The development of SiC for electronic applications has been a subject of intense research for nearly 40 years. Much of this research is motivated by the extraordinary combination of physical properties possessed by SiC, especially in the development of SiC-based devices for specific high-temperature, high-power, or high-frequency applications that are not suitable for Si- or GaAs-based devices. During the early years of SiC research and development, a significant amount of good, fundamental research was performed, but the development of commercially available SiC-based devices was retarded by low-quality bulk materials and inadequate epitaxial processes. In the late 1980s, research at academic institutions, such as North Carolina State University, and industrial laboratories, such as Westinghouse (now Northrup-Grumman), Advanced Technology Materials, Inc. (ATMI), and Cree Research, Inc., coupled with the commercial offering of highquality SiC wafers from Cree, created an opportunity for further advancement. Improvements in epitaxial processes and device processing strategies were also realized during this time. Together these factors have enabled the fabrication of high-quality device structures and have generated increased research and funding activities in SiC electronic devices.

While silicon carbide has been an industrial product for over a century, it is only now emerging as the semiconductor of choice for high-power, high-temperature, and high-radiation environments. From electrical switching and sensors for oil drilling technology to all-electric airplanes, SiC is finding a place which is difficult to fill with presently available Si or GaAs technology. In 1824 Jöns Jakob Berzelius published a paper which suggested there might be a chemical bond between the elements carbon and silicon. It is a quirk of history that he was born in 1779 in Linköping, Sweden where he received his early education, and now, 172 years later, Linkoping University is the center of a national program in Sweden to study the properties of SiC as a semiconductor.

To take advantage of the superior properties of SiC-based devices, the industry must be able to produce high-quality SiC homoepitaxial films on single-crystal SiC wafers. Silicon- and GaAs-based semiconductor devices have taken advantage of the industry's ability to produce largediameter, high-quality substrates to develop large-volume, low-cost devices. For the same reasons, the development of low-cost, high-quality single-crystal SiC substrates has been pursued since the first Lely (nonseeded growth process) platelets were used to make devices. Currently several groups (ATMI, USA; Northrop Grumman Research Center, USA; Okmetic Ltd., Finland; Nippon Steel Corporation, Japan; and our company, Cree Research, Inc., USA) are working toward SiC substrate fabrication. University researchers also play an important role in the search for solutions to fundamental outstanding issues. These efforts are important because the future success of the SiC industry is closely related to the successful development of the SiC bulk growth process.

SiC electronics research has been driven by the continued successful development of SiC technology for high-power and high-frequency semiconductor devices, and for service in high-temperature, corrosive, and high-radiation environments. The development of this technology has been accelerated by the introduction of commercially available SiC wafers, which have decreased in cost with time. The most recently demonstrated commercial SiC-based products include ultraviolet (uv)-flame sensors for terrestrial turbine engines and high-definition-television transmitter systems utilizing SiC-based transistors. Prototype microelectronic SiC devices include high-voltage Schottky rectifiers and power metal-oxide-semiconductor field-effect transistors, microwave and millimeter-wave devices, and high-temperature, radiation-resistant junction FETs (JFETs). These advancements in SiC-based device technology are attributable to both the successful development of commercially available, bulk SiC substrates and the recent advancements in SiC epitaxial layer growth technologies.

Since the commercial availability of SiC substrates in 1990, SiC processing technology has advanced rapidly. There have been demonstrations of monolithic digital and analogue integrated circuits, complementary metal-oxide-semiconductor (CMOS) analog integrated circuits, nonvolatile random-access memories, self-aligned polysilicon-gate metal-oxide-semiconductor field-effect transistors (MOSFETs), and buried-channel polysilicon-gate charge-coupled devices (CCDs). In this article, we review processing technologies for SiC.

Oxidation

A beneficial feature of SiC processing technology is that SiC can be thermally oxidized to form SiO2. When a thermal oxide of thickness x is grown, 0.5x of the SiC surface is consumed, and the excess carbon leaves the sample as CO. Shown in Figure 1 are the oxide thicknesses as a function of time for the Si-face and the C-face of 6H-SiC, and for Si. The oxidation rates are considerably lower for SiC than for Si. The oxidation rate of the C-face of 6H-SiC is considerably greater than that of the Si-face. Hornetz et al. have shown that the reason for the slower oxidation rate of the Si-face is due to a 1-nm Si4C4−xO2 (x < 2) layer that forms between the SiC and the SiO2 during oxidation of the Si-face. When oxidizing the Si-face, the Si atoms oxidize first, which inhibits the oxidation of the underlying C atoms that are 0.063 nm below the Si atoms. When oxidizing the C-face, the C atoms readily oxidize first to form CO, with no formation of the Si4C4−xO2 layer for temperatures above 1000°C.

In recent years, SiC has received a great deal of attention as a nearly ideal material for the fabrication of high-speed, high-power transistors. The high electric breakdown field of 3.8 × 106 V/cm, high saturated electron drift velocity of 2 × 107 cm/s, and high thermal conductivity of 4.9 W/cm K indicate SiC's potential for high-power, high-frequency operation. A wide bandgap should also allow SiC field-effect transistors (FETs) to have high radio-frequency (rf) output power at high temperatures.

These material qualities have been verified through outstanding device performance. Recent results for SiC metal-semiconductor field-effect transistors (MESFETs) have included superior frequency and power performance, with power gain at frequencies as high as 40 GHz and power densities as high as 3.3 W/mm. This represents significantly higher operating frequencies and power densities than current Si rf power FET technology, and nearly three times the power density of GaAs MESFETs, which are currently used in many commercial rf power applications. Similarly, SiC static induction transistors (SITs) have much higher power densities than their Si counterparts and have recently been demonstrated in modules with as much as 470-W total pulsed output power. This article describes microwave device operation, discusses material properties needed for rf power generation, and summarizes state-of-the-art SiC high-frequency device performance. Emphasis is placed on MESFETs and SITs since they are currently the most mature SiC-based device technologies.