5-5-5 SiC Insulators: Thermal Oxides and MOS Technology
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 orientations such as ( )
and ( ) that are obtained by making devices on wafers cut with different crystallographic orientations
(Section 5.2.1), have also yielded significantly improved 4H-SiC MOS channel properties .
One key step to obtaining greatly improved 4H-SiC MOS devices has been the proper introduction
of nitrogen-compound gases (in the form of ) during the oxidation and postoxidation
annealing process . These nitrogen-based anneals have also improved the
stability of 4H-SiC oxides to high electric field and high-temperature stressing used to qualify and
quantify the reliability of MOSFETs . However, as Agarwal et al. have pointed out, the wide
bandgap of SiC reduces the potential barrier impeding tunneling of damaging carriers through oxides
grown on 4H-SiC, so that 4H-SiC oxides cannot be expected to attain identical high reliability as
thermal oxides on silicon. It is highly probable that alternative gate insulators besides thermally grown
will have to be developed for optimized implementation of inversion channel 4H-SiC insulated
gate transistors for the most demanding high-temperature and high-power electronic applications. As
with silicon MOSFET technology, multilayer dielectric stacks will likely be developed to further enhance
SiC MOSFET performance .