Predictive microwave device design by coupled electro ... · University of Leeds, Leeds LS2 9JT,...
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Predictive microwave device design by coupled electro-thermal simulation based on a fully physical thermal model W. Batty ×+ , C. E. Christoffersen * , S. David × , A. J. Panks × , R. G. Johnson × , C. M. Snowden × and M. B. Steer * , × Institute of Microwaves and Photonics, * Dept. of Electrical and Computer Engineering, School of Electronic and Electrical Engineering, North Carolina State University (NCSU), University of Leeds, Leeds LS2 9JT, UK. Raleigh, NC 27695-7914, USA. + Contact: tel. +44 113 2332089, fax +44 113 2332032, e-mail [email protected]. Abstract Coupled electro-thermal simulations are performed to demonstrate predictive design of microwave devices. These simulations are based on an original, fully physical, thermal impedance matrix approach, capable of describing complex 3-dimensional systems, whilst requiring no model reduction for electro-thermal CAD. This thermal model is validated by thermal imaging of passive grid arrays repre- sentative of spatial power combining architectures. Fully physical simulations of power HEMT I-V characteristics are presented, by coupling the thermal resistance matrix approach to the Leeds Physical Model of MESFETs and HEMTs. Single-tone, two-tone and multi-tone harmonic balance simulations are presented for a MESFET ampli- fier, by implementing the thermal impedance matrix ap- proach in microwave circuit simulator, Transim (NCSU). I. Introduction The importance of self-heating, and of mutual thermal interaction between active elements, in power devices and circuits, is well known. Hence predictive design, based on electro-thermal simulation, has been pursued for over 30 years. Until recently, the state of the art in time-independent thermal simulation of power FETs and MMICs, for coupled electro-thermal CAD, has been represented by the hybrid finite element Green’s function approach of Bonani et al. This approach treated device structure such as surface metallisation, vias and partial substrate thinning. In time-dependent coupled electro- thermal CAD, thermal descriptions have been more prim- itive, and limited to simple rectangular multilayers. Work in this area is exemplified by that of Szekely et al., who generated a large range of semi-analytical thermal solu- tions for ICs, MCMs and microsystem elements. A vari- ety of thermal model reduction techniques have also been employed to make tractable the coupled electro-thermal simulation problem on CAD timescales. The problem of compact model development for thermal networks is cur- rently a highly active area of research. The authors have progressed the state of the art, by pre- senting a thermal resistance matrix model capable of de- scribing, nearly exactly, 3-dimensional time-independent heat flow in complex device structures, in a fashion com- patible with electro-thermal CAD [1], [2]. This model includes description of surface fluxes and treats all ther- mal non linearities. More recently, the authors have presented a thermal impedance matrix model, extending this description to treat the thermal time-dependent case, both steady-state and transient [3]. This time-dependent formulation allows interpretation of thermal subsystems in terms of generalised multiport network parameters, without requiring model reduction. Circuit level electro- thermal simulation was achieved by implementation of this thermal impedance matrix model in a network based microwave field and circuit simulator, Transim (NCSU). Fully physical electro-thermal device CAD, based on cou- pling of the thermal impedance matrix approach, to the rapid, quasi-2-dimensional, Leeds Physical Model of MESFETs and HEMTs, has been described by the au- thors elsewhere [4]. Application of the resulting fully physical electro-thermal model to optimisation of power transistor and MMIC design, by transient simulation, is described by David et al., (this conference, [5]). In contrast, the aim of this paper is to apply the ther- mal impedance matrix model to examine design rules for power devices embedded in amplifier circuits. This is achieved by electro-thermal harmonic balance and tran- sient simulation, particularly demonstrating thermal ef- fects on multi-tone intermodulation distortion and spec- tral regrowth. These represent an essential aspect of device optimisation for narrowband digital modulation applications such as CDMA for mobile communications. The ultimate intended application of the modelling capa- bility described here, is study and design of spatial power combining systems for use as high power sources at mil- limeter wavelengths. The thermal model is therefore vali- dated by thermal imaging of passive grid arrays represen- tative of one form of quasi-optical system architecture. II. Results Figs. 1–3 demonstrate validation of the thermal model, by comparison of simulation against thermal imaging, for a passive grid array representative of spatial power com- bining architectures. Figs. 4 and 5, demonstrate that fully physical prediction of device performance can be achieved by combining the thermal resistance matrix ap- proach with the Leeds Physical Model, and can be used for electro-thermal optimisation of device design. Fig. 6 depicts the simulated amplifier used for study of ther- mal effects on MESFET performance. Figs. 7 and 8 il- lustrate single-tone harmonic balance (HB) simulation. Figs. 9 and 10 illustrate two-tone HB, demonstrating in- termodulation distortion due to amplifier non linearity. Due to thermal inertia, thermal response is seen to be much greater at the 1 MHz difference frequency than at the ∼1 GHz fundamental frequencies. Figs. 11 and 12 il- lustrate the impact of thermal effects on multi-tone IMD. References [1] W. Batty, A. J. Panks, R. G. Johnson and C. M. Snowden, ‘Electro-thermal modelling and measurement for spatial power combining at millimeter wavelengths,’ IEEE Trans. Microwave Theory Tech., vol. 47, no. 12, pp. 2574–2585, 1999. [2] W. Batty, A. J. Panks, R. G. Johnson and C. M. Snowden, ‘Electro-thermal modelling of monolithic and hybrid microwave and millimeter wave IC’s,’ VLSI Design, vol. 10, no. 4, pp. 355–389, 2000. [3] W. Batty, C. E. Christoffersen, S. David, A. J. Panks, R. G. Johnson and C. M. Snowden, ‘Steady-state and transient electro-thermal simulation of power devices and circuits based on a fully physical thermal model,’ THERMINIC 2000, Bu- dapest, September 2000. [4] S. David, W. Batty, A. J. Panks, R. G. Johnson and C. M. Snowden, ‘Fully physical coupled electro-thermal modelling of transient and steady-state behaviour in microwave semiconduc- tor devices,’ 8th Gallium Arsenide Application Symp. (GAAS 2000), paper GAAS/P1 (3), Paris, October 2000. [5] S. David, W. Batty, A. J. Panks, R. G. Johnson and C. M. Snowden, ‘Electro-thermal modelling of microwave transistors and MMICs for optimised transient and steady-state perfor- mance,’ submitted to this conference, EDMO 2000.
Transcript of Predictive microwave device design by coupled electro ... · University of Leeds, Leeds LS2 9JT,...
Predictive microwave device design by coupled electro-thermalsimulation based on a fully physical thermal model
W. Batty×+, C. E. Christoffersen∗, S. David×,A. J. Panks×, R. G. Johnson×, C. M. Snowden× and M. B. Steer∗,
×Institute of Microwaves and Photonics, ∗Dept. of Electrical and Computer Engineering,School of Electronic and Electrical Engineering, North Carolina State University (NCSU),
University of Leeds, Leeds LS2 9JT, UK. Raleigh, NC 27695-7914, USA.+Contact: tel. +44 113 2332089, fax +44 113 2332032, e-mail [email protected].
AbstractCoupled electro-thermal simulations are performed todemonstrate predictive design of microwave devices.These simulations are based on an original, fully physical,thermal impedance matrix approach, capable of describingcomplex 3-dimensional systems, whilst requiring no modelreduction for electro-thermal CAD. This thermal model isvalidated by thermal imaging of passive grid arrays repre-sentative of spatial power combining architectures. Fullyphysical simulations of power HEMT I-V characteristicsare presented, by coupling the thermal resistance matrixapproach to the Leeds Physical Model of MESFETs andHEMTs. Single-tone, two-tone and multi-tone harmonicbalance simulations are presented for a MESFET ampli-fier, by implementing the thermal impedance matrix ap-proach in microwave circuit simulator, Transim (NCSU).
I. Introduction
The importance of self-heating, and of mutual thermalinteraction between active elements, in power devicesand circuits, is well known. Hence predictive design,based on electro-thermal simulation, has been pursuedfor over 30 years. Until recently, the state of the artin time-independent thermal simulation of power FETsand MMICs, for coupled electro-thermal CAD, has beenrepresented by the hybrid finite element Green’s functionapproach of Bonani et al. This approach treated devicestructure such as surface metallisation, vias and partialsubstrate thinning. In time-dependent coupled electro-thermal CAD, thermal descriptions have been more prim-itive, and limited to simple rectangular multilayers. Workin this area is exemplified by that of Szekely et al., whogenerated a large range of semi-analytical thermal solu-tions for ICs, MCMs and microsystem elements. A vari-ety of thermal model reduction techniques have also beenemployed to make tractable the coupled electro-thermalsimulation problem on CAD timescales. The problem ofcompact model development for thermal networks is cur-rently a highly active area of research.The authors have progressed the state of the art, by pre-senting a thermal resistance matrix model capable of de-scribing, nearly exactly, 3-dimensional time-independentheat flow in complex device structures, in a fashion com-patible with electro-thermal CAD [1], [2]. This modelincludes description of surface fluxes and treats all ther-mal non linearities. More recently, the authors havepresented a thermal impedance matrix model, extendingthis description to treat the thermal time-dependent case,both steady-state and transient [3]. This time-dependentformulation allows interpretation of thermal subsystemsin terms of generalised multiport network parameters,without requiring model reduction. Circuit level electro-thermal simulation was achieved by implementation ofthis thermal impedance matrix model in a network basedmicrowave field and circuit simulator, Transim (NCSU).Fully physical electro-thermal device CAD, based on cou-pling of the thermal impedance matrix approach, tothe rapid, quasi-2-dimensional, Leeds Physical Model ofMESFETs and HEMTs, has been described by the au-thors elsewhere [4]. Application of the resulting fully
physical electro-thermal model to optimisation of powertransistor and MMIC design, by transient simulation,is described by David et al., (this conference, [5]). Incontrast, the aim of this paper is to apply the ther-mal impedance matrix model to examine design rules forpower devices embedded in amplifier circuits. This isachieved by electro-thermal harmonic balance and tran-sient simulation, particularly demonstrating thermal ef-fects on multi-tone intermodulation distortion and spec-tral regrowth. These represent an essential aspect ofdevice optimisation for narrowband digital modulationapplications such as CDMA for mobile communications.The ultimate intended application of the modelling capa-bility described here, is study and design of spatial powercombining systems for use as high power sources at mil-limeter wavelengths. The thermal model is therefore vali-dated by thermal imaging of passive grid arrays represen-tative of one form of quasi-optical system architecture.
II. Results
Figs. 1–3 demonstrate validation of the thermal model, bycomparison of simulation against thermal imaging, for apassive grid array representative of spatial power com-bining architectures. Figs. 4 and 5, demonstrate thatfully physical prediction of device performance can beachieved by combining the thermal resistance matrix ap-proach with the Leeds Physical Model, and can be usedfor electro-thermal optimisation of device design. Fig. 6depicts the simulated amplifier used for study of ther-mal effects on MESFET performance. Figs. 7 and 8 il-lustrate single-tone harmonic balance (HB) simulation.Figs. 9 and 10 illustrate two-tone HB, demonstrating in-termodulation distortion due to amplifier non linearity.Due to thermal inertia, thermal response is seen to bemuch greater at the 1 MHz difference frequency than atthe ∼1 GHz fundamental frequencies. Figs. 11 and 12 il-lustrate the impact of thermal effects on multi-tone IMD.
References
[1] W. Batty, A. J. Panks, R. G. Johnson and C. M. Snowden,‘Electro-thermal modelling and measurement for spatial powercombining at millimeter wavelengths,’ IEEE Trans. MicrowaveTheory Tech., vol. 47, no. 12, pp. 2574–2585, 1999.
[2] W. Batty, A. J. Panks, R. G. Johnson and C. M. Snowden,‘Electro-thermal modelling of monolithic and hybrid microwaveand millimeter wave IC’s,’ VLSI Design, vol. 10, no. 4, pp.355–389, 2000.
[3] W. Batty, C. E. Christoffersen, S. David, A. J. Panks, R.G. Johnson and C. M. Snowden, ‘Steady-state and transientelectro-thermal simulation of power devices and circuits basedon a fully physical thermal model,’ THERMINIC 2000, Bu-dapest, September 2000.
[4] S. David, W. Batty, A. J. Panks, R. G. Johnson and C. M.Snowden, ‘Fully physical coupled electro-thermal modelling oftransient and steady-state behaviour in microwave semiconduc-tor devices,’ 8th Gallium Arsenide Application Symp. (GAAS2000), paper GAAS/P1 (3), Paris, October 2000.
[5] S. David, W. Batty, A. J. Panks, R. G. Johnson and C. M.Snowden, ‘Electro-thermal modelling of microwave transistorsand MMICs for optimised transient and steady-state perfor-mance,’ submitted to this conference, EDMO 2000.
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Fig. 1. Central temperature rise with time at turn-on, of a 10×10passive grid array dissipating 2 W. Substrates: FR-4 (experi-ment and simulation) and AlN (simulation). Cooling is purelyby radiation and convection (no heatsink mounting). Agree-ment between theory and experiment is good, validating thethermal model.
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Temperature (K)Fig. 2. Time dependent Inframetrics ThermaCam 280 measure-
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Temperature (K)Fig. 3. Time dependent thermal simulations of the 10× 10 passive
grid array: FR-4, 5 × 5 cm2, dissipating 2 W. Cooling purelyby surface radiation and convection (no heatsink mounting).Agreement between theory and experiment is good, validatingthe thermal model.
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10-finger: GaAs die + 6um metal
Fig. 4. Electro-thermally simulated I-V characteristics of a 10-finger Filtronic power HEMT with 6 µm surface metallisationand adiabatic surface boundary conditions. Finger width is 400µm; substrate thickness is 75 µm; heatsink temperature is 300K. Thermal droop is predicted fully physically, based solely onspecification of device structure and material parameters.
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Fig. 5. Electro-thermally simulated interface temperature plot ofthe 10-finger Filtronic power HEMT with 6 µm surface met-allisation and no convective surface fluxes. Finger width is 400µm; substrate thickness is 75 µm; heatsink temperature is 300K. Bias point is (VDS , VG) = (3.5 V, -0.2 V); power dissipationis 2.8 W. Temperature varies from 27 ◦C to 67 ◦C. Tempera-ture variation along and between gate fingers implies spectralregrowth.
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Vbias
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Fig. 6. Schematic of the simulated MESFET amplifier with thermalcircuit. The MESFET is described by the Curtice-Ettenberg cu-bic model with symmetric diodes and capacitances. The MMICdie is described by thermal 1-port network parameters.
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Fig. 7. Drain-source current ids (solid line) and drain-source voltagevds (dashed line) for a 5-finger power transistor, from single-tone HB analysis with fundamental frequency 10 MHz.
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Fig. 8. Power dissipation (solid line) and temperature variation(dashed line) for the 5-finger power transistor, from single-toneHB analysis with fundamental frequency 10 MHz.
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Fig. 9. Power dissipation in the frequency domain for a 50-fingerpower transistor, from two-tone HB analysis with fundamentalfrequency 1 GHz and difference frequency 1 MHz.
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Fig. 10. Temperature variation in the frequency domain for the 50-finger power transistor, from two-tone HB analysis with funda-mental frequency 1 GHz and difference frequency 1 MHz.
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Fig. 11. Current variation in the frequency domain for the 50-finger power transistor, from multi-tone HB analysis with 11fundamentals at frequency 0.5 GHz and difference frequencysteps of 0.5 MHz.
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Fig. 12. Current variation in the frequency domain for the 50-finger power transistor, from multi-tone HB analysis with 11fundamentals at frequency 0.5 GHz and difference frequencysteps of 0.5 MHz. Circles: without thermal effects. Crosses:with thermal effects.
