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SiC power devices for high voltage applications
Materials Science and Engineering B61 – 62 (1999) 330 – 338
SiC power devices for high voltage applications K. Rottner a,*, M. Frischholz a, T. Myrtveit a, D. Mou a, K. Nordgren a, A. Henry b, C. Hallin b, U. Gustafsson c, A. Scho¨ner c b
a ABB Corporate Research, Electrum 215, S-16 440, Stockholm-Kista, Sweden ABB Corporate Research, c/o IFM, Linko¨ping Uni6ersity, S-58183, Linko¨ping, Sweden c IMC, Electrum 233, S-16 440, Stockholm-Kista, Sweden
Abstract Silicon Carbide device technology is now evolving from a pure vision to a real alternative to silicon devices. The feasibility of SiC devices has been shown for many different types of devices, the development of a working production technology has started, yield, reliability and costs now being the key issues. At present the high substrate prices keep the manufacturing costs of SiC high, making it very difficult to enter the device market with SiC on economic terms. Prime applications are those for which SiC offers substantial benefits or even a technological breakthrough on the system level. The main application is power conversion where the latest development efforts on silicon based power switches (e.g. IGBT) allow utilisation of much higher switching frequencies, putting very high demands on the free wheeling diode. The system performance is to a large extent limited by the diode recovery charge—a major source of switching losses. Depending on the voltage range, different device concepts are of interest: In the lower voltage range the junction–barrier controlled Schottky (JBS) device is a promising candidate while at voltages beyond 2.5 kV the PIN diodes is the device of choice. Different system requirements — e.g. surge current capability — make the PIN junction superior to a Schottky device for certain applications. With progress in material and technology development the ‘world’s best’ result is becoming less and less important and reproducibility is the issue. Failure analysis of defective devices needs to be established with a high number of substrate defects being still an obstacle in SiC. High leakage/soft reverse characteristics are often encountered and can usually be attributed to localised defects. It is important to identify their origin and to separate process-induced defects from those already present in the epilayer or substrate. In order to use the high power handling capability of SiC reduction of margins (e.g. in epilayer thickness and doping) is necessary. This requires narrow bandwidth of process and material variations. For paralleling of SiC devices equal current sharing under static and dynamic conditions is a fundamental requirement from the system side. Top-down calculation gives material specifications, which the supplier has to meet. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Power devices; High-voltage devices; Converter; PIN diode; Junction barrier Schottky; Switching losses
1. Applications: market pull ABB’s main business interest in the high voltage range, which is generally defined by device blocking voltages above 1000 V, starts at power ratings of several hundred kilowatts and extends far into the megawatt range for traction and HVDC systems where device current ratings of several hundred to several thousand amperes are common. Virtually any power device in these applications is part of a power converter. One specific converter topol* Corresponding author. Tel.: + 46-8-752-1047; fax + 46-70-6101007. E-mail address: [email protected] (K. Rottner)
ogy is the six pulse voltage source converter shown in Fig. 1. It can operate as an inverter (d.c. to three-phase a.c.) or a rectifier (three-phase a.c. to d.c.). Back to back configurations are also used for e.g. coupling of a.c. grids with different frequencies (50/60 Hz) or d.c.– d.c. conversion for different bus voltages [1]. The basic function of an inverter is to form a sinusoidal waveform from a uni- or bipolar d.c. bus ( + d.c., 0, −d.c.). The dissipated power in switches can only be kept to a minimum in either fully open (full load current, as little voltage across the switch as possible) or fully blocking (full voltage, no current). Thus, the most feasible way of generating a sine wave is to use the pulse width modulation (PWM) mode of operation (Fig. 2). The carrier—or switching frequency of the
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Fig. 1. Voltage source converter.
PWM is normally 39 times the line frequency, which gives roughly 2 kHz for a 50 Hz a.c. grid. The switching speed of the transistors in the PWM operation is at least one order of magnitudes faster then the time constant of the carrier frequency. The former is normally expressed in terms of current and voltage time derivatives (dV/dt, dI/dt). These derivatives usually are limited by the semiconductor performance and have a strong impact on the losses and the system performance itself. Increasing the switching (carrier) frequency of the PWM converter can considerably reduce size, weight and cost of several passive components (transformers and filters). The current limitation of the switching frequency lies around 2 kHz due to the losses generated in the switches and diodes. One main source for these losses is the reverse recovery charge of the diodes, which severely impacts the whole converter design and device rating. The current and voltage waveforms for the diode and the transistor, respectively, are shown in Fig. 3. The reverse recovery charge causes huge current transients when the inductive load current is commutated from the transistor to the free wheeling diode. In worst
Fig. 2. PWM synthesis of sinusoidal waveform: carrier frequency 39 times a.c. frequency.
Fig. 3. Waveform of diode switch.
case these current transients can cause the destruction of the converter. In order to improve the system performance in terms of either loss reduction or increased switching frequency without increased losses, SiC diodes are highly desirable. Due to the material properties of SiC— mainly the high critical electric field—a reduction of the recovery charge in the diodes by more than 90% is possible. One possible strategy for commercialisation of SiC devices in the high power range is hybrid modules where SiC diodes operate together with Si transistors. 2. SiC devices Depending on the requirements of the application, the device of choice will be either a bipolar (p–n junction diode) or a unipolar device (Schottky diode). Especially if one is concerned about on-state losses, the criterion is the maximum blocking voltage required in a specific application. As a rule of thumb the Schottky diode is the device of choice below 2 kV, the p–n diode above about 3 kV. In addition to lowering the switching losses by reducing the recovery charge, which is an important issue for the Schottky diode as well as for the p–n diode, there are further requirements from the system side which have to be met by the devices. For traction as well as transmission applications, the ability to withstand surge current (typically ten times the nominal current) events for some tens of milliseconds is important. Stability against increased junction temperature during that event is a consequent need.
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2.1. De6ice structures 2.1.1. Bipolar-PIN rectifier 4H SiC P + N − N (PIN) diodes were fabricated with a device area of 0.8 mm2 consisting of the actual active anode area (0.50 mm2), a 2-zone junction termination extension (JTE) (zone width 50 mm each, total JTE area 0.28 mm2) and a channel stop separated from the JTE. The anode was formed by implantation of boron and aluminium, details have been reported at [2]. In order to achieve a good ohmic contact a highly doped p-type epilayer has been grown by CVD on top of the anode. The CVD layer is approximately 0.5 mm thick with an Al doping of around 3×1020 cm − 3. Boron was implanted for forming two concentric JTE zones. A schematic view of the device structure is shown in Fig. 4. The device structure was simulated with MEDICI predicting a theoretical breakdown voltage of about 3.5 kV. 2.1.2. Unipolar-junction barrier Schottky rectifier (JBS) When designing a Schottky rectifier for high-voltage applications there is a fundamental limit towards the surface electric field, which determines the potential
barrier for tunneling. If that barrier becomes to thin this will result in excessive leakage current due to the tunnel current. Utilising the full potential of SiC with a critical electric field of 3 MV cm − 1 the resulting barrier thickness for a typical Schottky barrier height of 1 eV is only about 3 nm which is a typical thickness for observing tunneling phenomena. In order to overcome the limitation of the Schottky contact a number of different concepts have been proposed such as the junction barrier Schottky rectifier, a combination of p–n junctions and Schottky contacts integrated in one device [3]. Figs. 5 and 6 show the principle of this concept: in reverse bias operation the depletion region of the p–n junction parts of the device merge at a certain voltage and shield the Schottky area from the high electric field (Fig. 6). In forward bias operation the Schottky area dominates due to the immediate onset of the forward current which is controlled by the low Schottky barrier height as compared to the large band gap of the semiconductor which determines the threshold voltage of the p–n junction (Fig. 5). The JBS concept is already successfully used for Si Schottky rectifiers for a number of years [3,4] and has also been demonstrated recently for SiC [5,6].
Fig. 4. Schematic device structure of a 3.5 kV PIN diode.
K. Rottner et al. / Materials Science and Engineering B61–62 (1999) 330–338
Fig. 5. Comparison of Schotty, PIN and JBS. (top) The JBS device is shown schematically in reverse and forward bias operation (for more details see e.g. [3,4]). (bottom) The current–voltage characteristics of the three devices in reverse and forward bias mode.
2.2. Experimental results— de6ice performance 2.2.1. Static characteristics
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Fig. 7. Forward current – voltage characteristic of a 3.5 kV PIN diode.
The capability to handle very high current densities was tested to up to current levels of 2000 A cm − 2 on the 3.5 kV pn diode. The result of this test, as seen in Fig. 8, allows from an application point of view to set a nominal current density to 200 A cm − 2 and still being able to handle ten times higher currents as is required for short periods of time (ms).
2.2.1.1. Forward characteristic (PIN diode). Fig. 7 shows the forward characteristic of our 3.5 kV PIN diode. The forward curve was measured using a set-up for pulsedmeasurements in order to avoid self-heating of the device during measurement. At room temperature, the forward voltage drop at 200 A cm − 2 was about 3.4 V. From this curve, the values for the threshold voltage and the series resistance were extracted as indicated in the figure. The resulting threshold voltage of 2.9 V compares favourably to the theoretical threshold voltage of about 2.8 V due to the bandgap of 4H SiC. The on resistance of 2.6 mV·cm − 2 is in accordance with the epi layer thickness and lifetime as confirmed by simulation of the actual device structure.
2.2.1.2. Re6erse characteristic (PIN diode). The static blocking characteristic is shown in Fig. 9 on a logarithmic scale up to 3 kV. The measurement of the reverse characteristic was carried out under d.c. conditions in an inert atmosphere. The current density is in the low 10 − 10 A cm − 2 range up to 2 kV and below 10 − 8 A cm − 2 up to more than 3 kV and thus does not contribute to any significant losses (power loss under blocking conditions are in the order of mW cm − 2). The shape of the curve indicates a trap-assisted band to band tunneling as reported by other groups [7].
Fig. 6. Comparison of the magnitude of the electric field as a function of the distance from the surface underneath the PIN and the Schotty part of the JBS device.
Fig. 8. Forward current – voltage characteristic to demonstrate the surge current capability of a 3.5 kV PIN diode.
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Fig. 11. Turn-off switching characteristic of a 3.5 kV PIN diode.
Fig. 9. Reverse current voltage characteristic of a 3.5 kV PIN diode.
2.2.1.3. Forward characteristic (JBS diode). The forward characteristic of a 1.8 kV JBS diode is shown in Fig. 10. In comparison to the PIN diode the forward voltage drop at 200 A cm − 2 is reduced from 3.2 V for the PIN diode to only 1.7 V for the JBS diode resulting in reduced on-state losses. The on resistance extracted from the curve is 5 mV·cm2. 2.2.2. Switching characteristics In order to determine the turn-off waveform, the PIN diode was tested in a chopper circuit where the load current was commutated between the diode to the transistor in single switching events to avoid selfheating. The measured switching waveform is shown for switching from the on state with a forward current IF of 0.5 A with a rate of change of the current dI/dt of 1
kA·cm − 2 ms − 1 into the blocking state with a reverse bias voltage VR of 500 V (Fig. 11). The waveform does not show the large current overshoot due to the reverserecovery charge observed for a conventional Si diode. Even though the base layer is significantly modulated under forward conduction for the switching speeds which were used here and which are typical for high power module applications, the bipolar SiC diode does not suffer from stored charge due to the very low minority carrier lifetimes in SiC. The reverse-recovery current in SiC is therefore not given mainly by the displacement current which is generated by the build up of the space charge region in the base layer. This is in fact the smallest recovery current that is possible for any rectifier and—in contrast to the conventional recovery charge in Si diodes—is not depending on temperature, forward current or current derivatives, but rather a function of the d.c. bus voltage.
2.3. Performance-limiting issues
Fig. 10. Forward current–voltage characteristic of a 1.8 kV JBS diode.
Progress in performance of SiC devices as demonstrated in this paper and also by many other groups during the past years has been tremendous, mostly due to continuous improvement in the material itself and the available technology, e.g. [3,5–17] and numerous references therein. As a consequence, the performance of SiC devices is now very close to matching the high expectations that have been raised in the past owing to the physical properties of SiC. Nevertheless there are still a number of issues that have to be addressed [18,19]. Among the issues there is the problem of paralleling a number of devices to be able to handle sufficiently high currents. This problem remains regardless of whether the active area of SiC devices, which is currently limited by the defect density, can be vastly increased in the future or not.
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Fig. 14. Reverse blocking characteristics of two 3.5 kV PIN diodes with sharp and soft breakdown behaviour, respectively. Fig. 12. Forward current–voltage characteristics of two 3.5 kV PIN diode with different series resistance.
2.3.1. Paralleling of de6ices Fig. 12 illustrates the problem posed by having to parallel several devices with different values for series resistance but the same turn-on voltage. At a fixed forward voltage drop the current sharing is not equal between the two diodes and, in the extreme case, one diode would conduct all the current — and in worst case be destroyed—while the other diode do not contribute to the current conduction at all. 2.3.1.1. Material Variation. The impact of fluctuations in the material properties such as lifetime and the thickness specification of substrate and epilayer on the problem of paralleling diodes is further illustrated in Fig. 13. As can be seen from the results of the simulation, the current sharing between the SiC diodes is not symmetric and the diode current density can vary by more than a factor of two between two SiC diodes. At some point the ultimate restrictions for the specification are then set by the ability of the package to withstand and spread the heat produced in the diodes.
Fig. 13. Influence of varying material specifications on the forward current density of a 3.5 kV PIN diode at a fixed forward voltage.
2.3.2. Re6erse characteristic— soft 6s. sharp breakdown Methods like statistical descriptions e.g. device yield produce only a very crude picture of the device performance and its limitations by describing an ensemble behaviour. Studying devices on an individual level by analysing leakage current and breakdown behaviour gives an already more detailed insight but is still limited by averaging over a large portion of the active device area. This is e.g. the case when studying the reverse blocking characteristics of rectifiers where one observes devices with a sharp increase in reverse current at breakdown indicating the onset of avalanche breakdown, but also devices with a ‘soft’ characteristic with slowly increasing leakage current well below the theoretical breakdown voltage (Fig. 14). The soft characteristic is often attributed to the formation of local microplasmas in the presence of material inhomogeneity or localised defects. The lack of information on whether the device is limited by localised phenomena or a uniform behaviour has been achieved is also a major problem when it comes to a comparison between simulations and experimental results. 2.3.2.1. Optical beam-induced current measurements (OBIC). OBIC has proven to be a valuable tool for collecting local information on power semiconductor devices in Si and SiC [20–23]. Due to the current sensitivity of the OBIC technique it can be applied to detect failures in the device that manifest themselves as peaks or ‘hot spots’ in the photocurrent distribution already at low voltages. The possibility to discriminate between electrically active defects and other non-active defects seen e.g. in an optical microscope allows identifying the cause of individual extrinsic device failures on a microscopic scale. Fig. 15 shows two OBIC images taken from devices with sharp (left) and soft breakdown behaviour (right), respectively. In the case of the device with a sharp breakdown the two zones of the JTE can be clearly distinguished as two concentric rings
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Fig. 15. Greyscale encoded OBIC images of two 3.5 kV PIN diodes, excitation wavelength 313 nm. (left) Device with sharp breakdown behaviour and uniform photocurrent distribution, Vrev = 500 V. (right) Device with soft breakdown with a ‘hot spot’ in the JTE region, Vrev = 300 V.
surrounding the darker speckled anode area by the different grey level belonging to each zone. The probe tip is discernible as the black feature pointing from the left side towards the centre of the diode. The diode with soft breakdown behaviour was measured at a reverse bias of 300 V. In this case a high photocurrent is flowing right at the anode edge, discernible as a bright ring surrounding the anode area. At the righthand side of the image, a bright area (hot spot) can be observed that stretches from the anode edge to the channel stopper. Whenever ‘hot spots’ are observed in OBIC images, this coincides with a steady increase of the leakage current in the I – V characteristic already at low voltages as compared to the design voltage is measured.
2.3.3. Long-term stability Another important issue is the long-term stability of devices, which, in a commercial system, have to function without failure over a period of 20 years. Most of the research and development so far has concentrated on demonstrating the capability of SiC devices in terms of absolute performance figures rather than on reproducibility and reliability. What the problems are those have to be solved become evident when the d.c. stability is measured. This is illustrated in Fig. 16 where the reverse leakage current at a constant reverse bias voltage of 2.9 kV of three diodes is plotted as a function of time. Although two of the diodes start at roughly the same leakage current level around 10 − 9 A after about 50 h of continuous operation in blocking mode the leakage current has risen by five orders of magnitude to 10 − 4 A. The other diode, however, starting with a leakage current of 10 − 5 A showed only little change in the leakage current, in contrast to the other diodes the leakage current even slightly decreased.
3. Economic considerations Once the feasibility of a new technology has been shown and even long before, the question arises whether the new technology will be successful on the market in terms of products. The two most obvious factors are cost and performance, which, for different markets, have different importance. In a typical niche market application performance is the key factor and the price of a component or module is basically irrelevant. This is the case where there exists no other solution to a specific problem than the new technology. On the other side, there are volume markets for which cost is essentially the only factor that really matters. This is usually the case if there is already an existing technology that can be used and the new technology offers a distinct advantage in cost.
Fig. 16. Reverse leakage current of three 3.5 kV PIN diodes as a function of time at a constant reverse-bias voltage of 2.9 kV. The devices were kept at a constant temperature of 50°C during the experiment
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Fig. 17. Development of the micropipe density of the best published values for SiC and production wafers. In addition the 40 and 90% lines for the micropipe-limited device applications are show.
What that means in reality for a SiC component used in a product shall in the following be exemplified for a typical power module (1.2 – 2.5 kV) comprising Si IGBTs and Si diodes with a module price of 1 USD/A. In this module we will later replace the Si components by SiC components. Subtracting the typical gross margin the manufacturing costs of this module are around 0.6 USD/A of which 0.3 USD/A is the cost of the semiconductor component, the rest of the cost is connected to packaging, material other than the semiconductor and testing. For a typical current density of 70 A cm − 2 that means a semiconductor value of 25 USD cm − 2. With a price of 100 USD for a 4 inch Si epiwafer that translates to 2 USD cm − 2 or 0.03 USD A − 1 (valid for high yields only). When we now replace Si by SiC components we will make the assumption that the current density of the SiC components will be approx. three times the current density of the Si components. The second assumption will be that the epiwafer will contain no micropipes limiting the yield. With a price of 1 –2 kUSD for the 2¦ SiC epiwafer we will then arrive at 60–120 USD cm − 2 or 0.3 – 0.6 USD A − 1. The material costs which where negligible in the case of Si will now double the manufacturing cost. Under these assumptions, the final price for the SiC module will under these assumptions be about 1.5 – two times the price of a Si module. This, however, may be permissible in a typical niche market application provided that the added value for the customer, which is offered by the SiC module in terms of switching speed and power losses, matches the increase in cost. After having answered the first question, we have to consider the validity of our assumptions, in particular the micropipe issue. When we follow the development of the density of micropipes over the years (Fig. 17) what we find is that when looking at best wafer results,
it has decreased exponentially over the first 4 years and now, since about 2 years the improvement has slowed down considerably. When it comes to production wafers, the situation we have to face is that, for certain areas such as optoelectronic and high-frequency applications, the currently available material quality is already sufficient. The micropipe-limited yield in 1998 is above 90% both for optoelectronic and high-frequency components and is therefore not a limiting factor for those markets any longer. Since the needs of these two volume market areas have been satisfied, they will no longer provide the driving force for the reduction of the micropipe density which is still needed for power supplies, drives and even more for high voltage applications. In both cases the micropipe density has to be decreased by a factor of ten and 100, respectively to reach 40% for the micropipe related yield, which is absolutely necessary for any SiC component to enter production. Given that the current speed of material improvement can be maintained, the date for a high voltage application entering the market will be in the beginning of the next millennium.
4. Conclusions Reduction of switching losses, which are the main limiting factor for the system performance, is the driving force for high-voltage power electronics application of SiC. SiC diodes can be produced with low reverse leakage current and a low forward voltage drop. The switching characteristics of PIN and JBS diodes show that SiC devices produced today are able to offer a substantial benefit on a systems level due to a large reduction of the reverse recovery charge. Remaining technological key issues are improvement of uniformity,
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long-term stability and reliability. In the high-voltage market, a SiC diode alone already offers a systems advantage due to a 50 – 90% decrease in switching losses and a hybrid module with a silicon transistor seems to be one possible path to commercialisation. Economical considerations, however, demand a reduction of micropipes for production wafers of about a factor of 50 in order to be able to enter production at an economic level. Extrapolating the material development of the past commercialisation seems to be possible around 2001–2003. The optoelectronic and high frequency market, however, will no longer provide the driving force for material development in order to reduce the number of micropipes as it had been in the past.
Acknowledgements The authors wish to thank the SiC Electronics group at IMC, Sweden for the processing of the PIN devices, Martin Hollander for the characterisation of JBS devices. Furthermore we want to thank the Daimler Chrysler in Frankfurt, Germany, in particular R. Held and E. Niemann, for the fabrication of the JBS diodes.
References [1] N. Mohan, T.M. Undeland, W.P. Robbins, Power Electronics — Converters, Applications and Design, 2nd edn, Wiley, New York, 1995. [2] K. Rottner, A. Scho¨ner, M. Frischholz, J.-O. Svedberg, U. Gustafsson, A. Ellison, E. Janze´n, O. Kordina, Proceedings of the ICSCIII-N97, Stockholm, Sweden September, 1997. [3] B.J. Baliga, IEEE Electron Dev. Lett. 5 (1984) 194.
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[4] H.R. Chang, B.J. Baliga, Solid State Electron. 29 (3) (1986) 359. [5] R. Held, N. Kaminski, E. Niemann, Mater. Sci. Forum 264–268 (1998) 1057. [6] F. Dahlquist, C.M. Zetterling, M. O8 stling, K. Rottner, Mater. Sci. Forum 264 – 268 (1998) 1561. [7] H. Mitlehner, W. Bartsch, M. Bruckmann, K.O. Dohnke, U. Weinert, ISPSD, 1997. [8] O. Kordina, J.P. Bergman, A. Henry, E. Janze´n, S. Savage, J. Andre´, L.P. Ramberg, U. Lindefelt, W. Hermansson, K. Bergman, Appl. Phys. Lett. 67 (11) (1995) 1561. [9] A. Itoh, T. Kimoto, H. Matsunami, IEEE Electron Device Lett. 17 (1996) 139. [10] K.J. Schoen, J.P. Henning, J.M. Woodall, J.A. Cooper Jr., M.R. Melloch, Mater. Sci. Forum 264 – 268 (1998) 945. [11] J.A. Cooper Jr., M.R. Melloch, J.M. Woodall, J. Spitz, K.J. Schoen, J.P. Henning, Mater. Sci. Forum 264 – 268 (1998) 895. [12] T. Kimoto, Q. Wahab, A. Ellison, U. Forsberg, M. Tuominen, R. Yakimova, A. Henry, E. Janze´n, Mater. Sci. Forum 264–268 (1998) 921. [13] P.M. Shenoy, B.J. Baliga, Mater. Sci. Forum 264–268 (1998) 993. [14] J. Spitz, M.R. Melloch, J.A. Cooper Jr., M.A. Capano, Mater. Sci. Forum 264 – 268 (1998) 1005. [15] J.B. Casady, A.K. Agarwal, L.B. Rowland, S. Seshadri, R.R. Siergiej, S.S. Mani, D.C. Sheridan, P.A. Sanger, C.D. Brandt, Mat. Res. Soc. Symp. Proc. 483 (1998) 27. [16] T.P. Chow, N. Ramungul, M. Ghezzo, Mat. Res. Soc. Symp. Proc. 483 (1998) 89. [17] K.G. Irvine, R. Singh, M.J. Paisley, J.W. Palmour, O. Kordina, C.H. Carter, Mat. Res. Soc. Symp. Proc. 512 (1999). [18] K. Hara, Mater. Sci. Forum 264 – 268 (1998) 901. [19] P.G. Neudeck, W. Huang, M. Dudley, Proceedings of the MRS Fall Meeting, December 1997, Boston, MA, 1997. [20] S.M. Lindsay, Phys. Stat. Sol. A 53 (1979) 311. [21] M. Frischholz, T. Mandel, R. Helbig, A. Hammerschmidt, G. Schmidt, Appl. Surf. Sci. 65/66 (1993) 784. [22] K. Rottner, S. Savage, J. Andre´, L. Ramberg, M. Frischholz, R. Helbig, Inst. Phys. Conf. Ser. 142 (1996) 721. [23] M. Frischholz, K. Nordgren, K. Rottner, J. Seidel, A. Scho¨ner, M. Bakowski, Mat. Res. Soc. Symp. Proc. 512 (1999).
SiC power devices for high voltage applications K. Rottner a,*, M. Frischholz a, T. Myrtveit a, D. Mou a, K. Nordgren a, A. Henry b, C. Hallin b, U. Gustafsson c, A. Scho¨ner c b
a ABB Corporate Research, Electrum 215, S-16 440, Stockholm-Kista, Sweden ABB Corporate Research, c/o IFM, Linko¨ping Uni6ersity, S-58183, Linko¨ping, Sweden c IMC, Electrum 233, S-16 440, Stockholm-Kista, Sweden
Abstract Silicon Carbide device technology is now evolving from a pure vision to a real alternative to silicon devices. The feasibility of SiC devices has been shown for many different types of devices, the development of a working production technology has started, yield, reliability and costs now being the key issues. At present the high substrate prices keep the manufacturing costs of SiC high, making it very difficult to enter the device market with SiC on economic terms. Prime applications are those for which SiC offers substantial benefits or even a technological breakthrough on the system level. The main application is power conversion where the latest development efforts on silicon based power switches (e.g. IGBT) allow utilisation of much higher switching frequencies, putting very high demands on the free wheeling diode. The system performance is to a large extent limited by the diode recovery charge—a major source of switching losses. Depending on the voltage range, different device concepts are of interest: In the lower voltage range the junction–barrier controlled Schottky (JBS) device is a promising candidate while at voltages beyond 2.5 kV the PIN diodes is the device of choice. Different system requirements — e.g. surge current capability — make the PIN junction superior to a Schottky device for certain applications. With progress in material and technology development the ‘world’s best’ result is becoming less and less important and reproducibility is the issue. Failure analysis of defective devices needs to be established with a high number of substrate defects being still an obstacle in SiC. High leakage/soft reverse characteristics are often encountered and can usually be attributed to localised defects. It is important to identify their origin and to separate process-induced defects from those already present in the epilayer or substrate. In order to use the high power handling capability of SiC reduction of margins (e.g. in epilayer thickness and doping) is necessary. This requires narrow bandwidth of process and material variations. For paralleling of SiC devices equal current sharing under static and dynamic conditions is a fundamental requirement from the system side. Top-down calculation gives material specifications, which the supplier has to meet. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Power devices; High-voltage devices; Converter; PIN diode; Junction barrier Schottky; Switching losses
1. Applications: market pull ABB’s main business interest in the high voltage range, which is generally defined by device blocking voltages above 1000 V, starts at power ratings of several hundred kilowatts and extends far into the megawatt range for traction and HVDC systems where device current ratings of several hundred to several thousand amperes are common. Virtually any power device in these applications is part of a power converter. One specific converter topol* Corresponding author. Tel.: + 46-8-752-1047; fax + 46-70-6101007. E-mail address: [email protected] (K. Rottner)
ogy is the six pulse voltage source converter shown in Fig. 1. It can operate as an inverter (d.c. to three-phase a.c.) or a rectifier (three-phase a.c. to d.c.). Back to back configurations are also used for e.g. coupling of a.c. grids with different frequencies (50/60 Hz) or d.c.– d.c. conversion for different bus voltages [1]. The basic function of an inverter is to form a sinusoidal waveform from a uni- or bipolar d.c. bus ( + d.c., 0, −d.c.). The dissipated power in switches can only be kept to a minimum in either fully open (full load current, as little voltage across the switch as possible) or fully blocking (full voltage, no current). Thus, the most feasible way of generating a sine wave is to use the pulse width modulation (PWM) mode of operation (Fig. 2). The carrier—or switching frequency of the
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Fig. 1. Voltage source converter.
PWM is normally 39 times the line frequency, which gives roughly 2 kHz for a 50 Hz a.c. grid. The switching speed of the transistors in the PWM operation is at least one order of magnitudes faster then the time constant of the carrier frequency. The former is normally expressed in terms of current and voltage time derivatives (dV/dt, dI/dt). These derivatives usually are limited by the semiconductor performance and have a strong impact on the losses and the system performance itself. Increasing the switching (carrier) frequency of the PWM converter can considerably reduce size, weight and cost of several passive components (transformers and filters). The current limitation of the switching frequency lies around 2 kHz due to the losses generated in the switches and diodes. One main source for these losses is the reverse recovery charge of the diodes, which severely impacts the whole converter design and device rating. The current and voltage waveforms for the diode and the transistor, respectively, are shown in Fig. 3. The reverse recovery charge causes huge current transients when the inductive load current is commutated from the transistor to the free wheeling diode. In worst
Fig. 2. PWM synthesis of sinusoidal waveform: carrier frequency 39 times a.c. frequency.
Fig. 3. Waveform of diode switch.
case these current transients can cause the destruction of the converter. In order to improve the system performance in terms of either loss reduction or increased switching frequency without increased losses, SiC diodes are highly desirable. Due to the material properties of SiC— mainly the high critical electric field—a reduction of the recovery charge in the diodes by more than 90% is possible. One possible strategy for commercialisation of SiC devices in the high power range is hybrid modules where SiC diodes operate together with Si transistors. 2. SiC devices Depending on the requirements of the application, the device of choice will be either a bipolar (p–n junction diode) or a unipolar device (Schottky diode). Especially if one is concerned about on-state losses, the criterion is the maximum blocking voltage required in a specific application. As a rule of thumb the Schottky diode is the device of choice below 2 kV, the p–n diode above about 3 kV. In addition to lowering the switching losses by reducing the recovery charge, which is an important issue for the Schottky diode as well as for the p–n diode, there are further requirements from the system side which have to be met by the devices. For traction as well as transmission applications, the ability to withstand surge current (typically ten times the nominal current) events for some tens of milliseconds is important. Stability against increased junction temperature during that event is a consequent need.
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2.1. De6ice structures 2.1.1. Bipolar-PIN rectifier 4H SiC P + N − N (PIN) diodes were fabricated with a device area of 0.8 mm2 consisting of the actual active anode area (0.50 mm2), a 2-zone junction termination extension (JTE) (zone width 50 mm each, total JTE area 0.28 mm2) and a channel stop separated from the JTE. The anode was formed by implantation of boron and aluminium, details have been reported at [2]. In order to achieve a good ohmic contact a highly doped p-type epilayer has been grown by CVD on top of the anode. The CVD layer is approximately 0.5 mm thick with an Al doping of around 3×1020 cm − 3. Boron was implanted for forming two concentric JTE zones. A schematic view of the device structure is shown in Fig. 4. The device structure was simulated with MEDICI predicting a theoretical breakdown voltage of about 3.5 kV. 2.1.2. Unipolar-junction barrier Schottky rectifier (JBS) When designing a Schottky rectifier for high-voltage applications there is a fundamental limit towards the surface electric field, which determines the potential
barrier for tunneling. If that barrier becomes to thin this will result in excessive leakage current due to the tunnel current. Utilising the full potential of SiC with a critical electric field of 3 MV cm − 1 the resulting barrier thickness for a typical Schottky barrier height of 1 eV is only about 3 nm which is a typical thickness for observing tunneling phenomena. In order to overcome the limitation of the Schottky contact a number of different concepts have been proposed such as the junction barrier Schottky rectifier, a combination of p–n junctions and Schottky contacts integrated in one device [3]. Figs. 5 and 6 show the principle of this concept: in reverse bias operation the depletion region of the p–n junction parts of the device merge at a certain voltage and shield the Schottky area from the high electric field (Fig. 6). In forward bias operation the Schottky area dominates due to the immediate onset of the forward current which is controlled by the low Schottky barrier height as compared to the large band gap of the semiconductor which determines the threshold voltage of the p–n junction (Fig. 5). The JBS concept is already successfully used for Si Schottky rectifiers for a number of years [3,4] and has also been demonstrated recently for SiC [5,6].
Fig. 4. Schematic device structure of a 3.5 kV PIN diode.
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Fig. 5. Comparison of Schotty, PIN and JBS. (top) The JBS device is shown schematically in reverse and forward bias operation (for more details see e.g. [3,4]). (bottom) The current–voltage characteristics of the three devices in reverse and forward bias mode.
2.2. Experimental results— de6ice performance 2.2.1. Static characteristics
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Fig. 7. Forward current – voltage characteristic of a 3.5 kV PIN diode.
The capability to handle very high current densities was tested to up to current levels of 2000 A cm − 2 on the 3.5 kV pn diode. The result of this test, as seen in Fig. 8, allows from an application point of view to set a nominal current density to 200 A cm − 2 and still being able to handle ten times higher currents as is required for short periods of time (ms).
2.2.1.1. Forward characteristic (PIN diode). Fig. 7 shows the forward characteristic of our 3.5 kV PIN diode. The forward curve was measured using a set-up for pulsedmeasurements in order to avoid self-heating of the device during measurement. At room temperature, the forward voltage drop at 200 A cm − 2 was about 3.4 V. From this curve, the values for the threshold voltage and the series resistance were extracted as indicated in the figure. The resulting threshold voltage of 2.9 V compares favourably to the theoretical threshold voltage of about 2.8 V due to the bandgap of 4H SiC. The on resistance of 2.6 mV·cm − 2 is in accordance with the epi layer thickness and lifetime as confirmed by simulation of the actual device structure.
2.2.1.2. Re6erse characteristic (PIN diode). The static blocking characteristic is shown in Fig. 9 on a logarithmic scale up to 3 kV. The measurement of the reverse characteristic was carried out under d.c. conditions in an inert atmosphere. The current density is in the low 10 − 10 A cm − 2 range up to 2 kV and below 10 − 8 A cm − 2 up to more than 3 kV and thus does not contribute to any significant losses (power loss under blocking conditions are in the order of mW cm − 2). The shape of the curve indicates a trap-assisted band to band tunneling as reported by other groups [7].
Fig. 6. Comparison of the magnitude of the electric field as a function of the distance from the surface underneath the PIN and the Schotty part of the JBS device.
Fig. 8. Forward current – voltage characteristic to demonstrate the surge current capability of a 3.5 kV PIN diode.
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Fig. 11. Turn-off switching characteristic of a 3.5 kV PIN diode.
Fig. 9. Reverse current voltage characteristic of a 3.5 kV PIN diode.
2.2.1.3. Forward characteristic (JBS diode). The forward characteristic of a 1.8 kV JBS diode is shown in Fig. 10. In comparison to the PIN diode the forward voltage drop at 200 A cm − 2 is reduced from 3.2 V for the PIN diode to only 1.7 V for the JBS diode resulting in reduced on-state losses. The on resistance extracted from the curve is 5 mV·cm2. 2.2.2. Switching characteristics In order to determine the turn-off waveform, the PIN diode was tested in a chopper circuit where the load current was commutated between the diode to the transistor in single switching events to avoid selfheating. The measured switching waveform is shown for switching from the on state with a forward current IF of 0.5 A with a rate of change of the current dI/dt of 1
kA·cm − 2 ms − 1 into the blocking state with a reverse bias voltage VR of 500 V (Fig. 11). The waveform does not show the large current overshoot due to the reverserecovery charge observed for a conventional Si diode. Even though the base layer is significantly modulated under forward conduction for the switching speeds which were used here and which are typical for high power module applications, the bipolar SiC diode does not suffer from stored charge due to the very low minority carrier lifetimes in SiC. The reverse-recovery current in SiC is therefore not given mainly by the displacement current which is generated by the build up of the space charge region in the base layer. This is in fact the smallest recovery current that is possible for any rectifier and—in contrast to the conventional recovery charge in Si diodes—is not depending on temperature, forward current or current derivatives, but rather a function of the d.c. bus voltage.
2.3. Performance-limiting issues
Fig. 10. Forward current–voltage characteristic of a 1.8 kV JBS diode.
Progress in performance of SiC devices as demonstrated in this paper and also by many other groups during the past years has been tremendous, mostly due to continuous improvement in the material itself and the available technology, e.g. [3,5–17] and numerous references therein. As a consequence, the performance of SiC devices is now very close to matching the high expectations that have been raised in the past owing to the physical properties of SiC. Nevertheless there are still a number of issues that have to be addressed [18,19]. Among the issues there is the problem of paralleling a number of devices to be able to handle sufficiently high currents. This problem remains regardless of whether the active area of SiC devices, which is currently limited by the defect density, can be vastly increased in the future or not.
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Fig. 14. Reverse blocking characteristics of two 3.5 kV PIN diodes with sharp and soft breakdown behaviour, respectively. Fig. 12. Forward current–voltage characteristics of two 3.5 kV PIN diode with different series resistance.
2.3.1. Paralleling of de6ices Fig. 12 illustrates the problem posed by having to parallel several devices with different values for series resistance but the same turn-on voltage. At a fixed forward voltage drop the current sharing is not equal between the two diodes and, in the extreme case, one diode would conduct all the current — and in worst case be destroyed—while the other diode do not contribute to the current conduction at all. 2.3.1.1. Material Variation. The impact of fluctuations in the material properties such as lifetime and the thickness specification of substrate and epilayer on the problem of paralleling diodes is further illustrated in Fig. 13. As can be seen from the results of the simulation, the current sharing between the SiC diodes is not symmetric and the diode current density can vary by more than a factor of two between two SiC diodes. At some point the ultimate restrictions for the specification are then set by the ability of the package to withstand and spread the heat produced in the diodes.
Fig. 13. Influence of varying material specifications on the forward current density of a 3.5 kV PIN diode at a fixed forward voltage.
2.3.2. Re6erse characteristic— soft 6s. sharp breakdown Methods like statistical descriptions e.g. device yield produce only a very crude picture of the device performance and its limitations by describing an ensemble behaviour. Studying devices on an individual level by analysing leakage current and breakdown behaviour gives an already more detailed insight but is still limited by averaging over a large portion of the active device area. This is e.g. the case when studying the reverse blocking characteristics of rectifiers where one observes devices with a sharp increase in reverse current at breakdown indicating the onset of avalanche breakdown, but also devices with a ‘soft’ characteristic with slowly increasing leakage current well below the theoretical breakdown voltage (Fig. 14). The soft characteristic is often attributed to the formation of local microplasmas in the presence of material inhomogeneity or localised defects. The lack of information on whether the device is limited by localised phenomena or a uniform behaviour has been achieved is also a major problem when it comes to a comparison between simulations and experimental results. 2.3.2.1. Optical beam-induced current measurements (OBIC). OBIC has proven to be a valuable tool for collecting local information on power semiconductor devices in Si and SiC [20–23]. Due to the current sensitivity of the OBIC technique it can be applied to detect failures in the device that manifest themselves as peaks or ‘hot spots’ in the photocurrent distribution already at low voltages. The possibility to discriminate between electrically active defects and other non-active defects seen e.g. in an optical microscope allows identifying the cause of individual extrinsic device failures on a microscopic scale. Fig. 15 shows two OBIC images taken from devices with sharp (left) and soft breakdown behaviour (right), respectively. In the case of the device with a sharp breakdown the two zones of the JTE can be clearly distinguished as two concentric rings
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Fig. 15. Greyscale encoded OBIC images of two 3.5 kV PIN diodes, excitation wavelength 313 nm. (left) Device with sharp breakdown behaviour and uniform photocurrent distribution, Vrev = 500 V. (right) Device with soft breakdown with a ‘hot spot’ in the JTE region, Vrev = 300 V.
surrounding the darker speckled anode area by the different grey level belonging to each zone. The probe tip is discernible as the black feature pointing from the left side towards the centre of the diode. The diode with soft breakdown behaviour was measured at a reverse bias of 300 V. In this case a high photocurrent is flowing right at the anode edge, discernible as a bright ring surrounding the anode area. At the righthand side of the image, a bright area (hot spot) can be observed that stretches from the anode edge to the channel stopper. Whenever ‘hot spots’ are observed in OBIC images, this coincides with a steady increase of the leakage current in the I – V characteristic already at low voltages as compared to the design voltage is measured.
2.3.3. Long-term stability Another important issue is the long-term stability of devices, which, in a commercial system, have to function without failure over a period of 20 years. Most of the research and development so far has concentrated on demonstrating the capability of SiC devices in terms of absolute performance figures rather than on reproducibility and reliability. What the problems are those have to be solved become evident when the d.c. stability is measured. This is illustrated in Fig. 16 where the reverse leakage current at a constant reverse bias voltage of 2.9 kV of three diodes is plotted as a function of time. Although two of the diodes start at roughly the same leakage current level around 10 − 9 A after about 50 h of continuous operation in blocking mode the leakage current has risen by five orders of magnitude to 10 − 4 A. The other diode, however, starting with a leakage current of 10 − 5 A showed only little change in the leakage current, in contrast to the other diodes the leakage current even slightly decreased.
3. Economic considerations Once the feasibility of a new technology has been shown and even long before, the question arises whether the new technology will be successful on the market in terms of products. The two most obvious factors are cost and performance, which, for different markets, have different importance. In a typical niche market application performance is the key factor and the price of a component or module is basically irrelevant. This is the case where there exists no other solution to a specific problem than the new technology. On the other side, there are volume markets for which cost is essentially the only factor that really matters. This is usually the case if there is already an existing technology that can be used and the new technology offers a distinct advantage in cost.
Fig. 16. Reverse leakage current of three 3.5 kV PIN diodes as a function of time at a constant reverse-bias voltage of 2.9 kV. The devices were kept at a constant temperature of 50°C during the experiment
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Fig. 17. Development of the micropipe density of the best published values for SiC and production wafers. In addition the 40 and 90% lines for the micropipe-limited device applications are show.
What that means in reality for a SiC component used in a product shall in the following be exemplified for a typical power module (1.2 – 2.5 kV) comprising Si IGBTs and Si diodes with a module price of 1 USD/A. In this module we will later replace the Si components by SiC components. Subtracting the typical gross margin the manufacturing costs of this module are around 0.6 USD/A of which 0.3 USD/A is the cost of the semiconductor component, the rest of the cost is connected to packaging, material other than the semiconductor and testing. For a typical current density of 70 A cm − 2 that means a semiconductor value of 25 USD cm − 2. With a price of 100 USD for a 4 inch Si epiwafer that translates to 2 USD cm − 2 or 0.03 USD A − 1 (valid for high yields only). When we now replace Si by SiC components we will make the assumption that the current density of the SiC components will be approx. three times the current density of the Si components. The second assumption will be that the epiwafer will contain no micropipes limiting the yield. With a price of 1 –2 kUSD for the 2¦ SiC epiwafer we will then arrive at 60–120 USD cm − 2 or 0.3 – 0.6 USD A − 1. The material costs which where negligible in the case of Si will now double the manufacturing cost. Under these assumptions, the final price for the SiC module will under these assumptions be about 1.5 – two times the price of a Si module. This, however, may be permissible in a typical niche market application provided that the added value for the customer, which is offered by the SiC module in terms of switching speed and power losses, matches the increase in cost. After having answered the first question, we have to consider the validity of our assumptions, in particular the micropipe issue. When we follow the development of the density of micropipes over the years (Fig. 17) what we find is that when looking at best wafer results,
it has decreased exponentially over the first 4 years and now, since about 2 years the improvement has slowed down considerably. When it comes to production wafers, the situation we have to face is that, for certain areas such as optoelectronic and high-frequency applications, the currently available material quality is already sufficient. The micropipe-limited yield in 1998 is above 90% both for optoelectronic and high-frequency components and is therefore not a limiting factor for those markets any longer. Since the needs of these two volume market areas have been satisfied, they will no longer provide the driving force for the reduction of the micropipe density which is still needed for power supplies, drives and even more for high voltage applications. In both cases the micropipe density has to be decreased by a factor of ten and 100, respectively to reach 40% for the micropipe related yield, which is absolutely necessary for any SiC component to enter production. Given that the current speed of material improvement can be maintained, the date for a high voltage application entering the market will be in the beginning of the next millennium.
4. Conclusions Reduction of switching losses, which are the main limiting factor for the system performance, is the driving force for high-voltage power electronics application of SiC. SiC diodes can be produced with low reverse leakage current and a low forward voltage drop. The switching characteristics of PIN and JBS diodes show that SiC devices produced today are able to offer a substantial benefit on a systems level due to a large reduction of the reverse recovery charge. Remaining technological key issues are improvement of uniformity,
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long-term stability and reliability. In the high-voltage market, a SiC diode alone already offers a systems advantage due to a 50 – 90% decrease in switching losses and a hybrid module with a silicon transistor seems to be one possible path to commercialisation. Economical considerations, however, demand a reduction of micropipes for production wafers of about a factor of 50 in order to be able to enter production at an economic level. Extrapolating the material development of the past commercialisation seems to be possible around 2001–2003. The optoelectronic and high frequency market, however, will no longer provide the driving force for material development in order to reduce the number of micropipes as it had been in the past.
Acknowledgements The authors wish to thank the SiC Electronics group at IMC, Sweden for the processing of the PIN devices, Martin Hollander for the characterisation of JBS devices. Furthermore we want to thank the Daimler Chrysler in Frankfurt, Germany, in particular R. Held and E. Niemann, for the fabrication of the JBS diodes.
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