The High Temperature Integrated Electronics and Sensor (HTIES)
Program at the NASA Lewis Research Center is currently developing
silicon carbide (SiC) for use in harsh conditions where silicon,
the semiconductor used in nearly all of today's electronics, cannot
function. Silicon carbide's demonstrated ability to function under
extreme high-temperature, high-power, and/or high-radiation conditions
will enable significant improvements to a far-ranging variety
of applications and systems. These range from improved high-voltage
switching for energy savings in public electric power distribution
and electric vehicles, to more powerful microwave electronics
for radar and cellular communications, to sensor and controls
for cleaner-burning, more fuel-efficient jet aircraft and automobile
engines.
For power distribution, SiC semiconductor switches offer the promise
of 10-fold to 100-fold performance improvements over present-day
silicon-based devices. Before these improvements can be realized,
however, present-day prototype SiC devices must be improved and
made reliable. One reliability requirement of modern power rectifiers
is that they must be able to withstand transient overvoltage glitches
that commonly occur in power system circuits. Silicon power rectifiers
in use today operate with high reliability in part because they
exhibit a stabilizing property known as a positive temperature
coefficient of breakdown voltage. This property permits them to
withstand and recover from overvoltage glitches without sustaining
permanent damage.
Prior to this work, prototype SiC rectifiers exhibited a negative
temperature coefficient of breakdown voltage; those rectifiers
could not reliably withstand overvoltage glitches. Such rectifiers,
no matter how well they outperformed silicon rectifiers in voltage,
current, and switching speed ratings, could not be incorporated
into aerospace power systems where reliable operation is critical.
The figure above shows a schematic cross section of the first SiC rectifiers to exhibit a positive temperature coefficient of breakdown voltage, enabling these Lewis-fabricated rectifiers to repeatedly withstand large overvoltage glitches. The next figure shows the current and voltage waveforms recorded when one of these devices was subjected to a 200-nsec overvoltage glitch pulse. The device clearly exhibits a positive temperature coefficient of breakdown voltage: as the rectifier self-heats over the pulse duration, the voltage across the device increases (top waveform) while the current through the device decreases (bottom waveform). If one ignores the unimportant displacement current spikes at the rising and falling edges of the pulse, the peak conduction current of ~2.5 A at 20 nsec corresponds to a current density in excess of 50,000 A/cm2.
This work demonstrates for the first time that robust SiC power
devices with excellent reliability and immunity from glitches
will be achievable as SiC technology matures.
Find out more about our research.
Lewis contact: Dr. Philip G. Neudeck, (216) 433-8902, Phil.Neudeck@grc.nasa.gov
Author: Dr. Philip G. Neudeck
Headquarters program office: OA
Previous articleLast updated May 5, 1997
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