5-6-3 SiC High-Temperature Signal-Level Devices

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 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 of a packaged 6H-SiC field effect transistor at 500°C in oxidizing air environment was recently demonstrated .

As further improvements to fundamental SiC device processing technologies (Section 5.5) are made, increasingly durable T > 300°C SiC-based transistor technology will evolve for beneficial use in harshenvironment applications. Increasingly complex high-temperature functionality will require robust circuit designs that accommodate large changes in device operating parameters over the much wider temperature ranges (as large as 650°C spread) enabled by SiC. Circuit models need to account for the fact that SiC device epilayers are significantly “frozen-out” owing to deeper donor and acceptor dopant ionization energies, so that nontrivial percentages of device-layer dopants are not ionized to conduct current near room temperature . Because of these carrier freeze-out effects, it will be difficult to realize SiC-based ICs operational at junction temperatures much lower than –55°C (the lower end of U.S. Mil-Spec. temperature range).

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