The vast majority of semiconductor-integrated circuit chips in use today rely on silicon metal-oxide– semiconductor field-effect transistors (MOSFETs), whose electronic advantages and operational device physics are summarized in Katsumata’s chapter and elsewhere . Given the extreme usefulness and success of inversion channel MOSFET-based electronics in VLSI silicon (as well as discrete silicon power devices), it is naturally desirable to implement high-performance inversion channel MOSFETs in SiC. Like silicon, SiC forms a thermal when it is sufficiently heated in an oxygen environment. While this enables SiC MOS technology to somewhat follow the highly successful path of silicon MOS technology, there are nevertheless important differences in insulator quality and device processing that are presently preventing SiC MOSFETs from realizing their full beneficial potential. While the following discourse attempts to quickly highlight key issues facing SiC MOSFET development, more detailed insights can be found in References 133–142. From a purely electrical point of view, there are two prime operational deficiencies of SiC oxides and MOSFETs compared to silicon MOSFETs. First, effective inversion channel mobilities in most SiC MOSFETs are lower than one would expect based on silicon inversion channel MOSFET carrier mobilities. This seriously reduces the transistor gain and current-carrying capability of SiC MOSFETs, so that SiC MOSFETs are not nearly as advantageous as theoretically predicted. Second, SiC oxides have not proven as reliable and immutable as well-developed silicon oxides, in that SiC MOSFETs are more prone to threshold voltage shifts, gate leakage, and oxide failures than comparably biased silicon MOSFETs. In particular, SiC MOSFET oxide electrical performance deficiencies are attributed to differences between silicon and SiC thermal oxide quality and interface structure that cause the SiC oxide to exhibit undesirably higher levels of interface state densities (), fixed oxide charges (), charge trapping, carrier oxide tunneling, and lowered mobility of inversion channel carriers. In highlighting the difficulties facing SiC MOSFET development, it is important to keep in mind that early silicon MOSFETs also faced developmental challenges that took many years of dedicated research efforts to successfully overcome. Indeed, tremendous improvements in 4H-SiC MOS device performance have been achieved in recent years, giving hope that beneficial 4H-SiC power MOSFET devices for operation up to 125°C ambient temperatures might become commercialized within the next few years. For example, 4H-SiC MOSFET inversion channel mobility for conventionally oriented (8° off (0001) c-axis) wafers has improved from <10 to >200 , while the density of electrically detrimental SiC– interface state defects energetically residing close to the conduction band edge has dropped by an order of magnitude . Likewise, alternative SiC wafer surface...
Hostile-environment SiC semiconductor devices and ICs are of little advantage if they cannot be reliably packaged and connected to form a complete system capable of hostile-environment operation. With proper material selection, modifications of existing IC packaging technologies appear feasible for nonpower SiC circuit packaging up to 300°C . Recent work is beginning to address the needs of the most demanding aerospace electronic applications, whose requirements include operation in highvibration 500–600°C oxidizing-ambient environments, sometimes with very high power . For example, some prototype electronic packages and circuit boards that can withstand over a thousand hours at 500°C have been demonstrated. Harsh-environment passive components such as inductors, capacitors, and transformers, must also be developed for operation in demanding conditions before the full system-level benefits of SiC electronics discussed in Section 5.3 can be successfully realized.
This section briefly summarizes a variety of SiC electronic device designs broken down by major application areas. SiC process and material technology issues limiting the capabilities of various SiC device topologies are highlighted as key issues to be addressed in further SiC technology maturation. Throughout this section, it should become apparent to the reader that the most difficult general challenge preventing SiC electronics from fully attaining beneficial capabilities is attaining long-term high operational reliability, while operating in previously unattained temperature and power density regimes. Because many device reliability limitations can be traced to fundamental material and junction/interface issues already mentioned in Sections 5.4 and 5.5, efforts to enable useful (i.e., reliable) SiC electronics should focus on improvements to these fundamental areas.
The wide bandgap of SiC is useful for realizing short-wavelength blue and ultraviolet (UV) optoelectronics. 6H-SiC-based pn junction light-emitting diodes (LEDs) were the first semiconductor devices to cover the blue portion of the visible color spectrum, and became the first SiC-based devices to reach high-volume commercial sales . Because SiC’s bandgap is indirect (i.e., the conduction minimum and valence band maximum do not coincide in crystal momentum space), luminescent recombination is inherently inefficient . Therefore, LEDs based on SiC pn junctions were rendered quite obsolete by the emergence of much brighter, much more efficient direct-bandgap Group III-nitride (III-N such as GaN, and InGaN) blue LEDs . However, SiC wafers are still employed as one of the substrates (along with sapphire) for growth of III-N layers used in high-volume manufacture of green and blue nitride-based LEDs. SiC has proven much more efficient at absorbing short-wavelength light, which has enabled the realization of SiC UV-sensitive photodiodes that serve as excellent flame sensors in turbine-engine combustion monitoring and control . The wide bandgap of 6H-SiC is useful for realizing low photodiode dark currents as well as sensors that are blind to undesired near-infrared wavelengths produced by heat and solar radiation. Commercial SiC-based UV flame sensors, again based on epitaxially grown dry-etch mesa-isolated 6H-SiC pn junction diodes, have successfully reduced harmful pollution emissions from gas-fired ground-based turbines used in electrical power generation systems . The low dark-currents of SiC diodes are also useful for X-ray, heavy ion, and neutron detection in nuclear reactor monitoring and enhanced scientific studies of high-energy particle collisions and cosmic radiation .
The main use of SiC RF devices appears to lie in high-frequency solid-state high-power amplification at frequencies from around 600 MHz (UHF-band) to perhaps as high as a few gigahertz (X-band). As discussed in far greater detail in References 5, 6, 25, 26, 159, and elsewhere, the high breakdown voltage and high thermal conductivity coupled with high carrier saturation velocity allow SiC RF transistors to handle much higher power densities than their silicon or GaAs RF counterparts, despite SiC’s disadvantage in low-field carrier mobility (Table 5.1). The higher thermal conductivity of SiC is also crucial in minimizing channel self-heating so that phonon scattering does not seriously degrade carrier velocity. These material advantage RF power arguments apply to a variety of different transistor structures such as MESFETs and static induction transistors (SITs) and other wide bandgap semiconductors (such as Group III-nitrides) besides SiC. The high power density of wide bandgap transistors will prove quite useful in realizing solid-state transmitter applications, where higher power with smaller size and mass are crucial. Fewer transistors capable of operating at higher temperatures reduce matching and cooling requirements, leading to reduced overall size and cost of these systems. SiC-based high-frequency RF MESFETs are now commercially available . However, it is important to note that this occurred after years of fundamental research tracked down and eliminated poor reliability owing to charge-trapping effects arising from immature semi-insulating substrates, device epilayers, and surface passivation . One key material advancement that enabled reliable operation was the development of “high-purity”semi-insulating SiC substrates (needed to minimize parasitic device capacitances) with far less charge trapping induced than the previously developed vanadium-doped semi-insulating SiC wafers. SiC MESFET devices fabricated on semi-insulating substrates are conceivably less susceptible. to adverse yield consequences arising from micropipes than vertical high-power switching devices, primarily because a c-axis micropipe can no longer short together two conducting sides of a high field junction in most areas of the lateral channel MESFET structure. SiC mixer diodes also show excellent promise for reducing undesired intermodulation interference in RF receivers . More than 20 dB dynamic range improvement was demonstrated using nonoptimized SiC Schottky diode mixers. Following further development and optimization, SiC-based mixers should improve the interference immunity in situations (such as in aircraft or ships) where receivers and high-power transmitters are closely located.
Most analog signal conditioning and digital logic circuits are considered “signal level” in that individual transistors in these circuits do not typically require any more than a few milliamperes of current and <20 V to function properly. Commercially available silicon-on-insulator circuits can perform complex digital and analog signal-level functions up to 300°C when high-power output is not required [163]. Besides ICs in which it is advantageous to combine signal-level functions with high-power or unique SiC sensors/MEMS onto a single chip, more expensive SiC circuits solely performing low-power signal-level functions appear largely unjustifiable for low-radiation applications at temperatures below 250–300°C . As of this writing, there are no commercially available semiconductor transistors or integrated circuits (SiC or otherwise)for use in ambient temperatures above 300°C. Even though SiC-based high-temperature laboratory prototypes have improved significantly over the last decade, achieving long-term operational reliability remains the primary challenge of realizing useful 300–600°C devices and circuits. Circuit technologies that have been used to successfully implement VLSI circuits in silicon and GaAs such as CMOS, ECL, BiCMOS, DCFL, etc., are to varying degrees candidates for T > 300°C SiC-integrated circuits. High-temperature gate-insulator reliability (Section 5.5.5) is critical to the successful realization of MOSFET-based integrated circuits. Gate-to-channel Schottky diode leakage limits the peak operating temperature of SiC MESFET circuits to around 400°C (Section 5.5.3.2). Therefore, pn junction-based devices such as bipolar junction transistors (BJTs) and junction field effect transistors (JFETs), appear to be stronger (at least in the nearer term) candidate technologies to attain long-duration operation in 300–600°C ambients. Because signal-level circuits are operated at relatively low electric fields well below the electrical failure voltage of most dislocations, micropipes and other SiC dislocations affect signallevel circuit process yields to a much lesser degree than they affect high-field power device yields. As of this writing, some discrete transistors and small-scale prototype logic and analog amplifier SiCbased ICs have been demonstrated in the laboratory using SiC variations of NMOS, CMOS, JFET, and MESFET device topologies . However, none of these prototypes are commercially viable as of this writing, largely owing to their inability to offer prolonged-duration electrically stable operation at ambient temperatures beyond the ~250–300°C realm of silicon-on-insulator technology. As discussed in Section 5.5, a common obstacle to all high-temperature SiC device technologies is reliable long-term operation of contacts, interconnect, passivation, and packaging at T > 300°C. By incorporating highly durable high-temperature ohmic contacts and packaging, prolonged continuous electrical operation...
The inherent material properties and basic physics behind the large theoretical benefits of SiC over silicon for power switching devices were discussed Section 5.3.2. Similarly, it was discussed in Section 5.4.5 that crystallographic defects found in SiC wafers and epilayers are presently a primary factor limiting the commercialization of useful SiC high-power switching devices. This section focuses on the additional developmental aspects of SiC power rectifiers and power switching transistor technologies. Most SiC power device prototypes employ similar topologies and features as their silicon-based counterparts such as vertical flow of high current through the substrate to maximize device current using minimal wafer area (i.e., maximize current density) . In contrast to silicon, however, the relatively low conductivity of present-day p-type SiC substrates (Section 5.4.3) dictates that all vertical SiC power device structures be implemented using n-type substrates in order to achieve beneficially high vertical current densities. Many of the device design trade-offs roughly parallel well-known silicon power device trade-offs, except for the fact that numbers for current densities, voltages, power densities, and switching speeds are much higher in SiC. For power devices to successfully function at high voltages, peripheral breakdown owing to edgerelated electric field crowding must be avoided through careful device design and proper choice of insulating/passivating dielectric materials. The peak voltage of many prototype high-voltage SiC devices has often been limited by destructive edge-related breakdown, especially in SiC devices capable of blocking multiple kilovolts. In addition, most testing of many prototype multikilovolt SiC devices has required the device to be immersed in specialized high-dielectric strength fluids or gas atmospheres to minimize damaging electrical arcing and surface flashover at device peripheries. A variety of edge-termination methodologies, many of which were originally pioneered in silicon highvoltage devices, have been applied to prototype SiC power devices with varying degrees of success, including tailored dopant and metal guard rings . The higher voltages and higher local electric fields of SiC power devices will place larger stresses on packaging and on wafer insulating materials, so some of the materials used to insulate/passivate silicon high-voltage devices may not prove sufficient for reliable use in SiC high-voltage devices, especially if those devices are to be operated at high temperatures.
The high-power diode rectifier is a critical building block of power conversion circuits. Recent reviews of experimental SiC rectifier results are given in References 3, 134, 172, 180, and 181. Most important SiC diode rectifier device design trade-offs roughly parallel well-known silicon rectifier trade-offs, except for the fact that current densities, voltages, power densities, and switching speeds are much higher in SiC. For example, semiconductor Schottky diode rectifiers are majority carrier devices that are well known to exhibit very fast switching owing to the absence of minority carrier charge storage that dominates (i.e., slows, adversely resulting in undesired waste power and heat) the switching operation of bipolar pn junction rectifiers. However, the high breakdown field and wide energy bandgap permit operation of SiC metal–semiconductor Schottky diodes at much higher voltages (above 1 kV) than is practical with siliconbased Schottky diodes that are limited to operation below ~200 V owing to much higher reverse-bias thermionic leakage.
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