- Email: [email protected]
GaN HEMTs with metal airbridges
Solid-State Electronics 51 (2007) 969–974 www.elsevier.com/locate/sse
Transient self-heating effects in multifinger AlGaN/GaN HEMTs with metal airbridges J. Kuzmik
a,f,*
, S. Bychikhin a, R. Lossy b, H.-J. Wu¨rfl b, M.-A. di Forte Poisson c, J.-P. Teyssier d, C. Gaquie`re e, D. Pogany a
a Institute for Solid-State Electronics, TU Vienna, Floragasse 7, A-1040 Vienna, Austria Ferdinand-Braun-Institut fuer Hoechstfrequenztechnik, Gustav-Kirchhoff-Strasse 4, 12489 Berlin, Germany c Alcatel-Thales III-V Lab/TIGER, 91404 Orsay, France d IRCOM CNRS University of Limoges, 7 rue Jules Valles, 19100 Brive, France e IEMN/III-V Lab/TIGER, av. Poincare´, BP 69, 59652 Villeneuve d’Ascq, France Institute of Electrical Engineering, Slovak Academy of Science, Dubravska cesta 9, 842 39 Bratislava, Slovakia b
f
Received 20 December 2006; accepted 10 April 2007 Available online 23 May 2007
The review of this paper was arranged by Prof. E. Calleja
Abstract Thermal behavior of a multifinger AlGaN/GaN/sapphire HEMT with a metal airbridge connecting five source contacts is investigated in the transient state using optical and electrical methods and using a 2D thermal model. The gate layout consists of eight fingers, each having 50 lm width/0.3 lm length, the pitch is 35 lm and the thickness of the electroplated airbridge is 7 lm. The device drain contact was pulsed by 10 ls long 10 V pulses, corresponding to 11.9 W/mm dissipating power density. The electrical characterization method shows that at the end of the pulse the temperature increase in the HEMT channel is 185 K while the transient interferometric mapping (TIM) optical method indicates that the airbridge structure serves also as a cooler removing approximately 10% of the heat energy. Surface temperature maps are constructed by using the TIM for a time window of 2–6 ls. An asymmetry in the temperature profiles was observed, e.g. the source contact was colder by 25% than the drain contact at t = 6 ls. 2007 Elsevier Ltd. All rights reserved. Keywords: AlGaN/GaN HEMT; Selfheating; Thermal characterization
1. Introduction Thermal management of high-power AlGaN/GaN high electron mobility transistors (HEMTs) is one of the main concerns of device designers. Various thermal aspects have been discussed in literature in order to reduce temperature in the active area: the selection of the proper substrate material for heat sinking [1–3], the thermal coupling at the substrate/GaN [3–5] or at the GaN/Au interfaces [5], * Corresponding author. Address: Institute for Solid-State Electronics, TU Vienna, Floragasse 7, A-1040 Vienna, Austria. Tel.: +43 1 58801 45123; fax: +43 1 58801 362 99. E-mail address: [email protected] (J. Kuzmik).
0038-1101/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2007.04.001
the device topology [6], and the flip-chip bonding [7–9]. It was shown that the heat removal either through the epoxy fill [7] or through properly designed bumps [8] of the flipchip mounted multifinger AlGaN/GaN/sapphire HEMTs can effectively decrease the device thermal impedance without using expensive SiC substrate. Most of the studies concentrated on DC thermal analysis [1–3,6]. Only recent papers report on transient thermal analysis [4,9]. However only single finger HEMT devices were investigated in the transient state, even though this type of investigation is urgently needed also for multifiger HEMT devices. In this work we investigate the transient temperature rise in multifinger AlGaN/GaN/sapphire HEMTs subjected to 10 ls voltage pulses. In particular we investigate
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the role of the airbridge structure in the device thermal management. The airbridge technology is used in highpower HEMTs to connect parallel source contacts in a multifinger layout. We use the transient interferometric mapping (TIM) [10,11] and electrical experimental methods [4] combined with a 2D thermal model to analyze the transient temperature increase and thermal cross talk between the fingers. We investigate the role of the airbridge metal in the device thermal management. 2. Devices and methods Samples were grown on 2-in. c-axis sapphire substrate by metal organic chemical vapor deposition. The epi-layers consist of (from the bottom) nucleation layer, 3 lm thick GaN buffer layer, 30 nm thick unintentionally doped AlGaN and 3 nm undoped GaN. The Al-content was 21%. The devices were processed using standard GaN HFET process [12]. The process is based on stepper and e-beam lithography, mesa isolation techniques of the active regions, Ti/Al/Ti/Au/WSiNx ohmic contacts with Ti/Au overlay, Pt/Ti/Au gate structures and SiNx passivation of the active device region. The source–drain distance is 2.5 lm. Finally electroplated air bridges (thickness 7 lm) completed the process. We used Agilent Technologies 8114 A programmable pulser to apply rectangular voltage pulses between the source and drain contacts of the HEMT. The current and voltage waveforms were recorded using a digital oscilloscope. To investigate heat dissipation we use the TIM technique [10,11], where the device is scanned (synchronized with voltage pulses) from the polished backside using an infrared laser beam (see Fig. 1). The phase shift Du of the probe beam reflected from the topside is caused by a temperature-induced change in the semiconductor refractive index n along the beam path and is proportional to dissipated energy in the device [10]. The scanning Du(x) signal provides thus a heat map of the device. Moreover, TIM method allows also to extract the instantaneous 2D power density P2D(x) distribution [11]. The P2D(x) map can localize the position of the heating source. If the device structure consists of different materials with various thermo-optical parameters (such as GaN on sapphire), P2D extraction leads to values of apparent 2D power density P2DA [4]. The qualitative meaning of P2DA is the same as for P2D, however the quantitative values differs since P2DA is influenced by the heterogeneity of the system [4]. The HEMT sample is scanned with 0.5 or 1 lm steps perpendicularly to the structure width and the heat spreading is obtained at selected time instants. Matlab toolbox was used for 2D numerical simulations of thermal effects. The dimension along the device gate width was neglected in the model. The toolbox finite element solver in conjunction with fully automated mesh generator provides an easy way to simulate the heat transport equation for device under transient conditions. To simplify the mesh generation, thickness of the sapphire substrate
Fig. 1. Backside infrared camera image of the multifinger AlGaN/GaN HEMT. Laser beam is visible in the center, scanning was performed along the x-axes.
was assumed to be 70 lm that covers the heat penetration depth during the 10 ls long pulse. The thermal conductivity values k were taken to be temperature independent. Adiabatic boundary conditions were assumed for calculations in the transient mode. The phase shift is calculated as a sum of two contributions arising from the GaN layer and sapphire substrate, similarly as in [4]. The HEMT channel temperature Tchannel (i.e. the temperature at the GaN surface Tsurface at the position of the HEMT channel) is determined also with a help of electrical method [4]. Initially a dependence of the source resistance RS on temperature T is independently calibrated. After extracting a self-heating induced current drop DIDS from the pulsed HEMT characteristics, we determine DTchannel(t) from the calibrated RS(T) and measured DIDS(t) dependencies, using the analytical expression DIDS(t) = gmIDSDRS, where gm is the HEMT transconductance and DRS the source resistance change [4]. 3. Results and discussions Fig. 2 shows the IDS(t) and VDS(t) waveforms of the stressed device. A 10 V pulse has been applied on the drain for 10 ls. A continuous rise in DIDS = IDS(0) IDS(t > 0) with time can be observed. To analyze its origin we performed stresses also for 4 and 19 V (of different duration), shown in the inset. It has been shown previously that traps on the device interfaces can influence IDS if the drain contact is subjected to pulses with >10 ls duration (the socalled drain lag phenomena) [13]. Previous measurements on devices grown on SiC, where the self-heating can be neglected, have shown the drain-lag effect was independent on VDS [13]. On the other hand in our case (GaN on sapphire), we obtained clear increase of DIDS with VDS, with almost DIDS = 0 for VDS = 4 V. This dependence clearly indicates the self-heating effect. Also for VDS = 19 V we observed a significant DIDS increase already for t as short as tens of ns, and that is a much shorter time scale than that expected and calculated for the electrical transients of the drain lag phenomena [13]. Therefore our results indicate
J. Kuzmik et al. / Solid-State Electronics 51 (2007) 969–974
Fig. 2. Current and voltage waveforms of the 10 ls long, 10 V pulse applied on the HEMT drain contact. Inset shows source–drain current drops DIDS for different pulse voltage levels and different pulse duration.
that for the case of our 10 ls pulses the drain lag effect can be neglected and we consider only thermal effects. Fig. 3 shows the evolution of phase shift profiles taken during a 10 ls/10 V pulse at t = 2, 6 and 10 ls, and after the pulse at t = 30 ls. As expected, Du(x) signal increases during the pulse due to heating and decreases after the pulse due to cooling, reflecting thus the heat dynamics in the device. The spreading of the peaks with time can also be observed due to lateral heat diffusion. Remarkable is the difference in the evolution of the signal magnitude in the valleys marked by arrows A, B (Fig. 3). At position ‘A’, a source contact with the electroplated thick gold metal on it is localized, while an ordinary drain contact is located at ‘B’. At t = 2 ls one observes nearly the same small amplitude DuB DuA, however for t > 2 ls the phase shift at the ordinary drain contact is higher DuB > DuA. The difference vanishes after the pulse is terminated (t = 30 ls). The trend of DuB > DuA can be clearly visible in Fig. 4 showing normalized TIM profiles at t = 2 ls and 10 ls. To interpret this result we take into account that
Fig. 3. Phase shift evolution (left side vertical scale) and extracted surface temperature increase (right side scale) profiles of the AlGaN/GaN HEMT stressed by 10 V/10 ls drain pulse. The temperature scale holds only for the data at t = 2 and 6 ls, the dashed line depicts phase shift signal after the pulse at t = 30 ls. Positions A mark the signal minimum at the source contacts with airbridge connections, positions B mark the signal at the ordinary drain contacts.
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Fig. 4. Normalized phase shift profiles of the HEMT of Fig. 3 at t = 2 and 10 ls.
the phase shift represents only the heat energy in the semiconductor and the substrate, but not that which is transferred from semiconductor to metallization [10]. Then the deeper valleys at position ‘A’ originate from a better heat sinking at the location of airbridges compared to the ordinary drain contact. This effect is negligible for the short time instants (t < 2 ls), and becomes more visible at longer time when the heat penetrates more effectively from the active region to the thick metal airbridge. To estimate the amount of the heat transferred via the airbridge at t = 10 ls, we calculate the difference of ‘‘areas’’ SA and SB below the Du signal at contacts, which are proportional to the total dissipated heat [10], see Fig. 5. The difference of areas was approx. 1.35 lm rad. Taking into account the total area STOT 68 lm rad and that the number of airbridge contacts is five, the airbridge cooling efficiency g i.e. the percentage of the total heat transferred from the device to the airbridge can be estimated as g = 5x(SB SA)/STOT 10%. A similar ratio of 10% was obtained also at t = 6 ls, but 3% at t = 2 ls. Thus it seems that after the initial increase, the cooling efficiency g of the airbridge contacts saturates. The saturation can be explained by merging of heat waves (Du signals) from neighboring channels at t 6 ls. A prerequisite for the proper thermal modeling is the knowledge of the size of the heating source. The size of
Fig. 5. Phase shift profiles of the HEMT of Fig. 3 at t = 10 ls with marked areas SA and SB below the source and the drain contacts.
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Fig. 6. Determination of the size of the HEMT heating source (single finger) using normalized P2DA obtained from TIM for different drain voltage values of 3, 4, 9 and 20 V. The size is approximated as a full width at half maximum. Channel (source–drain) area is marked by a solid rectangle. Gate was on a floating potential during the pulse.
Fig. 7. Comparisons of the experimental phase shift profiles of the HEMT of Fig. 3 with the simulations considering the heat removal through the airbridge. A two-side arrow marks the simulated area, which is mirrored for three channels. Heating sources assumed in the model are marked by solid rectangles.
the heating source LH is obtained from the analysis of the instantaneous power dissipation density P2DA [4] extracted from the Du(x) distribution. Fig. 6 shows P2DA(x) distribution in a one finger for various stress levels VDS = 3, 4 and 9 V, normalized to the maximal signal. LH was determined as a full width at half maximum. In contrary to the Du(x) time evolution (see Fig. 3), P2DA is almost constant with time as expected (not shown), however we observed a sharp reduction of LH as VDS increases from 3 to 4 V. This was because of the transition from the linear regime of the HEMT into the saturation. In the former case, large part of the VDS is dropped on the contact areas and the heat source is broad (LH 35 lm). On the other hand in the saturation (VDS > 3 V) a high-field domain is formed in the channel and thus LH is reduced to 16 lm for VDS = 4 V and to 11 lm for 9 V, respectively. No further LH reduction was observed for VDS = 20 V. We point out that even for the current saturation we observe that the heat dissipation region is broader than the gate-drain region where highfield domain is mostly expected. That is probably linked to the fact that no voltage was applied on the gate (floating gate). From the above we concluded that the heat dissipation region has a size of 11 lm which was further used in the 2D thermal modeling. For modeling of thermal properties of AlGaN/GaN heterostructure we used similar approach as in [4], but now instead of Si a sapphire substrate was considered. A thermal boundary resistance (TBR) of 1 · 10 7 K m2/W at the GaN/sapphire interface was assumed [3]. We used thermal conductivity of GaN and sapphire layer to be kGaN = 130 W/m K [14] and ksap = 30 W/m K [14], respectively. The thermo-optical coefficient of sapphire (dn/dT)sap = 1.3 · 10 5 K 1 was taken from [15] and the (dn/dT)GaN was taken as a fitting parameter. In the model we considered a 7 lm thick Au-based airbridge structure connected to the source contact, see the schematics of Fig. 7. We considered TBR at the GaN/metal interface to be 7 · 10 8 K m2/W [5]. A 0.5 lm thick combination of alloyed ohmic contact and sputtered barrier
system between the airbridge and GaN having kcontact 3 W/m K has also been taken into account [5]. In the simulation we used a translation symmetry of the system and the Neumann boundary condition to reduce the simulation area (see Fig. 7). The latter simplification is adequate for time scale = 10 ls and for the inner fingers where the system can be considered as adiabatic. Fig. 7 shows the calculated Du(x) distribution for three fingers obtained by mirroring the simulation data (dashed lines). A good coincidence between the experiments (solid lines) and simulation was obtained for (dn/dT)GaN = 4 · 10 5 K 1. The asymmetry in the Du(x) profiles in Fig. 7 is clearly visible, showing that the heat absorbed by the airbridge metal explains properly the lowering of the phase shift. Fig. 8 shows the calculated temperature profiles in the HEMT cross-section at the position of the channel. We see DTchannel 175 K at t = 10 ls on the GaN surface (depth z = 0 lm) and a considerable discontinuity at the GaN/sapphire interface due to TBR (z = 3 lm). Calculations indicate that if the airbridge is not considered, an additional temperature increase of DTchannel (10 ls) =
Fig. 8. Calculated temperatures in the HEMT cross-section at the position of the channel. GaN/sapphire interface is at z = 3 lm. Condition of the pulse is the same as for Fig. 3.
J. Kuzmik et al. / Solid-State Electronics 51 (2007) 969–974
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Fig. 9. Calibrated dependence of the HEMT source resistance on temperature.
change in the electron saturation velocity [4]) and the 2D thermal model. We note that the electrical method gives the temperature solely in the source-gate area and not in the whole source–drain channel, however due to significant lateral heat spreading during ls long pulses (Fig. 7) and due to observed size of the heating source (Fig. 6) this difference may be neglected. In the rest of the paper we present the approach how to determine temperature maps DTsurface(x) from the combination of TIM measurements and the knowledge of the DTchannel value extracted by the electrical method. We first relate the phase shift Du(x) to the GaN surface temperature DTsurface at the particular time instant. Fig. 11 shows a DTsurface(x)/Du(x) ratio at different time instants calculated by the 2D model. DTsurface(x)/Du(x) reflects the heat diffusion and distribution in the lateral and vertical directions. The profiles for t 2–6 ls are the most flat and the difference between the peak and the valley of the DTsurface(x)/ Du(x) distribution is <15%. Consequently if DTchannel is known in this time scale, the temperature profile maps can be obtained from TIM phase measurements, assuming a constant DTsurface(x)/Du(x) = DTchannel/Duchannel, i.e. DTsurface(x) = Du(x)xDTchannel/Duchannel, where Duchannel is taken as the TIM signal at the position of the channel. The DTchannel/Duchannel value is dependent on time, and on layout and material parameters of a particular device. Taking into account our results of the DTchannel determination (see Fig. 10) and TIM experiments (see Fig. 3, left Du scale) and provided that DTchannel/Duchannel 73 K/rad at both t = 2 and 6 ls, the phase shift scale of Fig. 3 can be directly transformed into the temperature scale (see right scale, consider only the data at t = 2 and 6 ls). At t = 6 ls one can observe a 25 K difference between the temperature in the center of the source (position A) and the drain (position B) contacts, representing a 25% change. We note that even though our presented experimental method of Du(x) to DT(x) conversions are justified only for a specific time windows, we provide an insight into thermal behavior of the multifinger GaN-based HEMTs in the regime of the pulsed operation.
Fig. 10. Calculated and experimental time evolution of the temperature increase in the channel of HEMT during the 10 V pulse.
Fig. 11. Calculated DTsurface/Du ratio profiles in the HEMT at different time instants. Condition of the pulse is same as for Fig. 3. Heating sources are marked by solid rectangles.
10 K would occur. We point out that the cooling efficiency g of the airbridge structure may be different for different device structures and may depend on many factors like gate pitch, thickness of the electroplated gold, etc. It was reported elsewhere that kGaN drops to 90 W/m K at 500 K [16]. To estimate the error of our model introduced by assumption of the constant kGaN we run also additional simulation assuming kGaN = 100 W/m K. This calculation showed additional 7% temperature increase at the end of the pulse if compared with the case of kGaN = 130 W/m K. The results of optical experiments and the 2D thermal model are further compared with the results of the electrical method for temperature determination. Fig. 9 shows a RS(T) dependence used for calibration. From the I–V waveforms shown in Fig. 2 and applying the procedure described elsewhere [4] we further extracted DTchannel(t) dependence (see Fig. 10). The experimental curve is in satisfactory agreement with the simulated transient showing 185 K temperature increase at the end of the pulse. The discrepancy between the model and the experiment can be explained by a non-ideality of the rectangular voltage pulse (see Fig. 2) as well as by simplifications introduced both by the experimental method (10% error due to the
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4. Conclusions We studied transient self-heating effects in the multifinger AlGaN/GaN/sapphire HEMT with a gate pitch of 35 lm and the airbridge structure connecting sources. For a dissipated power of 11.9 W/mm we determined a temperature rise 185 K in the HEMT channel at t = 10 ls. The simulation model matches well the experimental data. We have revealed that the airbridge structure absorbs the heat and the HEMT thermal design may be optimized for a given pulsed regime. Cooling efficiency of the airbridge structure increases with the time and saturates at 6 ls taking away a 10% of the heat. As a result, the temperature in the center of the source contacts at t = 6 ls is about 25% lower in comparison with the drain contact and for the best cooling performance we propose to increase the gate pitch with the pulse duration. We have also proposed the approach how to calculate the temperature distribution in the HEMT using the thermo-optical measurements and the electrical characterization. Acknowledgement This work was supported by TARGET – ‘‘Top Amplifier Research Groups in a European Team’’, and by ULTRAGAN – ‘‘InAlN/(In)GaN Heterostructure Technology for Ultra-High Power Microwave Transistors’’ projects of the Information Society Technologies Program of the EU under contracts IST-1-507893-NOE and No. 006903, respectively. References [1] Gaska R, Osinsky A, Yang JW, Shur MS. Self-heating in high-power AlGaN–GaN HFET’s. IEEE Electron Devices Letters 1998;19(3): 89–91. [2] Kuzmik J, Javorka P, Alam A, Marso M, Heuken M, Kordosˇ P. Determination of channel temperature in AlGaN/GaN HEMTs grown on sapphire and silicon substrates using DC characterization method. IEEE Transactions on Electron Devices 2002;49(8):1496–8. [3] Turin VO, Balandin AA. Performance degradation of GaN fieldeffect transistors due to thermal boundary resistance at GaN/ substrate interface. Electronics Letters 2004;40(1):81–3.
[4] Kuzmik J, Bychikhin S, Neuburger M, Dadgar A, Krost A, Kohn E, et al. Transient thermal characterization of AlGaN/GaN HEMTs grown on silicon. IEEE Transactions on Electronic Devices 2005; 52(8):1698–705. [5] Zhao Y, Zhu Ch, Wang S, Tian JZ, Yang DJ, Chen CK, et al. Pulsed photothermal reflectance measurement of the thermal conductivity of sputtered aluminium nitride thin films. Journal of Applied Physics 2004;96(8):4563–8. [6] Kuball M, Rajasingam S, Sarua A, Uren MJ, Martin T, Hughes BT, et al. Measurement of temperature distribution in multifinger AlGaN/GaN heterostructure field-effect transistors using microRaman spectroscopy. Applied Physics Letters 2003;82(1):124–6. [7] Sun J, Fatima H, Koudymov A, Chitnis A, Hu X, Wang M, et al. Thermal management of AlGaN–GaN HFETs on sapphire using flip-chip bonding with epoxy underfill. IEEE Electron Devices Letters 2003;24(6):375–7. [8] Das J, Oprins H, Ji H, Sarua A, Ruythooren W, Derluyn J, et al. Improved thermal performance of AlGaN/GaN HEMTs by an optimized flip-chip design. IEEE Transactions on Electron Devices 2006;53(11):2696–702. [9] Kuball M, Riedel G, Pomeroy JW, Sarua A, Uren MJ, Martin T, et al. Time-resolved temperature measurement of AlGaN/GaN electronic devices using micro-raman spectroscopy. IEEE Electron Devices Letters 2007;28(2):86–9. [10] Pogany D, Bychikhin S, Fu¨rbo¨ck Ch, Litzenberger M, Gornik E, Groos G, et al. Quantitative internal thermal energy mapping of semiconductor devices under short current stress using backside laser interferometry. IEEE Transations on Electron Devices 2002;49(11): 2070–8. [11] Pogany D, Bychikhin S, Litzenberger M, Groos G, Stecher M. Extraction of spatio-temporal distribution of power dissipation in semiconductor devices using nanosecond interferometric mapping technique. Applied Physics Letters 2002;81(15):2881–3. [12] Hilsenbeck J, Lenk F, Lossy R, Wu¨rfl J, Ko¨hler K, Obloh H. DC and Microwave Properties of 0.5 lm AlGaN/GaN High Electron Mobility Transistors Fabricated on 2-inch Line. Proceedings of the IEEE 27th International Symposium on Compound Semiconductors held in Monterey, California, October 2–5, 2000, pp. 351–356. [13] Meneghesso G, Verzellesi G, Pierobon R, Rampazzo F, Chini A, Mishra UM, et al. Surface-related drain current dispersion effects in AlGaN–GaN HEMTs. IEEE Transactions on Electron Devices 2004;51(10):1554–61. [14] Morkoc H. Nitride semiconductors and devices. Springer Series in Materials Science. Berlin Heidelberg New York: Springer-Verlag; 1999, p. 44. [15] Thomas ME, Andersson SK, Sova RM, Joseph RI. Frequency and temperature dependence of the refractive index of sapphire. Infrared Physics & Technology 1998;39:235–49. [16] Liu W, Balandin AA. Thermal conduction in AlxGa1 xN alloys and thin films. Journal of Applied Physics 2005;97(7):073710.
Transient self-heating effects in multifinger AlGaN/GaN HEMTs with metal airbridges J. Kuzmik
a,f,*
, S. Bychikhin a, R. Lossy b, H.-J. Wu¨rfl b, M.-A. di Forte Poisson c, J.-P. Teyssier d, C. Gaquie`re e, D. Pogany a
a Institute for Solid-State Electronics, TU Vienna, Floragasse 7, A-1040 Vienna, Austria Ferdinand-Braun-Institut fuer Hoechstfrequenztechnik, Gustav-Kirchhoff-Strasse 4, 12489 Berlin, Germany c Alcatel-Thales III-V Lab/TIGER, 91404 Orsay, France d IRCOM CNRS University of Limoges, 7 rue Jules Valles, 19100 Brive, France e IEMN/III-V Lab/TIGER, av. Poincare´, BP 69, 59652 Villeneuve d’Ascq, France Institute of Electrical Engineering, Slovak Academy of Science, Dubravska cesta 9, 842 39 Bratislava, Slovakia b
f
Received 20 December 2006; accepted 10 April 2007 Available online 23 May 2007
The review of this paper was arranged by Prof. E. Calleja
Abstract Thermal behavior of a multifinger AlGaN/GaN/sapphire HEMT with a metal airbridge connecting five source contacts is investigated in the transient state using optical and electrical methods and using a 2D thermal model. The gate layout consists of eight fingers, each having 50 lm width/0.3 lm length, the pitch is 35 lm and the thickness of the electroplated airbridge is 7 lm. The device drain contact was pulsed by 10 ls long 10 V pulses, corresponding to 11.9 W/mm dissipating power density. The electrical characterization method shows that at the end of the pulse the temperature increase in the HEMT channel is 185 K while the transient interferometric mapping (TIM) optical method indicates that the airbridge structure serves also as a cooler removing approximately 10% of the heat energy. Surface temperature maps are constructed by using the TIM for a time window of 2–6 ls. An asymmetry in the temperature profiles was observed, e.g. the source contact was colder by 25% than the drain contact at t = 6 ls. 2007 Elsevier Ltd. All rights reserved. Keywords: AlGaN/GaN HEMT; Selfheating; Thermal characterization
1. Introduction Thermal management of high-power AlGaN/GaN high electron mobility transistors (HEMTs) is one of the main concerns of device designers. Various thermal aspects have been discussed in literature in order to reduce temperature in the active area: the selection of the proper substrate material for heat sinking [1–3], the thermal coupling at the substrate/GaN [3–5] or at the GaN/Au interfaces [5], * Corresponding author. Address: Institute for Solid-State Electronics, TU Vienna, Floragasse 7, A-1040 Vienna, Austria. Tel.: +43 1 58801 45123; fax: +43 1 58801 362 99. E-mail address: [email protected] (J. Kuzmik).
0038-1101/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2007.04.001
the device topology [6], and the flip-chip bonding [7–9]. It was shown that the heat removal either through the epoxy fill [7] or through properly designed bumps [8] of the flipchip mounted multifinger AlGaN/GaN/sapphire HEMTs can effectively decrease the device thermal impedance without using expensive SiC substrate. Most of the studies concentrated on DC thermal analysis [1–3,6]. Only recent papers report on transient thermal analysis [4,9]. However only single finger HEMT devices were investigated in the transient state, even though this type of investigation is urgently needed also for multifiger HEMT devices. In this work we investigate the transient temperature rise in multifinger AlGaN/GaN/sapphire HEMTs subjected to 10 ls voltage pulses. In particular we investigate
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J. Kuzmik et al. / Solid-State Electronics 51 (2007) 969–974
the role of the airbridge structure in the device thermal management. The airbridge technology is used in highpower HEMTs to connect parallel source contacts in a multifinger layout. We use the transient interferometric mapping (TIM) [10,11] and electrical experimental methods [4] combined with a 2D thermal model to analyze the transient temperature increase and thermal cross talk between the fingers. We investigate the role of the airbridge metal in the device thermal management. 2. Devices and methods Samples were grown on 2-in. c-axis sapphire substrate by metal organic chemical vapor deposition. The epi-layers consist of (from the bottom) nucleation layer, 3 lm thick GaN buffer layer, 30 nm thick unintentionally doped AlGaN and 3 nm undoped GaN. The Al-content was 21%. The devices were processed using standard GaN HFET process [12]. The process is based on stepper and e-beam lithography, mesa isolation techniques of the active regions, Ti/Al/Ti/Au/WSiNx ohmic contacts with Ti/Au overlay, Pt/Ti/Au gate structures and SiNx passivation of the active device region. The source–drain distance is 2.5 lm. Finally electroplated air bridges (thickness 7 lm) completed the process. We used Agilent Technologies 8114 A programmable pulser to apply rectangular voltage pulses between the source and drain contacts of the HEMT. The current and voltage waveforms were recorded using a digital oscilloscope. To investigate heat dissipation we use the TIM technique [10,11], where the device is scanned (synchronized with voltage pulses) from the polished backside using an infrared laser beam (see Fig. 1). The phase shift Du of the probe beam reflected from the topside is caused by a temperature-induced change in the semiconductor refractive index n along the beam path and is proportional to dissipated energy in the device [10]. The scanning Du(x) signal provides thus a heat map of the device. Moreover, TIM method allows also to extract the instantaneous 2D power density P2D(x) distribution [11]. The P2D(x) map can localize the position of the heating source. If the device structure consists of different materials with various thermo-optical parameters (such as GaN on sapphire), P2D extraction leads to values of apparent 2D power density P2DA [4]. The qualitative meaning of P2DA is the same as for P2D, however the quantitative values differs since P2DA is influenced by the heterogeneity of the system [4]. The HEMT sample is scanned with 0.5 or 1 lm steps perpendicularly to the structure width and the heat spreading is obtained at selected time instants. Matlab toolbox was used for 2D numerical simulations of thermal effects. The dimension along the device gate width was neglected in the model. The toolbox finite element solver in conjunction with fully automated mesh generator provides an easy way to simulate the heat transport equation for device under transient conditions. To simplify the mesh generation, thickness of the sapphire substrate
Fig. 1. Backside infrared camera image of the multifinger AlGaN/GaN HEMT. Laser beam is visible in the center, scanning was performed along the x-axes.
was assumed to be 70 lm that covers the heat penetration depth during the 10 ls long pulse. The thermal conductivity values k were taken to be temperature independent. Adiabatic boundary conditions were assumed for calculations in the transient mode. The phase shift is calculated as a sum of two contributions arising from the GaN layer and sapphire substrate, similarly as in [4]. The HEMT channel temperature Tchannel (i.e. the temperature at the GaN surface Tsurface at the position of the HEMT channel) is determined also with a help of electrical method [4]. Initially a dependence of the source resistance RS on temperature T is independently calibrated. After extracting a self-heating induced current drop DIDS from the pulsed HEMT characteristics, we determine DTchannel(t) from the calibrated RS(T) and measured DIDS(t) dependencies, using the analytical expression DIDS(t) = gmIDSDRS, where gm is the HEMT transconductance and DRS the source resistance change [4]. 3. Results and discussions Fig. 2 shows the IDS(t) and VDS(t) waveforms of the stressed device. A 10 V pulse has been applied on the drain for 10 ls. A continuous rise in DIDS = IDS(0) IDS(t > 0) with time can be observed. To analyze its origin we performed stresses also for 4 and 19 V (of different duration), shown in the inset. It has been shown previously that traps on the device interfaces can influence IDS if the drain contact is subjected to pulses with >10 ls duration (the socalled drain lag phenomena) [13]. Previous measurements on devices grown on SiC, where the self-heating can be neglected, have shown the drain-lag effect was independent on VDS [13]. On the other hand in our case (GaN on sapphire), we obtained clear increase of DIDS with VDS, with almost DIDS = 0 for VDS = 4 V. This dependence clearly indicates the self-heating effect. Also for VDS = 19 V we observed a significant DIDS increase already for t as short as tens of ns, and that is a much shorter time scale than that expected and calculated for the electrical transients of the drain lag phenomena [13]. Therefore our results indicate
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Fig. 2. Current and voltage waveforms of the 10 ls long, 10 V pulse applied on the HEMT drain contact. Inset shows source–drain current drops DIDS for different pulse voltage levels and different pulse duration.
that for the case of our 10 ls pulses the drain lag effect can be neglected and we consider only thermal effects. Fig. 3 shows the evolution of phase shift profiles taken during a 10 ls/10 V pulse at t = 2, 6 and 10 ls, and after the pulse at t = 30 ls. As expected, Du(x) signal increases during the pulse due to heating and decreases after the pulse due to cooling, reflecting thus the heat dynamics in the device. The spreading of the peaks with time can also be observed due to lateral heat diffusion. Remarkable is the difference in the evolution of the signal magnitude in the valleys marked by arrows A, B (Fig. 3). At position ‘A’, a source contact with the electroplated thick gold metal on it is localized, while an ordinary drain contact is located at ‘B’. At t = 2 ls one observes nearly the same small amplitude DuB DuA, however for t > 2 ls the phase shift at the ordinary drain contact is higher DuB > DuA. The difference vanishes after the pulse is terminated (t = 30 ls). The trend of DuB > DuA can be clearly visible in Fig. 4 showing normalized TIM profiles at t = 2 ls and 10 ls. To interpret this result we take into account that
Fig. 3. Phase shift evolution (left side vertical scale) and extracted surface temperature increase (right side scale) profiles of the AlGaN/GaN HEMT stressed by 10 V/10 ls drain pulse. The temperature scale holds only for the data at t = 2 and 6 ls, the dashed line depicts phase shift signal after the pulse at t = 30 ls. Positions A mark the signal minimum at the source contacts with airbridge connections, positions B mark the signal at the ordinary drain contacts.
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Fig. 4. Normalized phase shift profiles of the HEMT of Fig. 3 at t = 2 and 10 ls.
the phase shift represents only the heat energy in the semiconductor and the substrate, but not that which is transferred from semiconductor to metallization [10]. Then the deeper valleys at position ‘A’ originate from a better heat sinking at the location of airbridges compared to the ordinary drain contact. This effect is negligible for the short time instants (t < 2 ls), and becomes more visible at longer time when the heat penetrates more effectively from the active region to the thick metal airbridge. To estimate the amount of the heat transferred via the airbridge at t = 10 ls, we calculate the difference of ‘‘areas’’ SA and SB below the Du signal at contacts, which are proportional to the total dissipated heat [10], see Fig. 5. The difference of areas was approx. 1.35 lm rad. Taking into account the total area STOT 68 lm rad and that the number of airbridge contacts is five, the airbridge cooling efficiency g i.e. the percentage of the total heat transferred from the device to the airbridge can be estimated as g = 5x(SB SA)/STOT 10%. A similar ratio of 10% was obtained also at t = 6 ls, but 3% at t = 2 ls. Thus it seems that after the initial increase, the cooling efficiency g of the airbridge contacts saturates. The saturation can be explained by merging of heat waves (Du signals) from neighboring channels at t 6 ls. A prerequisite for the proper thermal modeling is the knowledge of the size of the heating source. The size of
Fig. 5. Phase shift profiles of the HEMT of Fig. 3 at t = 10 ls with marked areas SA and SB below the source and the drain contacts.
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Fig. 6. Determination of the size of the HEMT heating source (single finger) using normalized P2DA obtained from TIM for different drain voltage values of 3, 4, 9 and 20 V. The size is approximated as a full width at half maximum. Channel (source–drain) area is marked by a solid rectangle. Gate was on a floating potential during the pulse.
Fig. 7. Comparisons of the experimental phase shift profiles of the HEMT of Fig. 3 with the simulations considering the heat removal through the airbridge. A two-side arrow marks the simulated area, which is mirrored for three channels. Heating sources assumed in the model are marked by solid rectangles.
the heating source LH is obtained from the analysis of the instantaneous power dissipation density P2DA [4] extracted from the Du(x) distribution. Fig. 6 shows P2DA(x) distribution in a one finger for various stress levels VDS = 3, 4 and 9 V, normalized to the maximal signal. LH was determined as a full width at half maximum. In contrary to the Du(x) time evolution (see Fig. 3), P2DA is almost constant with time as expected (not shown), however we observed a sharp reduction of LH as VDS increases from 3 to 4 V. This was because of the transition from the linear regime of the HEMT into the saturation. In the former case, large part of the VDS is dropped on the contact areas and the heat source is broad (LH 35 lm). On the other hand in the saturation (VDS > 3 V) a high-field domain is formed in the channel and thus LH is reduced to 16 lm for VDS = 4 V and to 11 lm for 9 V, respectively. No further LH reduction was observed for VDS = 20 V. We point out that even for the current saturation we observe that the heat dissipation region is broader than the gate-drain region where highfield domain is mostly expected. That is probably linked to the fact that no voltage was applied on the gate (floating gate). From the above we concluded that the heat dissipation region has a size of 11 lm which was further used in the 2D thermal modeling. For modeling of thermal properties of AlGaN/GaN heterostructure we used similar approach as in [4], but now instead of Si a sapphire substrate was considered. A thermal boundary resistance (TBR) of 1 · 10 7 K m2/W at the GaN/sapphire interface was assumed [3]. We used thermal conductivity of GaN and sapphire layer to be kGaN = 130 W/m K [14] and ksap = 30 W/m K [14], respectively. The thermo-optical coefficient of sapphire (dn/dT)sap = 1.3 · 10 5 K 1 was taken from [15] and the (dn/dT)GaN was taken as a fitting parameter. In the model we considered a 7 lm thick Au-based airbridge structure connected to the source contact, see the schematics of Fig. 7. We considered TBR at the GaN/metal interface to be 7 · 10 8 K m2/W [5]. A 0.5 lm thick combination of alloyed ohmic contact and sputtered barrier
system between the airbridge and GaN having kcontact 3 W/m K has also been taken into account [5]. In the simulation we used a translation symmetry of the system and the Neumann boundary condition to reduce the simulation area (see Fig. 7). The latter simplification is adequate for time scale = 10 ls and for the inner fingers where the system can be considered as adiabatic. Fig. 7 shows the calculated Du(x) distribution for three fingers obtained by mirroring the simulation data (dashed lines). A good coincidence between the experiments (solid lines) and simulation was obtained for (dn/dT)GaN = 4 · 10 5 K 1. The asymmetry in the Du(x) profiles in Fig. 7 is clearly visible, showing that the heat absorbed by the airbridge metal explains properly the lowering of the phase shift. Fig. 8 shows the calculated temperature profiles in the HEMT cross-section at the position of the channel. We see DTchannel 175 K at t = 10 ls on the GaN surface (depth z = 0 lm) and a considerable discontinuity at the GaN/sapphire interface due to TBR (z = 3 lm). Calculations indicate that if the airbridge is not considered, an additional temperature increase of DTchannel (10 ls) =
Fig. 8. Calculated temperatures in the HEMT cross-section at the position of the channel. GaN/sapphire interface is at z = 3 lm. Condition of the pulse is the same as for Fig. 3.
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Fig. 9. Calibrated dependence of the HEMT source resistance on temperature.
change in the electron saturation velocity [4]) and the 2D thermal model. We note that the electrical method gives the temperature solely in the source-gate area and not in the whole source–drain channel, however due to significant lateral heat spreading during ls long pulses (Fig. 7) and due to observed size of the heating source (Fig. 6) this difference may be neglected. In the rest of the paper we present the approach how to determine temperature maps DTsurface(x) from the combination of TIM measurements and the knowledge of the DTchannel value extracted by the electrical method. We first relate the phase shift Du(x) to the GaN surface temperature DTsurface at the particular time instant. Fig. 11 shows a DTsurface(x)/Du(x) ratio at different time instants calculated by the 2D model. DTsurface(x)/Du(x) reflects the heat diffusion and distribution in the lateral and vertical directions. The profiles for t 2–6 ls are the most flat and the difference between the peak and the valley of the DTsurface(x)/ Du(x) distribution is <15%. Consequently if DTchannel is known in this time scale, the temperature profile maps can be obtained from TIM phase measurements, assuming a constant DTsurface(x)/Du(x) = DTchannel/Duchannel, i.e. DTsurface(x) = Du(x)xDTchannel/Duchannel, where Duchannel is taken as the TIM signal at the position of the channel. The DTchannel/Duchannel value is dependent on time, and on layout and material parameters of a particular device. Taking into account our results of the DTchannel determination (see Fig. 10) and TIM experiments (see Fig. 3, left Du scale) and provided that DTchannel/Duchannel 73 K/rad at both t = 2 and 6 ls, the phase shift scale of Fig. 3 can be directly transformed into the temperature scale (see right scale, consider only the data at t = 2 and 6 ls). At t = 6 ls one can observe a 25 K difference between the temperature in the center of the source (position A) and the drain (position B) contacts, representing a 25% change. We note that even though our presented experimental method of Du(x) to DT(x) conversions are justified only for a specific time windows, we provide an insight into thermal behavior of the multifinger GaN-based HEMTs in the regime of the pulsed operation.
Fig. 10. Calculated and experimental time evolution of the temperature increase in the channel of HEMT during the 10 V pulse.
Fig. 11. Calculated DTsurface/Du ratio profiles in the HEMT at different time instants. Condition of the pulse is same as for Fig. 3. Heating sources are marked by solid rectangles.
10 K would occur. We point out that the cooling efficiency g of the airbridge structure may be different for different device structures and may depend on many factors like gate pitch, thickness of the electroplated gold, etc. It was reported elsewhere that kGaN drops to 90 W/m K at 500 K [16]. To estimate the error of our model introduced by assumption of the constant kGaN we run also additional simulation assuming kGaN = 100 W/m K. This calculation showed additional 7% temperature increase at the end of the pulse if compared with the case of kGaN = 130 W/m K. The results of optical experiments and the 2D thermal model are further compared with the results of the electrical method for temperature determination. Fig. 9 shows a RS(T) dependence used for calibration. From the I–V waveforms shown in Fig. 2 and applying the procedure described elsewhere [4] we further extracted DTchannel(t) dependence (see Fig. 10). The experimental curve is in satisfactory agreement with the simulated transient showing 185 K temperature increase at the end of the pulse. The discrepancy between the model and the experiment can be explained by a non-ideality of the rectangular voltage pulse (see Fig. 2) as well as by simplifications introduced both by the experimental method (10% error due to the
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4. Conclusions We studied transient self-heating effects in the multifinger AlGaN/GaN/sapphire HEMT with a gate pitch of 35 lm and the airbridge structure connecting sources. For a dissipated power of 11.9 W/mm we determined a temperature rise 185 K in the HEMT channel at t = 10 ls. The simulation model matches well the experimental data. We have revealed that the airbridge structure absorbs the heat and the HEMT thermal design may be optimized for a given pulsed regime. Cooling efficiency of the airbridge structure increases with the time and saturates at 6 ls taking away a 10% of the heat. As a result, the temperature in the center of the source contacts at t = 6 ls is about 25% lower in comparison with the drain contact and for the best cooling performance we propose to increase the gate pitch with the pulse duration. We have also proposed the approach how to calculate the temperature distribution in the HEMT using the thermo-optical measurements and the electrical characterization. Acknowledgement This work was supported by TARGET – ‘‘Top Amplifier Research Groups in a European Team’’, and by ULTRAGAN – ‘‘InAlN/(In)GaN Heterostructure Technology for Ultra-High Power Microwave Transistors’’ projects of the Information Society Technologies Program of the EU under contracts IST-1-507893-NOE and No. 006903, respectively. References [1] Gaska R, Osinsky A, Yang JW, Shur MS. Self-heating in high-power AlGaN–GaN HFET’s. IEEE Electron Devices Letters 1998;19(3): 89–91. [2] Kuzmik J, Javorka P, Alam A, Marso M, Heuken M, Kordosˇ P. Determination of channel temperature in AlGaN/GaN HEMTs grown on sapphire and silicon substrates using DC characterization method. IEEE Transactions on Electron Devices 2002;49(8):1496–8. [3] Turin VO, Balandin AA. Performance degradation of GaN fieldeffect transistors due to thermal boundary resistance at GaN/ substrate interface. Electronics Letters 2004;40(1):81–3.
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