The Transition from Silicon to Silicon Carbide in
Power Electronics
E SC 414M
Kenneth Myers
Introduction
Power electronic devices are one of the key technologies of our time. These
devices are what allow us to convert power, like cellphone and laptop chargers, or
practically anything that has a power cord with a “box” at the plug (AC-DC converters).
Power electronics are also used in many integrated circuits that require the control or
conversion of electric power. One of the main devices used in this field is called the
metal-oxide-silicon field effect transistor (MOSFET).
MOSFETs are a device that, when a certain voltage (threshold voltage) is applied
to the metal (gate), current is allowed to pass through the semiconducting layer. The
idea behind this is that the MOSFET is able to act as a switch that turns on when the
condition of threshold voltage is met.
For a very long time, plain silicon (Si) has been used as the material in the
semiconducting portion of a MOSFET, but recently a lot of research and effort has been
put into the fabrication and usage of silicon carbide (SiC). SiC is an important material in
this field due to the fact that it is much more resilient than standard Si. SiC has a wider
band gap, allowing it to take on higher fields and more power than Si. Another important
factor that makes SiC a better candidate than Si is its lower carrier density, which allows
a device made from SiC to work at higher temperatures without breaking down. [1]
The biggest issue in current SiC technology, however, is processing. This is an
issue due to the large amounts of interface defects generated between the SiC and the
oxide layer of the MOSFET. These defects are reducing the lifetime of the SiC charge
carriers as well as lowering their mobility.
[2]
Most work in SiC that goes on today is
research used to discover the reason that these states are generated as well as how to
prevent them or at least passivate them. [3]
The purpose of this paper is to review the benefits seen in the transition from silicon
carbide-based MOS devices, acknowledge the current issues with this technology, and
to discuss the methods taken to improve these shortcomings.
Literature Review: Advantages
One of the biggest advantages of the silicon carbide semiconductors is that they
are extremely tough. They easily handle working conditions that the standard silicon
semiconductors would fail no matter how well they were made. The two biggest of these
conditions are heat and radiation. The SiC devices are able to overcome these more
harmful working environments because of its wide band gap.
[1,4,5,6]
Compared to Si
devices with a band gap of 1.12 eV, the SiC material has a band gap of 3.2 eV. [1] The
reason that a wider band gap contributes to the resilience of a material is due to the
lower intrinsic carrier concentration and the resultant lowering of potential leakage
current. [1,4,5] A lower carrier concentration is beneficial because it essentially “spreads
out” the electrons (or holes) in the material more effectively. With the carriers farther
apart, the device is less capable to pass a current through these separated electrons
without being turned on. Shown in table 1, along with many other values, is the
difference in intrinsic carrier concentration. Because of this property, SiC can be used in
temperatures greater than 200°C, the limit of a Si device. [1]
Table 1: A comparison of Si and SiC
devices in terms of their physical
properties
Adapted from Roccaforte[1]
The next topic to be mentioned is SiC’s higher critical electric field, which is also
shown in table 1. [5,6] One major benefit of this increase in critical field is the ability to
create thinner substrates that are able to maintain the same breakdown voltage. [1] The
importance this property is the capability to make a smaller device without losing
desired properties. A second benefit is that with a thinner device having a higher doping
(which SiC is capable of) MOS devices can be created with significantly lower on-
resistance than silicon. [4,5] To put numbers behind this, silicon carbide has been shown
to have on-resistances of 50mΩ[4] and 80mΩ[5], compared to values in the 300-500mΩ
range[5]. The reason that a low on-resistance is desirable for a MOSFET related directly
to the nature of the device as a switch. When the device is switched on, the ideal case
would be to have zero resistance in the channel through which current passes. [1] In real
life situations, however, zero resistance is impossible. That is where minimizing this onresistance comes into play. By getting the value as close to zero as possible, the device
is minimizing the power lost when it is on. A recent improvement in SiC devices comes
from the General Electric (GE) Global Research Center, wherein the energy lost
through the device was recorded to be only 0.6mJ, which is five times less than their
current competitors and significantly more efficient than any Si device. [4]
Another improvement on the classic silicon material for silicon carbide is its
higher thermal conductivity (k). [1,4,5] Shown in table 1, the thermal conductivity for SiC is
over three times that of Si, which is a huge leap in this material property. This increase
in k does two things for SiC-based devices: 1) it allows the device to operate at higher
power and 2) there is less of a need for cooling systems.
[5]
The reason that these
devices are able to now operate at higher power is in part due to the lower resistances
mentioned above, creating less energy dissipated as heat. The other part is that the
increased thermal conductivity allows the material to radiate its own heat more quickly,
preventing damage that would normally be done in the case of silicon. In the same vein,
since the device itself is able to radiate off more internal heat, less hardware is required
to cool the devices that are being used.
The third and final improvement to be mentioned is the high saturation velocity
that is achieved by SiC.
[1,4]
As seen in table 1, the saturation velocity of SiC as
compared to Si is approximately double. The benefit of this property is that a device that
is made of silicon carbide will be able to switch between on and off at higher
frequencies than a device made from silicon.
[1,4]
This ability is useful in any high
frequency device that would benefit from an even higher operating frequency.
Literature Review: Issues
The biggest roadblock in implementing silicon carbide device into today’s
technology lies within the processing of these devices. Separately, the SiC and SiO 2
can be made without a problem; however, the catch for these two materials exists when
they are placed together. In processing, the interfacial region (the atomic layer between
the SiO2 and SiC) becomes ridden with defects that cause the devices to perform at a
less-than-optimal level. [3]
In comparison to silicon devices, which has an interface trap density (Dit) of
approximately 1010 defects/cm2 (density is put in two-dimensional units because the
interfacial region is considered to be approximately one atomic layer thick), the SiC
devices are shown to have a range of 1012 to 1013 defects/cm2.
[1]
This change in
density between devices is, essentially, the only reason that SiC has yet to overcome Si
in terms of MOSFET-based technology.
These defects cause many issues with silicon carbide devices, one of the main
problems being the reduction of channel mobility.
[6]
When considering an MOS device,
one of the main concepts is to bring carriers to the channel as quickly as possible in
order to have an almost zero turn-on time. A MOSFET is considered “on” when the
device is passing current through the channel. At this time, the mode of the device is
known as “inversion,” and this is when the problems with SiC are occurring. When the
device is placed into inversion, the carriers (electrons) normally line the interfacial
region and allow a current to pass, but due to these defects, electrons are unable to
perform this job. Some are taken up by the defects while others are scattered back into
the semiconducting layer. [3] The result of this trapping and scattering is an extreme loss
in the inversion channel mobility. Table 2 shows a few examples of “dry” (substrates
that have not been altered) SiC samples as well as some that have been changed
either in base processing or post-processing. These alterations will be discussed in a
future section of this paper. As it is shown, these dry samples have large interface trap
densities, which correlate to their extremely low mobility values.
Table 2: A comparison of physical property alterations in SiC through multiple processing methods
Adapted from Roccaforte[1]
A second issue created by these defects is known as time-dependent, biasstress-induced threshold voltage instability.
[7]
This phenomena is a known problem in
many devices that occurs in different circumstances dependent on the defects that are
involved. For SiC, this means that as you apply a bias to the gate of a MOSFET, the
threshold voltage will change depending on the amount of time spent biasing the
device. This instability is also seen in other devices, but the difference is that the
instability takes a significant amount of stress-time in order to occur, if it even occurs at
all. In these SiC devices, however, the shift in threshold voltage happens extremely fast
(after approximately 10ps).
[7]
An example of threshold voltage shift in reference to bias-
stress time is shown in figure 1, where three different bias voltages were applied over a
period of 10s to 10ks and extrapolated into the lines displayed. [7]
Figure 1: A comparison of threshold
voltage shifts over time in SiC for
gate biases of 5V, 10V, and 20V
Adapted from Lelis[7]
Literature Review: Steps to Improvement
In the case of the time-dependent, bias-stress-induced threshold voltage instability,
the defects responsible for the instability may be assumed to be the same ones
responsible for the same phenomena in silicon-based devices. These defects are called
“oxide traps” and are created from a broken Si-Si bond near the interfacial region,
creating a dangling bond that wants to be filled by an electron.
[7]
In order to remedy
these effects, a known solution was tested on the newer SiC devices: annealing in NO.
[7]
Shown in figure 2 is another threshold voltage shift versus bias-stress time analysis
which compares three sets of devices. The solid and dotted lines should be the ones to
pay most attention to, seeing as they are the same devices with the only difference
being the annealing process. As shown, the two shifts increase as time goes on at
approximately the same rate, but the annealed sample is at a significantly lowered shift
in threshold voltage.
Figure 2: A comparison of threshold
voltage shifts over time in SiC for
difference devices with different
annealing conditions
Adapted from Lelis[7]
As for the defects located in the interface, methods to lower their impact have
been created and tested. The first of these (as well as the most popular), is annealing
the SiC in either NO or N2O in order to passivate as many of the defects as possible. [1,6]
The differences in the inversion channel mobility from dry to nitrogen annealed are
shown in table 2. These changes in mobility range from slight to extreme depending on
the annealing temperature. On top of the mobility changes, it should also be noted that
the density of interface states has been reduced. The other method that has been
introduced more recently has been to anneal in POCl3.
[1,6]
Like the NO and N2O
anneals, the POCl3 anneal increases the inversion channel mobility while decreasing
interface state density as shown in table 2. What makes this process so great, however,
is that while it increases mobility more effectively than the NO and N 2O anneals, it can
be used in addition to these processes, furthering the reduction of interface states and
increasing mobility.
[6]
The reason that the POCl3 anneal has yet to overcome the
nitridation is due to the fact that is creating atomic phosphorous in the SiO 2 layer and in
turn creating more sites for electron trapping.
[6]
There are current investigations going
on to balance the amounts of NO (or N2O) and POCl3 in order to balance the benefits
and side effects.
[6]
Another suggestion has been to add an additional SiO2 layer that
acts as a barrier for the main SiO2 substrate. [6]
To conclude, silicon carbide is an amazing addition to the field of power
electronics. It is an extremely durable material that outclasses the current silicon
devices in almost every area. SiC can work at higher temperatures, at higher voltages,
with faster switching frequency, and with less care than Si. The only issue with this
material is how it is made. Due to the attention that this incoming material receives from
its respective field, the ability to analyze its defects accurately should be short work.
Once a method of passivating these interface states is created, SiC will see the market
and improve upon the current state of power electronics.
References:
[1] Roccaforte, F., Fiorenza, P., Greco, G., Nigro, R. Lo, Giannazzo, F., Patti, A., & Saggio, M.
(2014). Challenges for energy efficient wide band gap semiconductor power devices.
Physica Status Solidi (a), 211(9), 2063–2071. doi:10.1002/pssa.201300558
[2] Gruber, G., Hadley, P., Koch, M., Peters, D., & Aichinger, T. (2014). Interface defects in SiC
power MOSFETs - An electrically detected magnetic resonance study based on spin
dependent recombination, 165, 165–168. doi:10.1063/1.4865627
[3] Ettisserry, D. P., Goldsman, N., & Lelis, a. (2014). A methodology to identify and quantify
mobility-reducing defects in 4H-silicon carbide power metal-oxide-semiconductor fieldeffect transistors. Journal of Applied Physics, 115(10), 103706. doi:10.1063/1.4868579
[4] Stevanovic, L. D., Matocha, K. S., Losee, P. a., Glaser, J. S., Nasadoski, J. J., & Arthur, S. D.
(2010). Recent advances in silicon carbide MOSFET power devices. 2010 Twenty-Fifth
Annual IEEE Applied Power Electronics Conference and Exposition (APEC), 401–407.
doi:10.1109/APEC.2010.5433640
[5] Jordan, J., Esteve, V., Sanchis-Kilders, E., Dede, E. J., Maset, E., Ejea, J. B., & Ferreres, A.
(2014). A Comparative Performance Study of a 1200 V Si and SiC MOSFET Intrinsic
Diode on an Induction Heating Inverter. IEEE Transactions on Power Electronics, 29(5),
2550–2562. doi:10.1109/TPEL.2013.2282658
[6] Roccaforte, F., Fiorenza, P., Greco, G., Vivona, M., Lo Nigro, R., Giannazzo, F., … Saggio, M.
(2014). Recent advances on dielectrics technology for SiC and GaN power devices.
Applied Surface Science, 301, 9–18. doi:10.1016/j.apsusc.2014.01.063
[7] Lelis, A. J., Habersat, D., Green, R., Ogunniyi, A., Gurfinkel, M., … Goldsman, N. (2008). Time
Dependence of Bias-Stress-Induced Instability Measurements, 55(8), 1835–1840.