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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] J. M. Sanders, A. Kuthi, and M. A. Gundersen, “Optimization and implementation of a solid state high voltage pulse generator that produces fast rising nanosecond pulses,” IEEE Trans. Dielectr. Electr. Inusl., vol. 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.