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5.Silicon Carbide Technology
  • 5-6-4-1-1 SiC Schottky Power Rectifiers.

    2018-01-08

    4H-SiC power Schottky diodes (with rated blocking voltages up to 1200 V and rated on-state currents up to 20 A as of this writing) are now commercially available . The basic structure of these unipolar diodes is a patterned metal Schottky anode contact residing on top of a relatively thin (roughly of the order of 10 μm in thickness) lightly n-doped homoepitaxial layer grown on a much thicker (around 200–300 μm) low-resistivity n-type 4H-SiC substrate (8° off axis, as discussed in Section 5.4.4.2) with backside cathode contact metallization . Guard ring structures (usually p-type implants) are usually employed to minimize electric field crowding effects around the edges of the anode contact. Die passivation and packaging help prevent arcing/surface flashover harmful to reliable device operation. The primary application of these devices to date has been switched-mode power supplies, where (consistent with the discussion in Section 5.3.2) the SiC Schottky rectifier’s faster switching with less power loss has enabled higher frequency operation and shrinking of capacitors, inductors and the overall power supply size and weight . In particular, the effective absence of minority carrier charge storage enables the unipolar SiC Schottky devices to turn off much faster than the silicon rectifiers (which must be pn junction diodes above ~200 V blocking) which must dissipate injected minority carrier charge energy when turned off. Even though the part cost of SiC rectifiers has been higher than competing silicon rectifiers, an overall lower power supply system cost with useful performance benefits is nevertheless achieved. It should be noted, however, that changes in circuit design are sometimes necessary to best enhance circuit capabilities with acceptable reliability when replacing silicon with SiC components. As discussed in Section 5.4.5, SiC material quality presently limits the current and voltage ratings of SiC Schottky diodes. Under high forward bias, Schottky diode current conduction is primarily limited by the series resistance of the lightly doped blocking layer. The fact that this series resistance increases with temperature (owing to decreased epilayer carrier mobility) drives equilization of high forward currents through each diode when multiple Schottky diodes are paralleled to handle higher on-state current ratings .

  • 5-6-4-1-2 Bipolar and Hybrid Power Rectifiers

    2018-01-08

    For higher voltage applications, bipolar minority carrier charge injection (i.e., conductivity modulation) should enable SiC pn diodes to carry higher current densities than unipolar Schottky diodes whose drift regions conduct solely using dopant-atom majority carriers . Consistent with silicon rectifier experience, SiC pn junction generation-related reverse leakage is usually smaller than thermionicassisted Schottky diode reverse leakage. As with silicon bipolar devices, reproducible control of minority carrier lifetime will be essential in optimizing the switching-speed versus on-state current density performance trade-offs of SiC bipolar devices for specific applications. Carrier lifetime reduction via intentional impurity incorporation and introduction of radiation-induced defects appears feasible. However, the ability to obtain consistently long minority carrier lifetimes (above a microsecond) has proven somewhat elusive as of this writing, indicating that further improvement to SiC material growth processes are needed to enable the full potential of bipolar power rectifiers to be realized . As of this writing, SiC bipolar power rectifiers are not yet commercially available. Poor electrical reliability caused by electrically driven expansion of 4H-SiC epitaxial layer stacking faults initiated from basal plane dislocation defects (Table 5.2) effectively prevented concerted efforts for commercialization of 4H-SiC pn junction diodes in the late 1990s . In particular, bipolar electron–hole recombination that occurs in forward-biased pn junctions drove the enlargement of stacking disorder in the 4H-SiC blocking layer, forming an enlarging quantum well (based on narrower 3C-SiC bandgap) that effectively degrades transport (diffusion) of minority carriers across the lightly doped junction blocking layer. As a result, the forward voltages of 4H-SiC pn rectifiers required to maintain rated on-state current increase unpredictably and undesirably over time. As discussed in Section 5.4.5, research toward understanding and overcoming this material defect-induced problem has made important progress, so that hopefully SiC bipolar power devices might become commercialized within a few years . A drawback of the wide bandgap of SiC is that it requires larger forward-bias voltages to reach the turn-on “knee” of a diode where significant on-state current begins flowing. In turn, the higher knee voltage can lead to an undesirable increase in on-state power dissipation. However, the benefits of 100× decreased drift region resistance and much faster dynamic switching should greatly overcome SiC onstate knee voltage disadvantages in most high-power applications. While the initial turn-on knee of SiC pn junctions is higher (around 3 V) than for SiC Schottky junctions (around 1 V), conductivity modulation enables SiC pn junctions to achieve lower forward voltage drop for higher blocking voltage applications . Hybrid Schottky/pn rectifier structures first developed in...

  • 5-6-4-2 SiC High-Power Switching Transistors

    2018-01-08

    Three terminal power switches that use small drive signals to control large voltages and currents (i.e., power transistors) are also critical building blocks of high-power conversion circuits. However, as of this writing, SiC high-power switching transistors are not yet commercially available for beneficial use in power system circuits. As well summarized in References 134, 135, 172, 180, and 186–188, a variety of improving three-terminal SiC power switches have been prototyped in recent years. The present lack of commercial SiC power switching transistors is largely due to several technological difficulties discussed elsewhere in this chapter. For example, all high-power semiconductor transistors contain high-field junctions responsible for blocking current flow in the off-state. Therefore, performance limitations imposed by SiC crystal defects on diode rectifiers (Sections 5.4.5 and 5.6.4.1) also apply to SiC high-power transistors. Also, the performance and reliability of inversion channel SiC-based MOS fieldeffect gates (i.e., MOSFETs, IGBTs, etc.) has been limited by poor inversion channel mobilities and questionable gate-insulator reliability discussed in Section 5.5.5. To avoid these problems, SiC device structures that do not rely on high-quality gate insulators, such as the MESFET, JFET, BJT, and depletion-channel MOSFET, have been prototyped toward use as power switching transistors. However, these other device topologies impose non-standard requirements on power system circuit design that make them unattractive compared with the silicon-based inversion-channel MOSFETs and IGBTs. In particular, silicon power MOSFETs and IGBTs are extremely popular in power circuits largely because their MOS gate drives are well insulated from the conducting power channel, require little drive signal power, and the devices are “normally off” in that there is no current flow when the gate is unbiased at 0 V. The fact that the other device topologies lack one or more of these highly circuit-friendly aspects has contributed to the inability of SiC-based devices to beneficially replace silicon-based MOSFETs and IGBTs in power system applications. As discussed in Section 5.5.5, continued substantial improvements in 4H-SiC MOSFET technology will hopefully soon lead to the commercialization of 4H-SiC MOSFETs. In the meantime, advantageous highvoltage switching by pairing a high-voltage SiC JFET with a lower-voltage silicon power MOSFETs into a single module package appears to be nearing practical commercialization . Numerous designs for SiC doped-channel FETs (with both lateral and vertical channels) have been prototyped, including depletionchannel (i.e., buried or doped channel) MOSFETs, JFETs, and MESFETs . Even though some of these have been designed to be “normally-off” at zero applied gate bias, the operational characteristics of these devices have not (as of this writing) offered sufficient benefits relative to cost to enable commercialization. Substantia...

  • 5-6-5 SiC MicroElectromechanical Systems (MEMS) and Sensors

    2018-01-08

    As described in Hesketh’s chapter on micromachining in this book, the development and use of siliconbased MEMS continues to expand. While the previous sections of this chapter have centered on the use of SiC for traditional semiconductor electronic devices, SiC is also expected to play a significant role in emerging MEMS applications . SiC has excellent mechanical properties that address some shortcomings of silicon-based MEMS such as extreme hardness and low friction reducing mechanical wear-out as well as excellent chemical inertness to corrosive atmospheres. For example, SiCs excellent durability is being examined as enabling for long-duration operation of electric micromotors and micro jet-engine power generation sources where the mechanical properties of silicon appear to be insufficient . Unfortunately, the same properties that make SiC more durable than silicon also make SiC more difficult to micromachine. Approaches to fabricating harsh-environment MEMS structures in SiC and prototype SiC-MEMS results obtained to date are reviewed in References 124 and 190. The inability to perform fine-patterned etching of single-crystal 4H- and 6H-SiC with wet chemicals (Section 5.5.4) makes micromachining of this electronic-grade SiC more difficult. Therefore, the majority of SiC micromachining to date has been implemented in electrically inferior heteroepitaxial 3C-SiC and polycrystalline SiC deposited on silicon wafers. Variations of bulk micromachining, surface micromachining, and micromolding techniques have been used to fabricate a wide variety of micromechanical structures, including resonators and micromotors. A standardized SiC on silicon wafer micromechanical fabrication process foundry service, which enables users to realize their own application-specific SiC micromachined devices while sharing wafer space and cost with other users, is commercially available . For applications requiring high temperature, low-leakage SiC electronics not possible with SiC layers deposited on silicon (including high-temperature transistors, as discussed in Section 5.6.2), concepts for integrating much more capable electronics with MEMS on 4H/6H SiC wafers with epilayers have also been proposed. For example, pressure sensors being developed for use in higher temperature regions of jet engines are implemented in 6H-SiC, largely owing to the fact that low junction leakage is required to achieve proper sensor operation . On-chip 4H/6H integrated transistor electronics that beneficially enable signal conditioning at the high-temperature sensing site are also being developed . With all micromechanical-based sensors, it is vital to package the sensor in a manner that minimizes the imposition of thermomechanical induced stresses (which arise owing to thermal expansion coefficient mismatches over much larger temperature spans enabled by SiC) onto the sensing elements. Therefore (as mentioned previously in Section 5.5.6), advanced packaging is almost as critical as the us...

  • 5-7 Future of SiC

    2018-01-08

    It can be safely predicted that SiC will never displace silicon as the dominant semiconductor used for the manufacture of the vast majority of the world’s electronic chips that are primarily low-voltage digital and analog chips targeted for operation in normal human environments (computers, cell phones, etc.). SiC will only be used where substantial benefits are enabled by SiC’s ability to expand the envelope of high-power and high-temperature operational conditions such as the applications described in Section 5.3. Perhaps, the only major existing application area where SiC might substantially displace today’s use of silicon is the area of discrete power devices used in power conversion, motor control, and management circuits. The power device market, along with the automotive sensing market present the largest-volume market opportunity for SiC-based semiconductor components. However, the end consumers in both of these applications demand excruciatingly high reliability (i.e., no operational failures) combined with competitively low overall cost. For SiC electronics technology to have large impact, it must greatly evolve from its present status to meet these demands. There is clearly a very large discrepancy between the revolutionary broad theoretical promise of SiC semiconductor electronics technology (Section 5.3) versus the operational capability of SiC-based components that have actually been deployed in only a few commercial and military applications (Section 5.6). Likewise, a large discrepancy also exists between the capabilities of laboratory SiC devices compared with commercially deployed SiC devices. The inability of many “successful” SiC laboratory prototypes to rapidly transition to commercial product demonstrates both the difficulty and criticality of achieving acceptable reliability and costs.

  • 5-7-1 Future Tied to Material Issues

    2018-01-08

    The previous sections of this chapter have already highlighted major known technical obstacles and immaturities that are largely responsible for hindered SiC device capability. In the most general terms, these obstacles boil down to a handful of key fundamental material issues. The rate at which the most critical of these fundamental issues is solved will greatly impact the availability, capability, and usefulness of SiC semiconductor electronics. Therefore, the future of SiC electronics is linked to investment in basic material research toward solving challenging material-related impediments to SiC device performance, yield, and reliability. The material challenge that is arguably the biggest key to the future of SiC is the removal of dislocations from SiC wafers. As described previously in this chapter and references therein, the most important SiC power rectifier performance metrics, including device ratings, reliability, and cost are inescapably impacted by high dislocation densities present in commercial SiC wafers and epilayers. If mass-produced SiC wafer quality approached that of silicon wafers (which typically contain less than one dislocation defect per square centimeter), far more capable SiC unipolar and bipolar high-power rectifiers (including devices with kilovolt and kiloampere ratings) would rapidly become widely available for beneficial use in a far larger variety of high-power applications. Similar improvements would also be realized in SiC transistors, paving the way for SiC high-power devices to indeed beneficially displace silicon-based power devices in a tremendously broad and useful array of applications and systems (Section 5.3). This advancement would unlock a much more rapid and broad SiC-enabled power electronic systems “revolution” compared to the relatively slower “evolution” and niche-market insertion that has occurred since SiC wafers were first commercialized roughly 15 years ago. As mentioned in Section 5.4, recent laboratory results  indicate that drastic reductions in SiC wafer dislocations are possible using radically new approaches to SiC wafer growth compared to standard boule-growth techniques practiced by all commercial SiC wafer vendors for over a decade. Arguably, the ultimate future of SiC high-power devices may hinge on the development and practical commercialization of low dislocation density SiC growth techniques substantially different from those employed today. It is important to note that other emerging wide bandgap semiconductors besides SiC theoretically offer similarly large electrical system benefits over silicon semiconductor technology as described in Section 5.3. For example, diamond and some Group III-nitride compound semiconductors (such as GaN; Table 5.1) have high breakdown field and low intrinsic carrier concentration that enables operation at power densities, frequencies, and temperatures comparable to or exceeding SiC. Like SiC, however, electrical devices in these semiconductors...

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