Optimization of high voltage SiC PiN diode for operation in opening switch mode

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Optimization of High Voltage SiC PiN Diode for
Operation in Opening Switch Mode
Stanislav I. Soloviev*, Reza Ghandi, Ahmed Elasser
General Electric Global Research
Niskayuna, USA
*soloviev@ge.com
Jason. M. Sanders
Transient Plasma Systems, Inc.
Torrance, USA
jason@transientplasmasystems.com
Abstract—A high-voltage Silicon Carbide (SiC) PiN diode that
was primarily designed to operate as a rectifier in energy
conversion systems was evaluated in an opening switch mode by
testing it in inductive storage circuit. Based on obtained
experimental data, a mixed-mode simulation model which
includes the circuit topology and the device structure, was created
and validated. Numerical simulations of a diode structure
optimized for operation in an opening switch mode was
performed.
Keywords—semiconductor opening switch, PiN diode,
numerical simulation, Silicon Carbide, High Voltage.
I. INTRODUCTION
Pulse power generators with inductive storage and fast
current interrupters are of great interest for pulsed power
electronics. A silicon device called drift step recovery diode
(DSRD) was proposed and realized in 1983 by Grekhov at al. to
be used as a solid-state current interrupter [1]. The diode
structure consisted of p+/n/n+ layers and was capable of
interrupting current densities up to 160 A/cm2 with current
interruption times of about 2 ns and voltages on the order of 1
kV.
In 1991, Mesyats et al. from the Institute of Electro-physics
(Russia) discovered another semiconductor opening switch
(SOS) effect in silicon PiN-diode like structures. The p+/p/n/n+
silicon diodes could interrupt current densities of ~60kA/cm2.
This effect was used to develop high-power semiconductor
opening switches in intermediate inductive storage circuits [2].
The breaking power of the opening switch was as high as 5 GW
with interrupted currents up to 45kA, reverse voltages up to
450kV, and current interruption times between 10 ns and 60 ns.
Figure 1 (left scale) illustrates the comparison of the DSRD
and SOS devices among other solid-state current switchers,
where tc is the characteristic time either of the current build-up
through a device (FT-Fast Thyristor) or its cut-off (CSD-Charge
Storage Diode, DSRD, SOS), while Figure 1 (right scale) shows
the maximum working voltages achieved in the above-
mentioned semiconductor switches. The current interrupters
developed on the basis of DSRDs have a maximum operating
voltage of 20-30 kV [3].
Compared with other state of the art devices that are capable
of switching multi-kW to multi-MW electrical pulses in less
than 10 ns, these Si based diode switches are the best choice
when high pulse repetition rate, high average power, and long
lifetime are required.
Fundamental limitations of these devices, which primarily
result from material properties (electric field breakdown
strength, maximum junction temperature, carrier saturation
velocity, and dielectric constant) can potentially be extended to
realize a smaller, faster, higher voltage device made of Silicon
Carbide.
Basic physical properties of SiC open up a unique
opportunity to fabricate switches with 20 times faster switching
speed than corresponding Silicon devices with the same
operating voltage. Moreover, the average pulse repetition
frequency for SiC devices can be more than ten times higher
than that of Si devices under the same conditions of heat power
dissipation.
P. Ivanov et al. reported a mesa-epitaxial 4H-SiC p+/p/n/n+-
diode operating in pulse regimes similar to DSRD and SOS
modes [4]. It has been demonstrated that after short pumping the
diodes by a forward current pulse (5 ns duration, 200A/cm2 peak
current density) followed by applying a reverse voltage pulse
(rise time of 2 ns), the diodes were able to interrupt a reverse
current density of 3.5 - 25 kA/cm2 in a time less than 0.3 ns.
Fig.1. (left scale) Characteristic current density switching time tc, and (right
scale) working voltage, U, of semiconductor devices with different
mechanisms of current switching: CSD - Charge Storage Diode; DSRD -
Drift Step Recovery Diode; FT - Fast Thyristor; SOS - semiconductor
opening switch.
1E-2
1E-1
1E+0
1E+1
1E+2
1E+3
1E+0
1E+1
1E+2
1E+3
1E+4
1E+5
1E-10 1E-9 1E-8 1E-7 1E-6 1E-5
Peak Voltage, kV
Current Density, A/cm2
pulse time, s
SOS
DSRD
CSD
FT
tc, s

However, in spite of the fact that SiC DSRD and SOS
devices showed encouraging results that may qualify them as
ultrafast switches in high power pulse systems, many more
design and processing issues need to be addressed. Specifically,
a premature surface breakdown at SiC DSRD (SOS) chip edges
is still a major hurdle to clear as it has been reported in the ref.
[4,5]. Moreover, relatively short lifetimes of charge carriers in
SiC could be another limiting factor to implement SiC SOS
devices.
Although, the premature breakdown issue in SiC
power devices has been resolved to date by using specially
designed edge termination that enabled to achieve a breakdown
voltage close to the theoretical limit [6], charge carrier lifetimes
are subject for improvement in SiC technology.
The objectives of this work are to test a GE 6.5kV SiC PiN
diode with robust edge termination in a fast switching mode,
develop and validate numerical models based on obtained
empirical data and perform optimization of opening switch
diode.
II. EXPERIMENT
SiC PiN diodes used in this work were designed to operate
in energy conversion systems up to 6.5kV with fast recovery
switching waveforms and low reverse recovery losses [6].
Figure 2a shows a schematic cross-section of the device. The
diodes were fabricated on a 4H-SiC N-type substrate with a 70-
µm drift layer and 8x1014 cm−3 doping. The anode area of the
diodes was formed using reactive ion etching of the p-type layer
(4 µm, 1×1019 cm−3) grown on top of the lightly doped n-type
drift layer. An Aluminum-implanted junction termination
extension was used to achieve a high blocking voltage. Implant
activation was carried out at 1675◦C. Al/Ni/Al ohmic contacts
were patterned and annealed at 1050◦C. The chips were
designed to operate at a current density of 200 A/cm2.
The SiC diodes were packaged in ISOPLUS™ discrete
package (one 6x6 mm2 chip per package) as shown in Fig.2. (a)
Cross-section of GE SiC PiN diode and (b) Photograph of packaged SiC 6.5kV
PiN diode
The switching characteristic of this diode was tested using a
resonant circuit with a single pole admittance. The circuit,
shown in Fig.3Error! Reference source not found., was
realized using a SiC MOSFET, a 20 nF array of Class 2 ceramic
capacitors, and a 250 nH aircore inductor. The current through
the diode was measured using a current transformer with a 2 ns
usable risetime (Pearson, 6585) and the voltage switched by the
diode was measured using a high voltage 20 dB pad with 50 Ω
input impedance with a 500 ps usable risetime (TPS). The
waveforms were measured using a Tektronix MSO4034B (350
MHz front-end bandwidth, 2.5 GSa/sec). The measured results
are shown Error! Reference source not found.
Fig.3. Testing circuit diagram and a picture of the testing board set-up
Fig.4. Load resistor voltage and current waveforms
III. MODELING AND DEVICE OPTIMIZATION
Mixed-mode simulation was employed in order to model the
switching characteristics of the PiN diode shown in Figure 4.
The mixed-mode simulation includes a circuit model with a
a)
b)
Fig.2. (a) Cross-section of GE SiC PiN diode and (b) Photograph of packaged
SiC 6.5kV PiN diode.
N-drift layer, 8E14/cm3, 70 µm
anode
Edge
termination
Dielectric
passivation
N-SiC substrate
P-layer 1E19/cm3, 4 um
cathode

device structure model of the diode. The PiN device structure
shown in Figure 2a was simulated together with the circuit
shown in Figure 3. The results of the simulation are shown in
Figure 5, where experimental voltage waveforms across the load
resistor are added for comparison purposes. A good fit between
model and experimental curves for rise and fall times of the
voltage pulse was achieved, although some inconsistencies in
the waveforms at turn-on were observed, which we believe, are
associated with parasitic noise of the measurement tools. Carrier
lifetimes used in the model were 500 ns and 11 ns for electrons
and holes, respectively.
The tested PiN diode was designed to be used as a rectifier
with fast recovery, thus to have an SOS like diode, the device
structure must be modified in order to increase the speed of the
current interruption process during the second half part of the
reverse recovery process. Two structures are proposed for sharp
current interruption in SiC diodes: (a) p+-p-n+-substrate and (b)
p++-p-n--n+-substrate [4]. The first structure requires a deep dry
etch process which is challenging in SiC technology, thus, the
second structure is more promising at this point and it is modeled
in this work.
Following recommendations made in Ref [7] that a thickness
of p-plasma storage layer, Wp, should be around ~0.1Wn, where
Wn is the thickness of the n-drift layer, a numerical design of
experiment was performed. Figure 6 shows calculated current
and voltage waveforms for an optimized structure of a SiC
opening switch. Note, that the parameters of the circuit elements
and pumping pulse characteristics used are the same as in the
validation model of the PiN diode. Current and voltage
waveforms of the SiC PiN diode are shown in Figure 6 for
comparison purposes. A peak value of 2600V during reverse
recovery was achieved in the opening switch due to the sharp
current interruption, while in the PiN diode, the peak voltage
was only on the order of 600V at a maximum voltage of 500V
across the discharge capacitor.
Figure 7 shows the electric field distribution in the PiN diode
(Fig.7a) and opening switch diode (Fig.7b) at characteristic
transient times t1, t2, and t3, as marked in Fig.6. Note, that the
waveforms in Fig. 6 were synchronized with a pumping pulse.
Time t1 corresponds to the peak value of the reverse current in
the PiN diode. Electron-hole plasma in the PiN diode at this
instant has started to disassociate and a space charge region with
a peak electric filed at the p-n junction started to fill the drift
region, while a reverse current in the opening switch has not
reach its maximum value and an electric field has not formed
yet. When a reverse current in the opening switch diode reaches
its maximum value (time t2), the electric field distribution in the
SOS diode is similar to the field distribution in the PiN diode at
t1, however, in the PiN diode, the space charge region expanded
deeper into the drift layer and the peak of electric field has
increased. At time t3, an electron-hole plasma in the opening
switch diode is completely disassociated, the space-charge
region (SCR) is extended through the entire drift layer. The latter
causes a sharp current interruption and a voltage increase across
the load resistor. Transient currents in the PiN diode at the
corresponding time t3 are gradually decreasing since non-
equilibrium charge carriers have not completely recombined and
the SCR has not reached the n+ layer (Fig.7a).
IV. CONCLUSIONS
Mixed-mode simulations of an optimized diode structure
with an additional p-layer has demonstrated the feasibility of
making a SiC diode with a semiconductor opening switch effect
even with charge carrier lifetime values typical for modern SiC
technology. The model suggested that, e.g., with a 500V charged
capacitor, a 7ns pulse of 2600V could be achieved across the
load.
Further tuning of the SOS diode structure and optimization
of the circuit topology should improve both the risetime and the
voltage peak.
Figure 5 Experimental and simulated voltage waveforms of SiC diode on
load resistor per circuit of Figure 3.

Fig.3. Simulated voltage and current waveforms of PiN diode and opening
switch diode..
REFERENCES
[1] V. M. Tuchkevich, I. V. Grekhov. "New principles of switching high
power by semiconductor devices", Leningrad: Nauka, 1988 (in Russian).
[2] Y. A. Kotov, G. B. Mesyats, S. N. Rukin, A. L. Filatov, and S. K.
Lyubutin, “A Novel Nanosecond Semiconductor Opening Switch For
Megavolt Repetitive Pulsed Power Technology: Exp,” in Pulsed Power
Conference, 1993. Digest of Technical Papers., Ninth IEEE International,
1993, pp. 134–139.
[3] G. A. Mesyats, S. N. Rukin, S. K. Lyubutin, S. a. Darznek, Y. A. Litvinov,
V. a. Telnov, S. N. Tsiranov, and A. M. Turov, “Semiconductor opening
switch research at IEP,” Dig. Tech. Pap. Tenth IEEE Int. Pulsed Power
Conf., vol. 1, pp. 298–305, 1995.
[4] P. Ivanov and I. V. Grekhov, “Subnanosecond Semiconductor Opening
Switch Based on 4H-SiC Junction Diode,” Mater. Sci. Forum, vol. 740–
742, pp. 865–868, Jan. 2013.
[5] Kozlov, V. A. “New Generation of drift step recovery diodes for
subnanosecond switching and high repetition rate operation”. IEEE
Power Modulator Symposium, and High-Voltage Workshop. Conference
Record of the Twenty-Fifth International, 2002, pp. 441–444.
[6] Elasser, A., et al. “Static and Dynamic Characterization of 6.5-kV 100-A
SiC Bipolar PiN Diode Modules”, IEEE transactions on industry
applications, 50(1), 609–61, 2014.
[7] Ivanov P.A., Grekhov I.V. “High Voltage 4H-SiC Drift-Step Recovery
Diodes. Theory”, Zhurnal Tekhnicheskoi Phiziki, 2015, 85(6), 111–117
a)
b)
Fig.4. Electric field distributions at different transient times in (a) PiN diode
and (b) open switch diode.