An overview of cree silicon carbide power devices

An Overview of Cree Silicon Carbide Power Devices Jim Richmond, Sei-Hyung Ryu, Mrinal Das, Sumi Krishnaswami. Stuart Hodge Jr., Anant Agarwal and John Palmour Cree Inc

Abstract - The compelling system benefits of using Silicon Carbide (SIC) Schottky diodes have resulted in rapid commercial adoption of this new technology by the power supply industry. Silicon Carbide PM diodes, MOSFET’s, and BJT’s, a r e approaching the point of development that they could he transitioned to volume production. This paper reviews the characteristics of recently produced Sic devices including Schottky diodes, R N diodes, MOSFET’s, and BJT’s. A comparison of the static and dynamic performance of the Sic devices and typical silicon devices is performed. The results show the performance improvement available with SIC devices. The high temperature performance capabilities of SIC devices are also highlighted.

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I. INTRODUCTION American inventor Edward Acheson first made Silicon Carbide (SIC) in 1891 while trying to make diamond. In its polycrystalline form this material is widely used for grinding, sawing and polishing. It wasn’t until recently that single crystal Silicon Carbide growth techniques advanced sufficiently for the material to be used in the development of power semiconductor devices. As a semiconductor, Silicon Carbide has many unique properties that make it a near ideal material for high voltage or high temperature operation. Commercial shipments of Sic based Schottky rectifiers with 300, 600, and 1200V ratings have been achieved over the last few years. As with all new technologies. the limiting factor has been cost. Cree has been utilizing Silicon Carbide as a semiconductor material for more than 15 years in the production of blue and green LED’s. It has only been recently that the defect density has been reduced to a level low enough to allow the fabrication of large area power devices with an acceptable yield. With wafer sizes going up and defect densities going down, the possible device size is increasing. At the same time, material costs are dropping and a true competition with silicon power devices is evolving.

Fig. I Semiconductor properties for Sic, GaAs, and Si

The breakdown field of S i c is almost 9 times higher the breakdown field of silicon. This means that a similar device built in S i c will have 9 times the breakdown voltage rating of the Silicon counter part. The bandgap is defined as the energy difference between the valence band and the conduction band in the material. In wide handgap materials, (like SIC), the increase in current due to thermal generation is much lower than in silicon. This allows the creation of devices that can operate at much higher temperatures than their silicon counterparts. 111. COMMERCIAL SIC SCHO’ITKY DIODES

Due to the properties of Silicon Carbide, Cree, Inc. is able to manufacture high voltage Schottky diodes. These Schottky diodes are unipolar devices and thus have zero reverse recovery current and zero forward voltage recovery. The Cree S i c Schottky diode uses a Junction Bamer Schottky (JBS) structure. These structures add robustness to the design leading to higher device reliability. The basic JBS diode is created by merging Schottky diode and PiN diode structures in a single device. P‘ wells are created in the Schottky drift layer. Under reverse bias, a depletion region builds around the P’ wells effectively shielding the Schottky metal interface from high electric fields. This shielding has the added benefit of reducing possible increased field levels caused as a result of surface irregularities. Through careful design, it is ensured that these Pi wells never form a forward biased diode when the Schottky is conducting forward current. If this PN junction was allowed to tum on, the resulting reverse recovery current could have a disastrous effect on the rest of the power circuit. This is true

II. WHY SILICON CARBIDE? Silicon Carbide has several unique properties that lend themselves to the development of high voltage devices. Figure 1 shows some of lhe basic properties of Silicon Carbide compared with Silicon and Gallium Arsenide. Clearly, S i c has a strong advantage over Si and GaAs. The thermal conductivity of SIC is over ten times that of GaAs and three times that of Si. In fact, 4H-Sic has a higher thermal conductivity than copper.

0-7803-8538-1 /04/$20.00 02004 IEEE.

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not only during normal operation, but also during surge events which already place extreme stress on circuit components. Presently, Cree is manufacturing a series of JBS Schottky Diodes with voltage ratings of 300, 600, and 1200 volts with single die current ratings from 1 to I O Amps. Single die versions are available in D’-pak, D-pak, and TO-220 package configurations. Dual die versions of the diode are also available in a TO-247 package. Due to the positive temperature coefficient of the on-resistance of the S i c Schottky diode, devices can be paralleled without de-rating. This allows the dual die versions to be used as two separate devices with a common cathode, or to be paralleled and used at twice the single device rating. This ease of paralleling also allows the devices to be combined in semi-custom modules. These modules can consist of multiple S i c diode die to create high c u e n t rectifiers or a combination of SIC diodes and silicon MOSFETS or IGBTs to create high current switch assemblies. Module examples are shown in Figure 2. Due to the flexibility of combining off-tbeshelf devices in common modules, a large number of high current applications can be addressed. With this method, the system advantages of SIC Schottky diodes can be realized today in a cost-effective manner. P

lafer. Insert shows a diode under test. IV. DIODE ROAD MAP

Fig. 2. Examples of semi-custom Modules: (clockwise from top left) 1200 volt, 400 amp S i c diode; 300 volt, 120 amp SIC diode (Zx6OA); 1200 volt, 200 amp S i c diode with silicon IGBTs.

Cree is continuing to increase the Schottky diode product offering by releasing higher current devices. Recently 5.6 mm x 5.6 mm, 600 volt, 100 amp and 1200 volt, 50 amp S i c Schottky diodes (Figure 3) have been demonstrated. With a forward voltage of 1.7 V, reverse leakage below 200 pA and a capacitive charge of 171 IC,these devices show excellent characteristics. Higher temperature and higher voltage diode operation dictates a switch from a Schottky diode technology to a PiN diode technology. As the junction temperature increases, the Schottky diode forward voltage, and therefore the conduction losses, increase. The Schottky diode on-resistance increases as the diode blocking voltage is increased. Due to this, it is impractical to design a S i c Schottky diode to operate at greater than about 2500 volts. With the PIN diode, the forward voltage and conduction losses decrease with increasing temperature. Because this negative temperature coefficient is less pronounced than in Si, the S i c PiN diodes can be paralleled without forward current de-rating. As shown in Figure 4, the PIN diode has a knee voltage of 2.8 volts at 25°C. Due to this higher knee voltage, the S i c Schottky diode has a lower forward voltage drop than the S i c PiN at lower temperatures.

Since the PIN diode is a bipolar device it does have some stored charge and will not switch as fast as the Schottky diode. However the S i c PiN will switch approximately I O times faster than a silicon PiN diode. Cree has demonstrated [ l ] S i c PiN diodes with blocking voltages as high as 20 kV, which is the highest reported single junction diode blocking voltage reported to date. The largest S i c PiN diode made to date is a 8.5 mm x 8.5 mm device capable of blocking 9 kV when reverse biased and carrying 50 amps of forward current. The forward and reverse I-V's for this device are shown i n Figure 5. While 20 kV production diodes may still be a few years off, 4 kV to 6 kV S i c PiN diodes could he made today with reasonable yields. V. SWITCH ROAD MAP

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Cree is also working on unipolar and bipolar switches. The unipolar switch will he used at temperatures up to 200°C while the bipolar switch will be targeted for higher temperature and/or higher voltage applications. Cree's goal is to commercialize a 1100 V N-channel MOSFET that could replace Si IGBT's and Si MOSFET's of similar ratings. A SIC MOSFET will switch faster than a Si MOSFET and have lower on-state losses than a Si IGBT. The bipolar device is a 1200 V NPN BJT. The S i c BJT will switch faster and have lower conduction losses than a Si IGBT. Furthermore, the S i c BJT can easily operate at temperatures above 250°C. The output characteristics of a Sic MOSFET [2] show normal MOSFET characteristics (Figure 6). The gate threshold voltage is approximately 4 volts. The drain-source voltage of 1.5 volt at S amps yields a RDS(,) of 0.3 ohm. This RDS~..,is at least 33 times lower than the Roscon1of a commercially available 1200 volt Si MOSFET when normalized for die size. A comparable 12OOV Si Ultrafast IGBT has a VCE(sA~) of 3.2 volts at 5 amps which is more than twice that of the Sic MOSFET. When compared to either the Si IGBT or the Si MOSFET, the S i c MOSFET has significantly lower m (on-state) conduc .___--_-_. -- losses.

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Forward Voltage (volts)

Fig. 4. Comparison of 1200 volt, 5 Amp S i c PiN and S i c Schottky diode forward characteristics.

As shown in Figure 4,the forward voltage and therefore the conduction losses of the S i c PiN and Schottky diode will be equal at 175°C. This suggests that for higher temperature operation, the S i c PiN diode would offer lower conduction losses. For higher voltage devices this crossover will happen at lower temperatures

Fig. 6. 120OV S i c MOSFET Output Characteristics @ Ti = 25'C

Fig. 5. I-V characteristics of a 8Smm x 8.5, 9kV, 50 Amp S i c PIN diode.

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Fig. 9. Collector-emitter saturation voltage vs collector current for temperatures up to 325°C of a S i c BJT.

Fig. 7. 1200 volt, 5 Amp S i c MOSFET on-state characteristics at 25OC and 175"

Figure 9 shows the collector-emitter saturation voltage at collector currents up to 5 amps over the temperature range of 25°C to 325°C. in 50°C steps. The Vacsar, at 5 amp is shown to increase with temperature from 1.2 volts at 25°C to 2.4 volts at 175°C. Even at 325°C the VCE(SAT) is only about 5.15 volt. Figure 10 shows the BJT HE (current gain) at collector currents up to 5 amps over the temperature range of 25°C to 325"C, in 5 0 T steps. The current gain shows a decrease with temperature up to 275'C. Between 275°C and 325°C the gain begins to increase with temperature. The overall current gain goes from 20 to 13.4, a 35% reduction over the temperature range of 25OC to 275°C. The increase in V,E,S,T, and the reduction in current gain with temperature, indicate the BJT will parallel well without de-rating. Cree is currently scaling the BJT to a 25 - 30 m u device.

The on-state characteristics of the 1200 volt, 5 Amp S i c MOSFET are shown in Figure 7 for a VGS of 20 volts and junction temperatures of 2 5 T and 175OC. From 25 to 175°C the RoS.ONincreases by only 20%, from 0.3 ohm to 0.36 ohm. Our goal is to scale this device for a 0.1 ohm Ros.0~.which would allow I O amps of current at a voltage drop of only 1 volt. The S i c MOSFET may operate reliably at temperatures up to 200"C, which is beyond the range of Si switches. However gate dielectric reliability issues may keep this device from being used at the extreme temperatures required by many applications. To operate at very high temperatures a bipolar switch, such as a Silicon Carbide BJT, will offer the best performance bv offerina the lowest combined switching- and con ction losses. Figure 8 shows a typical 25°C CollectorEm a I-V characteristics for a 1.5 mm x 1.5 mm, 1200 volt, Sic JT [3]. This device has a common emitter current gain of 20, I a V,,,,, of 1.2 volt at 5 amps. .. . . I

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Fig, IO. Current gain vs collector current of a Sic BJT for temperatures up to 325°C.

Fig. 8. The output characteristics of a 1200 V SIC BJT at Tj = 25°C.

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VI. SWITCHING LOSS COMPARISON Figure 1 1 shows the turn-off voltage, current and instantaneous power measured at a junction temperature of 125°C for the Si ultrafast diode. This shows a peak reverse recovery current of 6 amps, a recovery time of 148 ns, and a peak instantaneous power of 2.8 kW. Figure 12 shows the tum-on voltage, current and instantaneous power measured at a junction temperature of 1 2 5 T for the IGBT with a Si ultrafast diode. During the IGBT turn-on the diode reverse recovery current is added to the IGBT current, resulting in a peak current of 11.7 amps. A peak instantaneous power of 1 1 kW is dissipated in the IGBT. Figure 13 shows the turn-off waveforms for the Sic Schottky at 125°C. It shows a peak reverse recovery current of 1 amp, a reduction of 83%, a recovery time of 30 ns, a reduction of 80% and a peak instantaneous power of 0.3 kW, a reduction of 89% when compared to the Si diode. Figure 14 shows the waveforms at a junction temperature of 125°C for the Si IGBT with a SIC Schottky diode. The use of the S i c Schottky diode results in a peak current of 6.7 amps, a 42 % reduction and a peak instantaneous power of 6.2 kW, a 44 % reduction when compared to the Si IGBT with the Si PiN diode case. A comparison of the switching parameters of the S i c Schottky diode with the Si ultrafast diode are shown for measurements taken at a junction temperature of 125°C in Table 1. All measured parameters show a major improvement with the S i c Schottky diode. The total switching losses are reduced from 1.74 ml (IGBT + Si PIN diode) to 0.89 nd (IGBT + S i c Schottky diode) or 49% at 125°C. In order to show the switching benefit of a S i c switch, the Si IGBT in the switching test circuit (see AECV 2003 [4]) was replaced by the SIC MOSFET. The losses were measured at 1000 V and 5 A with the temperature set to 125 'C. Figure 15 shows the turn-on waveforms for the S i c MOSFET at 125°C. While the peak instantaneous power is similar to the Si IGBT, the SIC MOSFET is turning on faster resulting in turn-on switching losses being reduced by 55% when compared to the Si IGBT. The remaining switching parameters for the Sic Schottky diode with the Sic MOSFET are shown for measurements taken at a junction temperature of 125°C in Table 1. Since the MOSFET does not have a turn-off tail like the IGBT, turn-off losses with the S i c MOSFET are significantly reduced. A reduction of 68% was measured. Total switching loss reduction (Sic MOSFET + S i c Schottky) is 60% at 125°C when compared to the Si IGBT with the Sic Schottky diode. The same switching test circuit was used to measure the switching losses of the S i c BJT. Measurements were made at 1000 V and 5 A with the temperature set to 125°C. The measurements were then repeated at a temperature of 25OOC. Figure 16 shows the SIC BJT turn-on waveforms at 250°C. The S i c BJT waveforms and the switching data in Table 1 show improved performance over the Si IGBT with the S i c Schottky diode. The total switching losses reduction (SIC BJT + SIC Schottky) was 25% at 1 2 5 T increasing to 31% at 250°C

To investigate the switching benefit of SIC devices, a SIC Schottky diode and a Si PIN diode were compared, along with their effect on the switching losses of a Si IGBT. Switching parameters were measured for an 8 A, 1200 V ultrafast soft recovery silicon diode and a 5 A, 1200 V SIC Schottky diode, along with the switching losses of a 1 I A, 1200 V IGBT. The losses were measured at a voltage of 1000 V and current of 5 A. The maximum temperature used in this testing was 125°C. The measurement details were presented at AECV 2003 [4].

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Fig. 11. Si Ultrafast diode at 125°C.

Fig. 12. IGBT w/ Si Ultrafast diode at 125°C.

Fig. 13. S i c Schottky diode at 125°C.

Fig. 14. IGBT w/ S i c Schottky at 125°C.

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allow extended high temperature operation. For extreme temperature operation, a SIC BIT switch will offer excellent performance. When the temperature is beyond the operating range of the S i c Schottky diode, a SIC PiN diode will keep conduction losses low while having minimal impact on switching speed. While silicon devices are struggling to achieve performance improvements of fractions of a percent, S i c devices, even at this early stage of development, are showing both conduction and switching losses reductions of 50% and greater.

when compared to the Si IGBT with the S i c Schottky diode. The improvement at 250°C may be due to a reduction in the inteinal base resistance with temperature. These results indicate that the S i c BJT can switch effectively at elevated temperatures. VII. CONCLUSION S i c devices exhibit exceptional conduction and switching perfoimance when compared to silicon devices. The use of the S i c Schottky diodes available today and the SIC MOSFETs when they become available will reduce system losses and

and Related Materials (ECSCRM), Bologna, Italy, Sept 1 - 4, 2004.

ACKNOWLEDGMENT The authors would like to thank Mr. J. Weimer, Dr. J. Scofield and Mr. C. Severt of the Air Force Research Lab, and Dr. C. Scozzie of the Army Research Lab for their support.

[21 Sumi Krishnaswami, Anant Aganval, Craig Capell, Jim Richmond, Sei-Hyung Ryu, John Palmour, Santosh Balachmdran, T. Paul Chow, Stephen Bayne, Bruce Geil, Kenneth A. Jones and Charles Scozzie '1000 V, 30 A S i c Bipolar Junction Transistors and Integrated Darlington Pairs", presented at the 5th European Conference on Silicon Carbide and Related Materials (ECSCRM), Bologna, Italy, Sept 1 - 4, 2004.

REFERENCES

[ I ] M.K. Das, J.J. Sumakeris, B.A. Hull, J. Richmond, S . Krishnaswami, and A.R. Powell: Drift-Free, 50 A, I O kV, 4HS i c PIN Diodes with Improved Device Yields (European Conference on Silicon Carbide and Related Materials, Bologna, Italy, Sept. 2004).

[41 Jim Richmond, "Application of S i c Schottky diodes for Increased Power Converter Efficiency", Proceedings of the 5' International All Electric Combat Vehicle (AECV) Conference, Angers, France, June 2 - 5,2003.

[ I ] Sei-Hyung Ryu, Sumi Krishnaswami, Mrinal Das, James Richmond, Anant Agarwal, John Palmour, and James Scofield, "4H-Sic DMOSFETs for High Speed Switching Applications", presented at the 5th European Conference on Silicon Carbide

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