure, deformation potentials, and carrier mobility in strained Si, Ge and SiGe alloys&quot;,

High-performance Si/SiGe n-type modulationdoped transistors.&quot;

Investigation of Si/SiGe-Based FET Geometries for High Frequency Performance by Computer Simulation&quot;,

The Monte Carlo Method for Semiconductor Device Simulation’,

RF performance of strained Si MODFETs and MOSFETs on "virtual" SiGe substrates: A Monte Carlo study

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RF performance of strained Si MODFETs and MOSFETs on "virtual" SiGe substrates: A Monte Carlo study

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