5-6-2 SiC RF Devices
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.