IEEE Electrification Magazine - December 2013 - 65
in the evolution of power electronics technology to obtain
high power density, high efficiency, and superior performance for various applications. the overall system efficiency will also increase due to the improved properties
of semiconductor materials at low temperatures. the
performance of semiconductor devices, down to liquid
nitrogen (ln2) temperatures (63-77.2 K), has been shown
to improve with the decrease in temperature due to the
improved thermal, electrical, and electronic properties of
the materials. specifically, the field-effect semiconductor
devices operating at low temperatures have shown
important advantages over those
operating at room temperature,
including lower power dissipation,
improved switching characteristics,
reduced interconnect resistance, and
increased carrier mobility.
the operation of power electronic
and control equipment at cryogenic
temperatures has been investigated
for a number of applications, not
only for controlling superconducting
machines and devices but also for
use in satellites and spacecraft to
remove the need for the auxiliary
heaters that are frequently used
to keep the electronics warm. in
addition, low-temperature electronics have potential uses in
deep space and terrestrial
applications that include magnetic levitationbased transportation systems, military all-electric vehicles (EVs), medical diagnostics, cryogenic
instrumentation, and superconducting magnetic
energy storage systems. Furthermore, superconducting motors offer a significant advantage to
cruise and cargo ships to significantly expand
their capacity. Because of the higher efficiency of
superconducting motors and cryogenic power
electronics, the fuel consumption is reduced. this
leads to hundreds of thousands of dollars in fuel
savings per year for an average cargo ship. the
Hts system provides superior degaussing over legacy copper coils, and it is more efficient and weighs
significantly less than copper systems.
operated at room temperature. this is because the semiconductor materials seem to demonstrate better electrical
and thermal properties at lower temperatures up to about
50 K. they also have higher carrier mobility and saturation
velocity at low temperatures, resulting in high-speed operation. it was also found that the thermal conductivities of
the device and substrate materials improve significantly at
lower temperatures, leading to simpler thermal management, lower on-state power loss, and improved reliability. in
addition to the conduction losses, the switching losses of
power devices also decrease at cryogenic temperatures,
leading to increased overall power
conversion efficiency.
significant improvements in performance have been reported for
many power devices when operated at
cryogenic temperatures: for power
metal-oxide-semiconductor fieldeffect transistors (MosFEts) the onstate resistance falls by about four to
five times; for the diode, the reverse
recovery is reduced by an order of
magnitude; and for insulated-gate
bipolar transistors (iGBts), the tail current effects are reduced. it has been
demonstrated that MosFEt operation
at low temperatures provides advantages such as reduced physical size,
enhanced reliability, and higher current density. it is also reported that the
MosFEt threshold voltage and transconductance increase
at low temperatures. at 77 K, the threshold voltage has
been found to increase by 1 V due to carrier concentration
reduction when compared to room temperature, and the
breakdown voltage of the power MosFEts reduces up to
23%. the simulation results of operating silicon power
MosFEts at room temperature and at liquid nitrogen temperature show that when operating at liquid nitrogen temperature, the channel mobility increased ten times, the
drain current capability increased three times, and the onstate resistance reduced two to three times, from 300 K to
77 K for that particular device, because of higher carrier
mobility at lower temperatures.
the commercially available MosFEts in plastic or
metal packages have also been found to work well if they
are immersed in a bath of liquid nitrogen, although these
devices have not been designed for cryogenic applications,
according to a study by Mueller. they can be operated at
much higher current levels with lower conduction losses;
hence, high-efficiency power converters could be
designed. it was also shown that heat sinks other than the
liquid nitrogen may not be required. in a previous work,
Mueller showed that the on-resistance of commercially
available high-voltage MosFEts (500-1,000 V) decreases by
a factor of 10-30 or more depending on the drain current if
cooled down to 77 K. these results show that even when
The use of
cryogenically cooled
power converters
will change the way
lightweight, highpower converters
are designed and
manufactured for
various applications.
cryogenic Power electronics
Power Devices
the understanding of the characteristics and operation of
power semiconductor devices at cryogenic temperatures
is necessary for integrating the power electronics with the
superconducting power applications. as reported in several
research studies, the operation of power semiconductor
devices at cryogenic temperatures results in improved
switching speed and lower on-state voltage than when
IEEE Elec trific ation Magazine / d ec em be r 2 0 1 3
65
Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2013
IEEE Electrification Magazine - December 2013 - Cover1
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