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High Current Switching Capabilities of a 3000 V SiC
Thyristor for Fast Turn-on Applications
Jason M. Sanders
Ahmed Elasser and Stanislav Soloviev
Transient Plasma Systems, Inc.
Torrance, CA USA
GE Global Research
Niskayuna, NY USA
Abstract— High voltage, pulse power Silicon Carbide (SiC)
thyristors have been under development at GE Global Research
Center for the past decade. Previous tests of these relatively
mature devices have shown that they can reliably hold off high
voltages (> 3 kV) at extremely low leakage currents (nanoamps),
and carry high pulse currents (> 1.5 kA) on microseconds
timescales, while surviving millions of cycles at a 1Hz repetition
rate. To evaluate the performance of these devices on faster time
scales and higher power densities, Transient Plasma Systems, Inc.
(TPS) conducted brass-board testing to evaluate the hardswitching capabilities of these devices on sub-μs timescales. These
tests were designed to baseline dv/dt, di/dt, instantaneous power
density, and switching efficiency of the SiC device and compare
that against current state-of-the-art Silicon (Si) devices.
Compared to a commercially available Si thyristor with a
comparable specified current rating, the SiC thyristor turned on
seven times faster and was more than 200% more efficient. The
high voltage, high peak current, high di/dt, and low losses of these
devices makes them an attractive, near-term solution for compact
pulsed power systems, which frequently rely on arrays of
series/parallel stacked IGBTs or MOSFETs that operate at
significantly lower power densities.
Keywords—Pulsed Power Switch, High Voltage Switch, High
Power Switch, thyristor, SiC thyristor, Wide band gap device
I. INTRODUCTION
Triggered solid state devices capable of holding off more
than 1 kV and surging multiple kAs of current are increasingly
a viable alternative for pulsed power applications that previously
required triggered spark gap switches or other devices based on
gaseous electronics. For a number of pulsed applications
requiring high surge currents at low to moderate pulse repetition
rates, thyristor devices offer superior performance compared to
MOSFETs or IGBTs; however, thyristors typically feature
slower turn-on time, which is governed by a diffusion limited
process referred to as the base transit time. This relatively slow
turn on transient results in high losses when hard-switching on
timescales below 1 μs. The high saturated electron drift velocity
and superior critical electric field strength offered by a wide
band gap material has the potential to reduce turn-on time and
increase efficiency, potentially reducing losses on time scales
below 1 μs to an acceptable level. Results will be presented from
an investigation designed to evaluate the pulsed capabilities of a
6 mm  6 mm, 3 kV, 35 A asymmetrical thyristor fabricated on
ultra-low micropipe density 4H-SiC 4” wafers by GE Global
Research Center. Previous investigations into the pulsed
Work conducted at TPS was supported by the Air Force Office of Scientific
Research as part of a Phase II STTR grant. Contract Number: FA9550-15-C0051. Program Manager: Dr. Jason Marshall.
capability of these devices have been focused on microsecond
duration pulses at moderate peak currents of 200-800 Amperes
and di/dt less than 1 kA/μs. Initial results show that these
devices are capable of surging high currents at di/dt greater than
10 kA/μs with a current rise time of 150 ns. Switching
efficiencies more than 2 times higher than Si-based systems
have been recorded, indicating that failure mechanisms related
to local heating in the resistive drift layer of the device may be
less prevalent with SiC compared with Si devices.
Solid state switches like this SiC thyristor are critical devices
for compact pulsed power systems capable of long-lived,
reliable operation at high pulse repetition rates (PRR).
Depending on the required switching voltage, peak current, and
charge transfer time, this component will most likely be either a
thyristor, IGBT, or MOSFET. For state of the art Si devices, the
relatively slow switching time of the thyristor compared to the
IGBT or MOSFET typically prevents effective use of the
thyristor for applications requiring pulses with durations less
than hundreds of nanoseconds, unless downstream compression
is utilized [1]. The relatively long period of time it takes a Si
thyristor to reach its fully conductive state typically necessitates
the use of IGBTs or MOSFETs, which operate at lower power
density and oftentimes must be arrayed in series/parallel
combinations.
Previous investigations into the pulsed power performance
of these devices had been focused on microsecond duration
pulses at moderate peak currents of 200-800 Amperes and di/dt
less than 1 kA/μs [2, 3]. The data presented in this paper
represents the findings of an initial investigation conducted at
TPS into the pulsed capabilities of the devices on timescales
between 100 to 500 ns at significantly higher pulsed currents
and di/dt than previously reported. The intention of this effort
is to identify and understand limitations and failure mechanisms
of these devices when operating at high current densities and
switching at high di/dt. These tests have shown that these SiC
thyristors are capable of achieving di/dt above 10 kA/μs for
currents greater than 1 kA.
SiC
Fig. 1. Device cross-section of the 3 kV, 35 A SiC thyristor currently under
evaluation for high pulsed di/dt operation. It is expected that future devices
will be a refined version of this structure, optimized for the targeted
specifications.
II. SIC THYRISTOR DEVICE AND TEST SETUP
The thyristor is grown on an 4H-SiC N-substrate, featuring
a stack of P-N-P layers formed by epitaxial growth. The
resulting structure, shown in Fig. 1, has an N-gate layer, thereby
requiring a negative gate-anode current to turn it on. The
structure is formed by first etching through the anode layer
down to the N-base in closely spaced lines to create a highly
interdigitated pattern of gate and anode connections. The highly
interdigitated design enables the high di/dt and dv/dt needed for
a high performance pulsed power device. The lightly doped Ptype blocking layer is ~ 30 µm thick and is doped at ~ 1015 cm3
to safely achieve a 3000 V blocking voltage.
TABLE I. RESONANT CIRCUIT COMPONENTS AND CIRCUIT DYNAMICS
RCH
2 MΩ
D
GE-GRC 6.5 kV SiC PiN Diode
C
100 nF Film Capacitor
L
1 μH Aircore Inductor
Q
GE-GRC SiC Thyristor
fo
500 kHz
Z = √(L/C)
3Ω
T/4
500 ns
Si
Fig. 2. Top: A schematic of the circuit used to evaluate the hard-switching
capability of GE-GRC’s SiC thyristor on sub-μs timescales. Bottom-left:
GE-GRC’s SiC thyristor. Bottom-right: Brass-board test circuit.
A second, sloped etch is performed through the N- base at
the edge of the device for the edge termination. Subsequent
thick oxide layers are patterned and used as ion implant masks
to create highly-doped regions for p-type and n-type contacts
for the anode and gate, respectively, and to form the junction
termination extension (JTE). The JTE is used to achieve high
forward blocking voltages while being robust to process
variations. The devices are asymmetrical, blocking high voltage
only in the forward direction, which is sufficient for this
application. Although the total chip area is 36 mm2, most of the
area is consumed by the terminations and contact pads.
Interconnect metal and interlayer dielectrics are used to connect
the interdigitated fingers together while keeping the gate and
anode isolated from each other. Thick pad metal is used to
enable high pulse currents, and high-voltage polyimide
passivation is used for device protection. The device P-contact
anode resistance has been optimized to reduce the forward drop
during pulse conditions.
a state-of-the-art Si thyristor. The tests were conducted by
charging a capacitor to a DC voltage and then discharging the
capacitor through the thyristor and an anti-parallel diode into a
resonant LC circuit. The energy circulates through a parallel
combination of the thyristor and an anti-parallel diode until the
energy is dissipated. The components shown in Fig. 2 were
chosen according to the values listed in Table 1 in order to
achieve a current risetime of 500 ns.
III. TEST RESULTS
The results comparing the SiC device to the Si device are
shown in Fig. 3. The fast anode-cathode fall exhibited by the
SiC thyristor is critical for efficient sub-μs hard-switching. The
dv/dt of the SiC device is measured to be seven times faster than
that of the Si device. The fast turn-on capability of the SiC
device results from the fact that the critical field for electrical
breakdown of SiC is 10 times higher than Si, enabling
significantly narrower drift layer thicknesses compared with Si
devices.
Switching efficiency was calculated by windowing the
datasets and numerically integrating the traces to calculate the
energy dissipated in the device during the first half-cycle. The
SiC device was shown to be 220% more efficient than the Si
device for current rise times of 500 ns. Switching performance
and efficiency results are shown in Table 2. The 70% switching
efficiency measured for the SiC thyristor is adequate for a
number of low average power pulsed applications. It is
expected that this performance can be improved by optimizing
the device design and possibly the gate drive circuitry as well.
To test the device’s surge capability, the resonant circuit was
replaced by a non-inductive 50 mΩ current sense resistor
(Stackpole Electronics, MR5FT50L0, 1% tolerance, 5 W). The
voltage across the current sense resistor was measured with a
standard Tektronix 10 probe (Tektronix TPP0201, 10, 300 V,
DC – 200 MHz). Preliminary results show that this device is
capable of surging up to currents above 1 kA in less than 150 ns,
as shown in Fig. 3. The current sense resistor (CSR) used to
measure the peak current of 3 kA shown in Fig. 3 was a noninductive design; however, the effective inductance from the
magnetic field generated azimuthally along the length of the
CSR has an impedance on the order of 50 mΩ for 100-200 ns
TABLE 2. PERFORMANCE COMPARISON OF SI THYRISTOR TO SIC
THYRISTOR
Fig. 3. Top: A direct comparison of the performance of the SiC thyristor to
the Si thyristor. Middle: To compare energy dissipated in the Si thyristor
to the SiC thyristor, the data sets were windowed as shown in the plot above.
Bottom: Surge testing indicates this SiC device can switch a high current in
150 ns, at power densities that are significantly higher than commercially
available Si thyristors, IGBTs, and MOSFETs.
To test the pulsed capabilities of the device on sub-μs
timescales, two identical pulsed resonant circuits were designed
as a test bed to allow a direct comparison of the SiC thyristor to
Si Thyristor
SiC Thyristor
Charge Voltage (V)
600
600
Anode Current (A)
300
384
Voltage Falltime (ns)
322
79
Current Risetime (ns)
272
245
dv/dt (kV/μs)
1.86
7.59
di/dt (kA/μs)
1.10
1.57
Stored Energy (mJ)
18
18
Dissipated Energy (mJ)
12.2
5.4
Efficiency
32%
70%
transients. These measurements will be revisited in future work
using an impedance controlled, high bandwidth CSR. These
tests were limited to a maximum charging voltage of 600 V due
to the limits of the DC power supply available for testing. In
spite of this relatively low charging voltage compared with the
device’s hold-off voltage, these initial tests indicate that this
device is promising for pulsed power applications for which fast
turn-on and instantaneous power density are critical. The
measured current density is significantly higher than
commercially available Si IGBTs with turn-on times faster than
250 ns, making this device a potential drop-in replacement
candidate to increase power density in applications where
system size and weight must be minimized.
Planning for future pulsed power tests of these devices is
underway. These tests will continue to explore the limits of
instantaneous power density of these devices as a function of the
number of shots fired. Additionally, tests to explore the
feasibility of modularizing these devices in series/parallel arrays
are also planned. The relatively simple triggering mechanism of
these current fed devices has the potential to significantly reduce
the complexity of triggering a series stack of these devices
compared with their voltage driven Si IGBT and Si/SiC
MOSFET counterparts. The realization of series stack of these
devices that is simply triggered by a single pulse transformer,
with one primary winding and multiple secondaries, has promise
for a low-cost, highly dense switch capable of pulsing tens of
kAs of current at hold-off voltages of 10 kV or higher, resulting
in a highly dense, fast switch.
IV. CONCLUSION
GE Global Research has developed a relatively mature SiC
thyristor, which has been shown to take advantage of the wide
bandgap properties of this material to achieve high hold-off
voltage and peak current capabilities this material offers
compared to Si. These features, in addition to the material’s
high carrier mobility, narrower drift region, and high
temperature properties, make it a suitable candidate for pulsed
power applications that require high power density and fast turnon capability.
[1]
[2]
[3]
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implementation of a solid state high voltage pulse generator that produces
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18, no. 4, pp. 1228-1235, 2011.
S. Soloviev, A. Elasser, S. Katz, S. Arthur, Z. Stum, L. C. Yu,
"Optimization of holding current in 4H-SiC thyristors," Materials Science
Forum, Vols. 740-742, pp. 994-997, 2013
A. Elasser, P. Losee, S. Arthur, Z. Stum, J. Garrett, and M. Schutten,
“3000V, 25A pulse power asymmetrical highly interdigitated SiC
thyristors,” in Twenty-Fifty Annual IEEE Applied Power Electronics
Conference and Exposition (APEC), Feb. 2010, pp. 1598-1602.