Emerging Silicon Carbide Power Electronics Components

J. C. Zolper Defense Advanced Research Projects Agency

3701 North Fairfax Drive Arlington, VA 22203

Abstruct--Power electronics engineers seek to optimize the performance of their circuits and systems to maximize efficiency, reduce size and cost, and perfect power quality. To support this, the developers of power electronics components have driven the performance of the underlying silicon-based switches and diodes to reduce on-state and switching losses, increase frequency of operation, and expand the integration of control electronics. However, some silicon power electronics components are facing fundamental limits in performance that may not support future system requirements. This paper will describe the emergence of a new class of power electronics components based on the wide bandgap semiconductor silicon carbide (SIC) that will extend the design space for future power electronic engineers.

I. INTRODUCTION

Just as silicon-based digital electronics manufacturers continue to improve circuit performance by advancing the core transistor technology to follow Moores Law, manufacturers of silicon power electronics components continuously improve on the component performance through enhanced device designs and more aggressive processing techniques. However, just as high speed silicon CMOS transistors are facing fundamental limits in scaling that may force the industry to radically different transistor concepts, power component performance is also faced with the intrinsic limitations of silicon technology leading to the emergence of a new class of power electronics components based on silicon carbide (Sic) material.

The ideal power switch should have low on-state resistance, IOW switching loss, high operation frequency, and good thermal capability. In selecting the ideal switch for an application, circuit designers may trade-off between the switching speed and the on-state voltage drop. Other important considerations include: high temperature capability; radiation hardness; ease of current control; ease of protection under abnormal modes of operation; and whether the device is normally-on or normally-off [I] . Beyond these technical considerations, the designerslproduct engineers also seek to minimize costs.

Power devices made with Sic are expected to show great performance advantages compared to those made with Si. S i c has a ten times higher critical electric field and three times higher thermal conductivity (attributed to the strong silicon to carbon covalent bond). Both these material

0-7803-8975-1/05/$20.00 02005 IEEE.

properties can be exploited in power devices. High breakdown electric fiefds allow Sic power devices to be fabricated with thinner and higher doped blocking laycrs reducing on-state resistance. As seen in Fig. 1, for a given blocking voltage, the on-state resistance for S i c components can be 400 times lower compared to a Si device [2]. Superior on-state performance leads to lower switching loss that in turn can be traded off to enable higher switching frequencies. The high thermal conductivity of S i c readily dissipates heat enabling an increase in the applied power to the device for a given junction temperature. In addition, the large bandgap of the semiconductor is exponentially inversely proportional (I E exp(-E,kT)) to the generation of background carriers. This results in a much higher operating temperature and higher radiation hardness for S i c devices.

This paper begins with an overview of the fundamental limits and technological challenges that Sic materials and devices face in order to expand their penetration into the commercial and military markets. Then, a few applications will be profiled in voltage ranges between 300 V to 10 kV using the current state of the art devices, while identifying areas for improvement. Finally, the benefits of utilizing Sic devices for each application will be discussed.

102 103 104 Blocking Voltage (V)

Figure 1. Theoretical on-state resistance versus blocking voltage for Si and SIC power electronics components.

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11. Sic MATER~ALS

The attractivc intrinsic matcrial propertics of S i c have been known for many years, but the obstacle for cornmercialization has been the poor quality of S i c wafers, resulting in low device yields. Ln particular, S i c substrates used to produce power devices have been plagued, until recently, by a defect known as a micropipc or open core screw dislocation. However, recent advances in wafcr quality, including the reduction in micropipe density, have resulted in the emergence of the first commercially available power components.

Fundamentally, device yield is a function of defects, such as micropipes existing in the wafer. Fig. 2 represents the calculated device yield vs. current at different micropipe densities; this plot assumes micropipes are the sole defect affecting device breakdown and that the devices are operating at a current density of 150 A/cm2. A reasonable ideal yield of 87% for commercially attractive 20 A rated device is expected when the micropipe density is less than licm. Additionally, if StC devices operate at the higher current densities, yicld improvements are also observed. For example, a yield of 95% for a 20 A device operating at 350 Ncm can be expected for the same defect density of l/cm2 [l].

To achieve high device yield, a recent program hnded by the Defense Advanced Research Projects Agency (DARPA)* Wide Bandgap High Power Electronics (WBG- HPE) Phase 1 demonstrated SIC substrates with micropipe densities reduced to 0.2/cm2. This has led to commercial high quality three inch n-type 4H-Sic wafers, albeit at a higher per unit cost than silicon 131. Due to their current expense, Sic components will first enter the market for use in high value applications where the superior performance is demanded.

111. sic POWER SEMICONDUCTORS DEVtCES

A . Sic Schof tb Diode S i c Schottky Barrier Diodes (SBD) are commercially

available today from two primary suppliers with ratings of 600-1200 V / 1-10 A [4] and 300 V - 1700 V I 4 -25 A IS]. The prime benefits of the Sic S3D lie in its ability to switch fast [< 50 ns) with almost zero reverse recovery charge, independent of forward current and temperature. Fig. 3 shows the reverse characteristics of the 10 A I 600 V SBD with a leakage current of 50 pA at 25 C and 70 FA at 200 C - a very nominal increase for such a wide temperature range (see Fig. 3). These Zero Stored Charge diodes are finding application in high performance power supplies and power factor correction units [ 6 ] . The remaining challenges for Sic SBDs are to achieve the lowest possible on-state voltage drop and high breakdown voltage for a given voltage blocking epitaxial layer and to further reduce cost by increasing device manufacturing yield [7].

B. 4 H - S i c PiN Diode The 4H-Sic diodes are attractive choices as high power

rectifiers compared to Si PiNs due to their extremely high blocking voltage potential (2 10 kV) and 10 times faster switching speeds than a silicon device [8]. However, growth of pure, low defect density epitaxial layers with sufficiently high minority carrier lifetimes has been a challenge for commercialization of such high voltage rectifiers.

Recently, high quality I00 pm and 200 pm epitaxial drift layers have been grown for 10 and 20 kV PiN devices with forward voltage drops 2 3.9 and 6.3 V, respectively, at IO0 A h 2 (See Fig. 4). This level of reverse blocking has never been achieved with silicon diodes. However, these early demonstrations have suffered from device instabilities due to defect generation under forward bias operation, and from poor device yield due to inconsistent reverse leakage in large devices. These limitations are being overcome with reccnt results showing 10 kV diodes with stable forward voltage (V,) characteristics (less than 100 mV voltage drift) over 100 hrs [ 9 ] . This has been accomplished by further improvements in the material growth technology.

50

40 0 2 4 6 8 10 12 $4 16 16 20 22 24 26 28 30

Current (A)

FIgure 2. Calculated yictds as a function of current rating and defect densities in Sic.

Figure 3 . The reverse characteristics of a 10A1600 V 4H-Sic SBD.

.MWP Lfurnia/ian abuiir DARPA is uvuilable al mw.durpo.niil

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Improvements in the repeatability of the reverse blocking characteristics have also been demonstrated through advances in device edge termination as was done in the early days of silicon device development [I. Work continues to optimize the performance of these devices for commercial and military applications. During Phase I WBG-HPE, Sic PiN diodes were demonstrated with reverse blocking of > 9 kV with IO0 gm drift layers and Vf = 4.2 V (drift of < 100 mV) at 20-50 A (current density of 100 A/cm2) [3]. The focus o f the next phase of the program is developing 10 kV PiN Diodes with Vr 5 4 . 2 5 V and die yield of more than 30%, operating at junction temperature of 200 C [ 101.

C. 4 f f - S i c MOSFET One of the most significant advantages of Sic is the

ability to grow a stable thermal oxide, much in the same way as on Si, enabling fabrication of MOS-based device structures. S i c MOSFETs are promising in high-power applications due to their low on-state voltage drop (at lower current densifies), high switching speeds, lower drift layer charge compared to Si, and high temperature stabitity. However, S i c MOSFETs are still far away from commercialization due to their extremely low electron inversion channel mobilities (-10 cm2Ns) [ l l ] .

3 c

E a U I U 0 a .-

0.0 0.5 1.0 1.5 2 0 2 5 3.0 3.1 4.0

Forward Voltage M

0 1 2 3 4 5 6 7 Forward Voltage (V)

Figure 4. Forward conduction of high power 4H-Sic PiN diodes fabricated on a) 100 pin and b) 200 pm epilayers. Forward voltage drops of 3.9 and 6.3 V are observed. Reverse blocking is nominally IO and 20 kV, respectiveiy

Low channel mobilities increase on-resistance to values that prevent operating at low on-state voltage drops. If channel mobilities improve to 50 cm*Ns, the potential of Sic MOSFETs begin to be realized [ 11,

Significant work is ongoing to improve the devices specific on-resistance through surface analysis, S O 2 growth conditions, and post-implantation annealing [ 121. For example, work done in WBG-HPE Phase I showed an excellent low on-state resistance of 123 mR-cm2 for a blocking voltage of 10 kV DMOSFETs with active area = 4 . 2 ~ 1 0 . ~ cm2 and guard ring termination (See Fig. 5) [,13]. The Phase I1 extension will work on developing 10 kV MOSFETs with on-state resistance of 5 0.25 a-cm with a die yield of > 30% at 200 C junction temperature [I .

D. 4H-Sic IGBT IGBTs combine the current density and the low loss of

bipolar transistors with the speed and high impedance of the MOSFETs. S i c bipolar devices show 20-50 times lower switching losses, and also lower on-state voltage drop (even with order of magnitude smaller carrier lifetimes in the drift region) as compared to similarly rated Si bipolar devices. Low on-state voltage drop enables > 20 kV devices to be more reliable and thermally stable.

IGBTs are classified into two categories, N-channel and P-channel. Compared to N-channel, P-channel lGBTs have higher an-state power losses (due to lower inversion layer hole mobility), but have lower turn-off losses because of the use of NPN transistor [I] . In general, for 4H-Sic fabrication a P-channel IGBT is more suitable than a N- channel IGBT because highly conductive P-type substrates are not currently available. To circumvent the Iack of P- substrates, alternative fabrication techniques have been suggested using N-substrates for N-channel devices. The WBG-HPE Phase 11 goal will evaluate the construction of

z E

E n

Y

C .-

L

180

140

100

60

20

1 3 5 7

Drain-Source

Figure 5 . On-state characteristics for a lOkV Sic MOSFETs with active area = 4 .2~10 cm.

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P- and N- channel IGBTs to achieve 10-15 kV devices with VF 5 4.25 V and die yield of more than 50% at junction temperature of 200 "C [ I . To date little research has been done on Sic IGBTs, but further improvements in materials quality and device design arc expected to dramatically improve the operating characteristics.

E. 4H-Sic Field Cutitrolled Thyristor: Higher breakdown electric field, carrier saturation

velocities, and thermal conductivity of S i c enable S i c thyristors to have superior performance - fast switching, low voltage drop at high current density, higher tcmpcrature operation (up to 527 "C) and higher radiation hardness - compared to Si thyristors [14]. High voltage applications in the range of 2-5 kV, such as traction control and DC transmission, are potential candidates to benefit from S i c thyristors. In these applications, one single SIC thyristor can replace multiple Si thyristors.

To date, the most promising S i c thyristor structure is p-n- p-n, which permits the use of low-resisitivity N-type substrate. These devices showed blocking voltages of 600 V both in forward and reverse bias [IS]. This device has a rated forward current of 1.8 A (563 A/cm2) with a voltage drop of 3.7 V.

IV. sic APPLICATION INSERTION OPPORTUNTIES Currently, there are significant efforts to accelerate the

development and insertion of the new high voltage and high frequency SIC devices needed for commercial and military applications. The frontier or power electronics, and S i c components in particular, i s to realize high power devices to enable high voltage systems to employ the same power conversion and distribution concepts available today only at lower voltages with silicon. For example, research is underway to fabricate components at 2 10 kV for solid-state power substations to be used in commercial utilities or naval shipboard power distribution. A number of other applications can benefit from Sic electronics to overcome the limitations of current technologies as discussed in the following sections.

A . Commercial hybrid electric vehicles (HEV) are being

developed to minimize the dependence on oil-refined fuels and supplies and to help curb the destruction of the environment from the burning of fossil fuels. HEV use both the usual internal combustion engine (ICE) powered by gasoline plus an electric motor (EM) with a high voltage battery. Fig. 6 displays the 2004 Ford Escape Hybrid and the schematic for the HEV system revealing the interconnection between the different drive trains [ 161. As seen in the figure, the battery is connected to an ac-dc inverter providing power to the traction motor and generator converting electrical energy into the mechanical drive.

Current HEV inverter designs use Si IGBT switches linked together as i i module to achieve the required power levels. Individual IGBTs have breakdown and threshold

Hybrid electric vehicle- 300-600 V

vokages of 650 V and 3.5 V at 250 pA rcspectivcly and an on-slate currcnt of 50 A. The power module (IGBTs + Schottky diode) has on-state current of 400A, and thermal resistance of 0.107-0.1 1 I "CIW for ICBT and 0.137-0.127 "CIW for the diode.

Next generation designs will demand significantly tnorc performance from the power dcvices specifically in the areas of: increased current capabilities, minimized losses, minimized parasitics, good electrical isolation, and thcrmal and mechanical ruggedness [ 171. As suggested previously, Si deviccs will not meet the demands; therefore new devices with more advanced capabilities, such as S i c power modules, will be required. Advantages include: higher breakdown voltagcs; lower on- resistances; larger thermal conductivities; operation at temperature up to 600 *C; reduced switching losses and electromagnetic interference; and radiation hardness [18,19]. In replacing Si power devices with Sic , improvements in space, weight, and power (SWAP) are realized - a 10% improvement in traction drive efficiency and a 66% reduction in heatsink size [ZO].

In a HEV traction drive simulation performed with Si and S i c MOSFETs, the switching losses were approximately the same, while the big difference was observed in the conduction losses - these conduction losses wcre around 6000 times less than observed when using Si devices. With the six MOSFETs integrated together into an inverter, the total device losses wcre 66% less in the SIC based inverter as compared to the Si system. Further advantages will be realized using Sic devices for HEV electric drives including higher operation temperatures requiring smaller heat sinks decreasing the volume and weight of these devices. This reduction will appear as improved HEV efficiency [IS-].

* * M W Tncdon t, nmkdl d A

Figure 6. 2004 Ford Escape Hybrid vehicle displaying the location of the LCE under the hood and the EM in the center of the car, with the HEV schematic (0 2004 IEEE).

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B. Power Fuctor Correction (PFC) Circuirs- 600- I200V Power factor (PF) reflects how efficiently devices utilize

electricity - essentially comparing the amount of useful work realized from the total amount of electrical energy supplied. PF is the ratio of "usefid" power, measured in watts (W), divided by the "apparent" power, measured in volt-amps (VA) - PF = W N A [21,22]. A PF of unity indicates that all the supplied power is converted to useful work (resistive). In reactive devices (devices using inductive coils or capacitors such as electric motors, transformers, etc.), a portion of the supplied power goes to create an electro-magnetic field and not to resistive work. PFCs are compensatory electronic devices that attempt to make the electric system more resistive in nature [I.

PFC circuits are boost converters that are driven in Discontinuous Conduction Mode (DCM) or Continuous Conduction Mode (CCM). DCMs do not require high- speed diodes, but suffer from de-rating of circuit components and instability under light loads. CCM circuits offer IOW RMS currents, stability under light load operation, but require ultrafast diodes

CCM PFC circuits utilize Si PiNs. Si Schottky barrier diodes are not used due to their large on-state voltage drops observed in the 600 V range. However, PiNs have reverse recovery charge values around 100-500 nC, which increase significantly with larger forward currents and higher temperatures, and switch times of 100 ns. This places a tremendous burden on the other switching elements in terms of larger losses and forward safe operation areas.

S i c SBD offer prime benefits to be used in PFC circuits. These benefits include: fast recovery (50 ns) and almost zero reverse recovery charge even at high junction temperature operation. Additional advantages for PFC circuits include: reduction in the number of MOSFETs by SO%, elimination or size reduction of the active or passive snubbers, and reduction of the EM1 filter size and other passives [,]

The introduction of S i c based circuits will positively impact PFC circuits by improving circuit efficiency, enabling the operation o f the power distribution systems at higher temperatures and frequencies, and decreasing the total number of circuits required without sacrificing performance. Fig. 7 compares power efficiencies at different loads for Si and SIC diodes. The efficiency of circuits made with S i c is - 5% better at 40% load compared to Si [I.

significant amount of additional circuitry is required for turn-off, increasing component cost and decreasing the drive reliability. IGBTs are advantageous due to use of MOS-gate, fast switching, and minimum cooling requirements, but have high conduction losses at medium voltage levels and require series connection of multiple IGBTs [23]. ABB introduced Insulated-Gate-Commutated Thyristors (IGCTs), a modification to GTOs, delivering fast switching with inherently low losses, less complex circuitry with improved reliability [24J.

The superior performance of S i c diodes makes it possibfe to replace Si IGBTs or thyristors in motor drive applications. For the voltage range of 1.5-3 kV, it is possible to use Sic Schottky-PiN hybrid rectifiers, which combines the advantages of both Schottky and PiN diodes, like the Junction Barrier Schottky (JBS) and Merger PiN Schottky (MPS) diodes. These hybrid diodes have excellent characteristics including: low on-state voltage drop, low off- state leakage, fast switching, and good high temperature performance. Beyond 3 kV, Sic PiN diodes show great potential because of their exceptionally fast switching, comparable on state voltage at sufficiently high current densities, and high-temperature operation [25]. Therefore, Sic-based drives will be smaller, more efficient, cost effective, and handle higher power [26].

D. Electromagnetic Aircraft Launch System ( E M LS) - 3-6kV The U S Navy is presently pursuing EMALS as a

replacement to the existing steam catapult aircraft launching system in carriers. There are a number of factors driving this change. First is decreasing the size of the launching equipment; existing catapult systems are 1133 m' and weigh 486 metric tons. Such volume assumes significant amount of below-deck space and the top-side weight impacting the ship's stability and righting ability. Second, hture platforms demand launching larger, heavier, and more sophisticated aircrafts; therefore higher energy launching systems will be required. Current steam catapult systems, with cfficiencies -5%, are not sufficient to provide the anticipated launching power. Finally, steam catapults lack feedback control which imparts a sizable amount of transient stresses onto the airframes during launch, leading to diminished equipment lifetimes 127,281,

C. Ac and dc traction applications (pumps, fans, and

compressors) use high torque and low speeds for high efficiency conversion of electrical energy into rotational energy; they currently utilize Si-based Gate-Turn Off Thyristors (GTOs) or IGBTs for switching functionality. These technologies have advantages and limitations associated with their use. GTO technology is reliable and conduction losses are at acceptable levels. However, a

Indusdrial Drives and Traction 2-5kY

h d Ubb Figure 7. Efficiency comparison of PFC circuit with Si and Sic diodes

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The Navy's EMALS design uses an impulse-power linear induction launch motor (LIM) that will accelerate aircrafts to speeds in excess of 130 knots (67 &sec). The system consists of four elements: energy-storage subsystem; power- conversion subsystem; LIM; and control consoles to set launch parameters and monitor the entire system. Fig. 8 provides a schematic of the envisioned EMALS system [29]. LlMs are comprised of a row of stator coils energized by very-high-power silicon-controlled rectifiers (SCRs). The SCRs are situated independently along the length of the launch track and handle tens of thousands of amps at several thousand volts. Active cooling is required to keep the individual elements from overheating and effectively remove the heat buildup during launch [28J.

S i c devices have been considered as a new approach for EMALS design. A preliminary study performed under WBG-HPE Phase I investigated the advantages of utilizing S i c MOSFETs compared to Si IGBTs or IGCTs. The main conclusion from the siinulations was an enormous EMALS volume reduction (80%) arising from savings in device and heat sink volumes, and the elimination of the inductors (see Table 1) [30]. This added space is greatly demanded for integrating advanced tactical functionalities in future carriers [28].

Topology

Total D e v i c e Vo lume (cm')

H eatsink Total

[GBT I G C T Sic M O S F E T (% volume reduction)

3 - L e v e l 3 - L e v e l NPC 2-level VSI

148,300 124,200 39.210 ( - 6 8 )

( -66)

N P C VSI V S I

2 @4,4 3 0 154,800 51,120

E. High voltage transmission equipment is used to transmit

power from generating locations to points where the electricity can be used. GTOs, in early high power applications, enabled the building of efficient converters but this came at the cxpense of power losses and the need for additional power protection devices. The introduction of IGCTs simplified the design and size of the converters and significantly decreased the losses observed when using GTOs. Currently, IGCTs represent an optimal cost- effective choice in many high power applications requiring turn-off devices.

There is a need for devices capable of handling high current densities with low forward voltage drops for power transmission systems. Until now, the most successful

High Power Trailmission Utility - 10 kV+

Sic MOSFET number I I I O

Figure 8. Projected EMALS system.

Transformer Size (m')

Device Weieht ltonsl Total Size (m')

candidate has been Si IGBTs. Even as these devices continue to be improved, the power handling density and thermal stability are being approached due to material limitations. To protect the devices, the switching frequencies are reduced or additional circuitry are integrated, however these options increase costs or heighten power losses [3 11.

S i c promises to be a potential enhancement for the power utility industry because of its capability to handle larger voltages, current densities, and to operate at higher switching frequencies. A detailed study performed during WBG-HPE Phase I revealed the size and weight benefits S i c offers in solid state power substation (SSPS) designs. In replacing the conventional 2.7 MVA 60 Hz power transformer-based down converter with a 20 16-I~ S i c SSPS converter using MOSFETs, a savings of more than 50% in weight and volume was predicted (See Table 11) [30]. Additional features expected are: improved power quality factor; digital control and reconfigurability; and superior control of power sag, flicker, and harmonics [32]. With the reduced size and additional features of Sic, high voltage equipment can be used in applications where smaller size substations are desired.

0.313 1.724* 10 0.0185

TABLE 11 SIZE AND WEIGHT COMPARISONS BETWEEN Sic-BASED SSPS

AND CONVENTIONAL POWER TRANSFORMER.

I 13.8 kV to4160 V Converter I SSPS I Conventional 1

I I . r I I

Total Weight (tons) 2.63** 6 1 * ** Estimated weight

Estimated 4 m'when fully packaged

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v. CONCLUSfONS ACKNOWLEDGMENTS

This paper reviewed the representative high power electronics application areas that can benefit from the higher breakdown electric field, carrier saturation velocities, and thermal conductivity of S i c compared to Si devices. As a result of the advantages in the fundamental device performance, tremendous savings in weight, volume and cost can be realized at the system level. However, in order to commercialize S i c devices and fully exploit their potential in power electronics (especially for high voltage applications), more improvements at the material and device levels are essential.

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[TZO] B. Ozpineci and L.M. Tolbert, Comparison of Wide-Bandgap

[2 I ] http://w.webcom.com/tps/index.html, Thunderbyrd Power

[22] I . Francis, How to Measure Power Factor.

[24] DrivefACS LOO0 Medium Voltage Drives for Speed and Torque

[32] J . Zolper, Trunsformorional Opporruriifiesfor High Power

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