IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 56, NO. 2, FEBRUARY 2009 337

Maximum Junction Temperaturesof SiC Power Devices

Kuang Sheng, Senior Member, IEEE

Abstract—This paper presents a detailed physical analysis onthe junction temperatures, thermal stabilities, and thermal run-away effects of self-heating unipolar SiC power devices. Resultsreveal that the risk of thermal runaway could limit the usablejunction temperature of these SiC devices to substantially lowerthan 200 ◦C, regardless of the device size and the cooling methodused.

Index Terms—BJT, high temperature, JFET, MOSFET, powerdevice, Schottky barrier diodes (SBD), SiC.

I. INTRODUCTION

MANY PAPERS have reported the operation of nonpowerSiC devices at temperatures as high as 600 ◦C and

pulsed testing of power SiC devices at 300 ◦C−400 ◦C, demon-strating the ability of SiC material and the physical junctions towithstand high temperatures [1]–[4]. However, detailed analy-sis and investigation have not been available on the capability ofSiC power devices in a realistic power electronic circuit wherethey need to dissipate a significant amount of heat. In such acircuit, due to the self-heating effect, the junction temperatureof a power device is significantly higher than the ambienttemperature. The maximum current and power ratings of apower device are typically limited by the maximum junctiontemperature of the device. Such self-heating imposed limit canhave the following two different physical mechanisms.

1) Switching and conduction losses of the power devicecause its junction temperature to increase beyond thematerial temperature limit (> 800 ◦C for SiC). At thosehigh temperatures, excessive and exponentially increas-ing leakage current quickly damages the integrity of thesemiconductor or other materials used to fabricate thepower device.

2) Device switching and conduction losses cause its junctiontemperature to rise. An increasing junction temperatureleads to more device losses and hence sets up a positivefeedback mechanism. A thermal runaway can be trig-gered at a temperature that is much lower than criticaltemperatures of the materials.

Most silicon power devices fail with mechanism “1)”described earlier, and their current/power ratings are set

Manuscript received October 2, 2008. Current version published January 28,2009. The review of this paper was arranged by Editor M. A. Shibib.

The author is with the Department of Electrical and Computer Engineer-ing, Rutgers University, Piscataway, NJ 08854 USA (e-mail: ksheng@ece.rutgers.edu).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TED.2008.2010605

accordingly. For SiC power devices, however, mechanism “1)”is unlikely going to be the reason that is limiting their current-/power-handling capability due to its high intrinsic temperature.They will more likely be limited by the following: i) ohmic/Schottky contact metal temperature limit; ii) passivation/packaging materials used; or iii) mechanism “2).”

In this paper, the effect of mechanism “2)” described previ-ously on the current-/power-handling capability and maximumjunction temperature of SiC power devices will be analyzed anddiscussed in detail. It will be shown that this mechanism can,in some cases, seriously limit our ability in fully realizing theSiC material potential on power devices. Possible approachesthat can be taken to alleviate such limitation will also bediscussed.

II. THERMAL STABILITY OF A SELF-HEATING

POWER DEVICE

A typical cross-sectional view of a packaged SiC powerdevice mounted on a heat sink is shown in Fig. 1(a). Its steady-state thermal equivalent circuit is included in Fig. 1(b). For thethermal circuit to stabilize, the following equation has to besatisfied:

TJ − Tamb = Ploss(TJ) ∗ θJ−A (1)

where TJ is the device junction temperature, Tamb is theambient temperature, Ploss(TJ) is the junction temperature-dependent device power loss, and θJ−A is the total junction-to-ambient thermal resistance. While detailed discussion may leadto some variations, θJ−A is taken as temperature independentin this paper.

A. Temperature Dependence of Device Loss

The total power device loss under a given circuit load condi-tion [Ploss(TJ ) in Fig. 1(b)] comprises the current conductionloss and switching loss. The conduction loss of a power devicecan be written as

PCON(TJ) = I2LOADRON(TJ )D = ILOADVON(TJ)D (2)

where ILOAD is the current going through the device, RON(TJ)is the effective device resistance, VON(TJ ) is the ON-statevoltage, and D is the current conduction duty cycle. The deviceresistance (or ON-state voltage) usually increases quickly with

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338 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 56, NO. 2, FEBRUARY 2009

Fig. 1. (a) Cross-sectional view of a typical thermal system for a SiC power device and (b) its steady-state equivalent thermal circuit.

Fig. 2. Amount of power generated by a 4H-SiC epitaxial resistor (0.17 Ω at300 K) versus junction temperature at different current levels. Also shown inthe figure is the power dissipation line with θJ_A = 20 K/W.

junction temperature. As an example, resistance of a uniformlydoped N-type 4H-SiC epitaxial layer is given by [5]

RON(TJ) = RON_300K

(T

300

)2.4

(3)

where RON_300K is the resistance at room temperature. Anepitaxial resistor is used in the following example, as it providesthe intrinsic structure for all unipolar SiC power devices.

B. Junction Temperatures of a Self-Heating Power Device

With (1)–(3), the junction temperature of an intrinsic 4H-SiCunipolar power device can be calculated for a given set of devicestructural parameters and application conditions. In Fig. 2, theamount of power generated by an epitaxial resistor (R = 0.17 Ωat 300 K) is plotted against junction temperature when conduct-ing different current levels. Clearly, due to electron mobilitydegradation, the resistor generates more power with increasingjunction temperature. Also plotted in the same figure is a linearline depicting the amount of power that can be continuouslydissipated by the cooling system with a total thermal resistance(θJ−A) of 20 K/W. The intersecting points between this linear

Fig. 3. Steady-state temperature versus current for a 4H-SiC epitaxial resistor(0.17 Ω at 300 K).

power dissipation line and the power generating curves are thesteady-state operating points for those current levels. It is seenthat when the current increases to above a certain level, anintersection point between the curve and the linear line cannotbe found, indicating a thermal runaway condition. As shown inFig. 2, the highest current that the considered epitaxial resistorcan conduct is 4.15 A, corresponding to a junction temperatureof 238 ◦C.

A better illustration is shown in Fig. 3 where the steady-state device junction temperature is plotted against its cur-rent level for the resistor described earlier. The steady-statedevice junction temperature increases at an accelerated ratewith increasing current. While 4.15 A and 238 ◦C are thetheoretical maximum device current and junction temperature,respectively, one cannot use the device at this level in apractical application, as safety margins will be needed. If a10% current safety margin is applied (I = 90% ∗ 4.15 A), thedevice junction temperature drops sharply to 110 ◦C. This ismuch lower than the temperature potential of SiC materialwhich promises to remain reliable at temperatures as highas 600 ◦C.

The thermal runaway temperature of this device can alsobe calculated by substituting (2) and (3) into (1) and set

SHENG: MAXIMUM JUNCTION TEMPERATURES OF SiC POWER DEVICES 339

Fig. 4. Steady-state temperature versus current with different junction-to-casethermal resistances and ambient temperatures for a 4H-SiC epitaxial resistor(0.17 Ω at 300 K).

∂ILOAD/∂TJ = 0. Simple mathematical derivation will lead tothe following thermal runaway condition:

TJ_max =α

α − 1Tamb

Pmax =(α − 1)α−1

αα

Tamb

θJ−A(4)

where α = 2.4 is the exponent used in (3), TJ_max is thethermal runaway temperature in kelvins, Tamb is the ambi-ent temperature in kelvins, and Pmax is the maximum powerthat can be dissipated continuously. This result is surprisinglysimple and indicates that the thermal runaway temperature isindependent of the device resistance and cooling method. It isonly related to the ambient temperature.

To verify (4) and study the effect to a fuller extent, themaximum junction temperatures of the SiC epitaxial resistorunder other packaging and ambient conditions have also beenstudied by evaluating junction temperature against current withdifferent θJ−A values (20, 10, and 5 K/W) and at ambienttemperatures (−50 ◦C, 27 ◦C, and 100 ◦C). The results areshown in Fig. 4, from which a few interesting observations canbe made.

First, as predicted, as in (4), the maximum junction tempera-ture (TJ_max) remains constant for a given ambient temper-ature, regardless of how the device is cooled (θJ−A). Bettercooling only increases the device current capability but not itsTJ_max. The TJ_max values are 108 ◦C, 238 ◦C, and 365 ◦Cfor ambient temperatures of −50 ◦C, 27 ◦C, and 100 ◦C, respec-tively. This is in good agreement with (4). With a similar 10%current safety margin, the maximum usable device junctiontemperature for these three ambient temperatures will drop to15 ◦C, 110 ◦C, and 209 ◦C, respectively. This is surprisinglylow for SiC devices.

Fig. 5. Maximum junction temperature and current for a 4H-SiC epitaxialresistor (0.17 Ω at 300 K) at different ambient temperatures. A 10% margin formaximum current is also considered. θJ_A = 5 K/W.

Second, the maximum junction temperature rise aboveambient (ΔT = TJ − Tamb) is higher at a higher ambienttemperature. This indicates that thermal runaway is less likelyin a hotter environment and that the device can dissipate morepower in those ambient temperatures.

In Fig. 5, the maximum junction temperatures and currentsat thermal runaway are plotted against ambient temperature.Also included in the figure are the maximum temperatures andcurrents when a 10% margin is considered for the current in apractical application. The big gap between device temperaturesat Imax and 90% ∗ Imax, as explained previously, is clearlyevident.

III. THERMAL STABILITY OF SiC POWER DEVICES

The temperature dependences of losses from SiC switch-ing devices (MOSFET, BJT, and JFET) in a practical powerelectronics application can differ from that of a resistor. Theswitching losses of these devices can be generally considered tobe temperature independent due to a lack of conductivity mod-ulation. This even includes BJT [3] since no reliable evidenceof significant conductivity modulation has been reported in theliterature (despite a few such claims). On the other hand, theconduction losses of conductivity-modulation-free devices aremainly determined by the carrier (most likely electron) mobilityand hence increase significantly with junction temperature.Due to the unique structure of each device, their temperaturedependence differs from each other and will be discussed indetail hereinafter.

A. Junction Temperatures of SiC Power Devices inConduction-Loss Dominant Mode

In many power electronic circuits, power device lossesare dominated by their conduction loss. The following para-graphs discuss the maximum junction temperatures for 4H-SiCSchottky barrier diodes (SBDs), JFETs, BJTs, and MOSFETs.

340 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 56, NO. 2, FEBRUARY 2009

Fig. 6. Steady-state temperature versus current with different junction-to-case thermal resistances and ambient temperature for a 4H-SiC SBD (1.2 kVand 10 A).

1) 4H-SiC SBDs: The forward-voltage drop of an SBDcomprises an offset voltage related to Schottky barrier (VB) anda resistive component proportional to the current, as describedin the following:

VON = VB(TJ ) + ILOAD ∗ RON(TJ ). (5)

The differential resistance RON arises from the drift regionand it increases with temperature in the same way as the epitax-ial resistor discussed in Section II. VB decreases somewhat withtemperature, and its temperature dependence can be empiricallyfitted based on [6]

VB(TJ) = VB0 + C1TJ + C2T2J (6)

where VB0, C1, and C2 are fitting constants. Based on (3), (5),and (6), the SBD junction temperature can be analyzed based on(1) for various ambient temperatures and θJ−A. The results areshown in Fig. 6. While the figure is similar to Fig. 6, there area few key differences. First, the thermal runaway takes place ata higher temperature, particularly when the thermal resistanceθJ−A is high. Second, TJ_max is dependent on θJ−A. With a10% current margin considered, the maximum usable junctiontemperatures that such a 1.2-kV 10-A device can operate withθJ−A = 5 K/W are 226 ◦C (8.8 A), 143 ◦C (10.3 A), and71 ◦C (12.5 A) for ambient temperatures of 100 ◦C, 27 ◦C, and−50 ◦C, respectively. This is somewhat higher than a pure SiCresistor analyzed in Section II but still significantly lower thanthe SiC material potential.

The differences between the SiC SBD and epitaxial resistorcan be attributed to the offset voltage (VB) that decreases athigher temperature. It helps to slow down the increase of deviceloss with temperature, and hence, it makes the SBD less proneto thermal runaway.

2) 4H-SiC JFETs and BJTs: Due to the lack of conductivitymodulation and other nonresistive voltage-drop contributor,the forward I–V curves of 4H-SiC JFETs and BJTs undertypical operating current densities behave just like a resistor.The voltage-drop temperature dependences of these devicesalso follow the 2.4 power law shown in (3) [7]–[9]. As a

result, the thermal runaway temperatures of these two devicescan be expected to be the same as what have been shown inFigs. 3–5. (It is assumed that the SiC JFET and BJT consid-ered also have a 0.17-Ω resistance at 300 K. However, thethermal runaway temperatures would remain the same for otherdevice sizes.)

3) 4H-SiC MOSFETs: As the most favored transistors inSiC, 4H-SiC MOSFETs have experienced great difficulty inperfecting their MOS interface for high channel electron mobil-ity and good device reliability and stability. On the other hand,the imperfect MOS interface has also given rise to a channelelectron mobility that increases with increasing temperature,which is exactly the opposite of the trend for that in other de-vices (e.g., Si MOSFET, SiC JFET, etc.). Since MOSFET chan-nels reported by different groups have different imperfections,the temperature coefficients of the overall device resistancecan be positive or negative, depending on the manufacturerand the temperature range considered [7], [10]. Nevertheless,resistances of all of these MOSFETs do not increase as quicklyas those of the epi-resistance discussed previously. As a result,they are significantly less prone to the thermal runaway problemhighlighted earlier. Quantified analysis on their maximum junc-tion temperatures is not included here due to the large variationof MOSFET performances among different groups and theirfast-changing nature at this point.

B. Junction Temperatures of SiC Power Devicesin Switching Mode

In power electronics applications other than those mentionedin Section III-A, SiC device switching losses cannot be ignored.As discussed in the beginning of Section III, switching lossesfor the devices considered can be treated as independent ofjunction temperature. As a result, thermal runaway conditionsfor these applications will be different from the conduction-loss-dominated case presented in Section III-A. If the to-tal device loss is dominated by the temperature-independentswitching losses, then the positive feedback described in mech-anism “2)” in the introduction of this paper will not take place,and thermal runaway will not happen.

Therefore, the maximum device junction temperature andthermal runaway analysis will depend on the relative percent-ages of conduction and switching losses, which can differgreatly from application to application. However, there havebeen reports in the literature that conduction loss and switchingloss should be the same if an optimum device size is chosen[11]. Based on this assumption, the thermal stability analysis ofvarious SiC power devices presented in Section III-A will bereexamined in this section.

1) 4H-SiC SBDs: During transient operation, SBDs operatein much the same way as a nonlinear parallel-plane capacitor.With only displacement current involved, no energy loss can beresulted in this device during such transients (note that the sameis not true for p-i-n diodes).

As a result, the power loss of a SiC SBD is always conductionloss dominated (neglecting leakage loss). The thermal stabilityand maximum junction temperature of the SiC SBDs in switch-ing circuit are the same as that presented in Section III-A.

SHENG: MAXIMUM JUNCTION TEMPERATURES OF SiC POWER DEVICES 341

Fig. 7. Steady-state temperature versus current with different junction-to-casethermal resistances and ambient temperatures for 4H-SiC JFETs and BJTs. Theon-resistance at 300 K is set to 0.17 Ω, and the device switching loss is assumedto be the same as the conduction loss at 300 K.

2) 4H-SiC JFETs and BJTs: As explained before, tempera-ture dependences of the conduction loss and switching loss ofSiC JFETs and BJTs are virtually the same. The two devicesare discussed together in this paper on the thermal stabilityaspect. The steady-state junction temperatures of a SiC JFET(or BJT) are recalculated in the same approach as that inSection II by including switching losses. To make it comparableto the analysis based on SiC epitaxial resistor, the JFET/BJTanalyzed here is also assumed to have a resistance of 0.17 Ω at300 K. Based on the optimum assumption cited earlier, the de-vice switching loss is assumed to be the same as the conductionloss at 300 K.

The junction temperature versus current curves are shown inFig. 7 with different junction-to-case thermal resistances andambient temperatures. With 10% current margin, SiC JFETsand BJTs can be used safely up to temperatures of 243 ◦C,157 ◦C, and 72 ◦C for the three ambient temperatures con-sidered. This is substantially higher than the correspondingTJ_max (209 ◦C, 110 ◦C, and 15 ◦C) when switching loss wasnot considered. It is clear that SiC JFETs and BJTs can be usedfor higher junction temperatures when switching loss is present.The thermal stability of these devices will be even better ifthe switching loss accounts for a higher percentage of the totallosses.

3) 4H-SiC MOSFETs: The thermal stability of MOSFETswith switching losses considered is also expected to be betterthan the conduction-loss-dominated case. For the same reasonas in Section III-A, quantified analysis for MOSFETs is notincluded here.

It is worth noting that the discussion here assumes high-frequency repetitive device switching and that the junctiontemperature fluctuation within a switching cycle is neglected.For applications with low duty-cycle current pulses and largejunction temperature fluctuation, dynamic thermal impedancewill need to be considered. The junction temperature can have

excursions above TJ_max predicted in this analysis withoutcausing thermal runaway.

IV. INTERPRETATION AND DISCUSSION OF RESULTS

The analysis in Section III shows that, depending on thedevice type and whether switching loss is significant in thecircuit, the SiC power devices analyzed in this paper could havesubstantially different thermal runaway effect.

SiC JFETs and BJTs are predicted to encounter this prob-lem at surprisingly low temperatures (e.g., TJ_max = 238 ◦Cwhen Tamb = 27 ◦C) when their switching losses are negligiblecompared to conduction losses. This happens for all devicesizes, packages, and cooling methods as long as the junction-to-ambient thermal resistance (θJ−A) is constant (true formost practical cases). The picture becomes drastically bleakerfor a designer when a 10% (or more) margin is consideredwhen choosing the maximum device current to avoid thermalrunaway. The usable device junction temperature has to belimited to 110 ◦C (Tamb = 27 ◦C), a level far below whatis expected from a SiC device. Such constraints on devicejunction temperature may be relieved when device switchinglosses account for a significant portion of the total device loss.For a given application where the load current and device gatedriver are fixed, thermal stability can be improved by adoptinga larger device die size which will decrease conduction loss andincrease switching loss.

SiC SBDs are less prone to the thermal runaway problemdue to the negative temperature coefficient on its forward I–Vcurve offset voltage. It encounters the thermal runaway problemat temperatures around 200 ◦C–300 ◦C, depending on the totalthermal resistance (θJ−A). The usable junction temperaturewith 10% current margin is typically around 150 ◦C and be-comes worse when θJ−A is lower. One disadvantage of SiCSBD on this aspect is that the thermal stability issue cannot berelieved by introducing more frequent switching action becauseits switching loss is always negligible.

Among these devices, SiC MOSFETs are projected to bemuch less prone to the thermal stability problem, thanks tothe negative temperature coefficient of the imperfect channelresistance.

It should be noted that the limitation on SiC device TJ_max

does not prevent them from handling high power density bysimply adopting better cooling methods (a smaller θJ−A). Itshould also be noted that all analyses in this paper are car-ried out by assuming a temperature-independent θJ−A. Thesituation can be even worse if θJ−A is dominated by thermalconduction and hence increases with the amount of power beingdissipated.

V. SUMMARY AND CONCLUSION

In this paper, the thermal stability and thermal runaway prob-lems of SiC power devices including SBDs, JFETs, BJTs, andMOSFETs have been analyzed in detail. Based on fundamentalphysical models and experimental data, steady-state junctiontemperatures of these devices have been calculated at differentdevice currents, switching losses, junction-to-ambient thermalresistances, and ambient temperatures.

342 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 56, NO. 2, FEBRUARY 2009

The results reveal that, with the exception of MOSFETs,these devices may become thermally unstable (runaway)at junction temperatures as low as < 200 ◦C, which aresubstantially lower than the superior capability of SiC materialitself (> 800 ◦C). Such limitation is particularly serious whenconduction loss is dominant over switching loss and/or ambienttemperature is low, regardless of the device size, packaging,and cooling methods used. The issue investigated in this paperwarrants careful consideration and attention from researchersand engineers designing and using SiC power devices.

The analysis carried out suggests that the maximum junctiontemperature of these devices can be improved by the followingways: 1) increasing the percentage of switching loss in the totallosses; and/or 2) operating at a higher ambient temperature;and/or 3) using heat sinks whose thermal resistances decreasewith increasing power dissipation and minimizing thermal re-sistance attributed to thermal conduction.

REFERENCES

[1] G. W. Hunter, P. G. Neudeck, R. S. Okojie, G. M. Beheim, J. A. Powell,and L. Chen, “An overview of high-temperature electronics and sensordevelopment at NASA Glenn Research Center,” J. Turbomach., vol. 125,no. 4, pp. 658–664, Oct. 2003.

[2] P. G. Neudeck, D. J. Spry, L. Y. Chen, R. S. Okojie, G. M. Beheim,R. Meredith, and T. Ferrier, “SiC field effect transistor technologydemonstrating prolonged stable operation at 500 ◦C,” Mater. Sci. Forum,vol. 556/557, pp. 831–834, 2007.

[3] K. Sheng, L. C. Yu, J. Zhang, and J. H. Zhao, “High temperature char-acterization of SiC BJTs for power switching applications,” Solid StateElectron., vol. 50, no. 6, pp. 1073–1079, Jun. 2006.

[4] J. Richmond, S. H. Ryu, S. Krishnaswami, A. Agarwal, J. Palmour,B. Geil, D. Katsis, and C. Scozzie, “400 watt boost converter utilizingsilicon carbide power devices and operating at 200 ◦C baseplate temper-ature,” Mater. Sci. Forum, vol. 527–529, pp. 1445–1448, 2006.

[5] M. Roschke and F. Schwierz, “Electron mobility models for 4H, 6H, and3C SiC,” IEEE Trans. Electron Devices, vol. 48, no. 7, pp. 1442–1447,Jul. 2001.

[6] Datasheet, Cree Inc., Durham, NC, 2008. [Online]. Available: http://www.cree.com

[7] H. Zhang, “Electro-thermal modeling of SiC power electronic systems,”Ph.D. dissertation, Univ. Tennessee, Knoxville, TN, 2007.

[8] Y. Zhang, K. Sheng, M. Su, J. H. Zhao, P. Alexandrov, X. Li, L. Fursin,and M. Weiner, “Development of 4H-SiC LJFET-based power IC,” IEEETrans. Electron Devices, vol. 55, no. 8, pp. 1934–1945, Aug. 2008.

[9] Y. Tang, J. B. Fedison, and T. P. Chow, “High temperature characterizationof implanted-emitter 4H-SiC BJT,” in Proc. IEEE/Cornell Conf. HighPerform. Devices, 2000, pp. 178–181.

[10] B. A. Hull et al., “Status of 1200 V 4H-SiC power DMOSFETs,” in Proc.ISDRS, 2007, pp. 1–2.

[11] A. Q. Huang, “New unipolar switching power device figures of merit,”IEEE Electron Device Lett., vol. 25, no. 5, pp. 298–301, May 2004.

Kuang Sheng (M’99–SM’08) received the B.Sc.degree in electrical engineering from Zhejiang Uni-versity, Hangzhou, China, in 1995 and the Ph.D.degree in electrical engineering from Heriot–WattUniversity, Edinburgh, U.K., in 1999.

He was a Researcher with Cambridge University,Cambridge, U.K., for three years and is currently anAssistant Professor with the Department of Electri-cal and Computer Engineering, Rutgers University,Piscataway, NJ. His research areas include variousaspects of power semiconductor devices and ICs on

SiC, Si, and SOI. He has authored or coauthored around 80 technical papers ininternational journals and conferences, and he is the holder of a patent.