12.9 kV SiC PiN diodes with low on-state drops and high carrier lifetimes Siddarth Sundaresana, Charles Sturdevant, Madhuri Marripelly, Eric Lieser and Ranbir Singh GeneSiC Semiconductor, 43670 Trade Center Pl, Suite 155, Dulles, Virginia 20166, USA. a [email protected], *corresponding author Keywords: SiC PiN diode, high-voltage, on-resistance, switching, OCVD, breakdown voltage. Abstract. Sharp avalanche breakdown voltages of 12.9 kV are measured on PiN rectifiers fabricated on 100 µm thick, 3 x 1014 cm-3 doped n- epilayers grown on n+ 4H-SiC substrates. This equates to 129 V/µm. Optimized epilayer, device design and processing of the SiC PiN rectifiers result in a > 60% blocking yield at 10 kV, ultra-low on-state voltage drop and differential onresistance of 3.75 V and 3.3 mΩ-cm2 at 100 A/cm2 respectively. Open circuit voltage decay (OCVD) measured carrier lifetimes in the range of 2-4 µs are obtained at room temperature, which increase to 14 µs at 225 °C. Excellent stability of the forward bias characteristics within 10 mV is observed for a long-term forward biasing of the PiN rectifiers at 100 A/cm2. A PiN rectifier module consisting of five parallel large area 6.4 mm x 6.4 mm 10 kV PiN rectifiers is connected as a freewheeling diode with a Si IGBT and 1100 V/100 A switching transients are recorded. Data on the current sharing capability of the PiN rectifiers is also presented. Introduction At 10 kV -20 kV voltage ratings, 4H-SiC PiN rectifiers offer the best trade-off between onstate voltage drop, switching losses and high-temperature performance as compared to Si PiN and SiC Schottky/JBS rectifiers. Optimized 2.4 mm x 2.4 mm (active area = 0.79 mm2) and large-area 6.4 mm x 6.4 mm (active area = 25 mm2) were designed and fabricated at GeneSiC with 10 kV – 13 kV blocking voltage capabilities. A detailed investigation of the on-wafer and packaged forward bias, blocking voltage, switching, carrier lifetime and long-term forward bias stability of these PiN rectifiers is presented in this paper. Device Design and Fabrication SiC PiN rectifiers with 2.4 mm x 2.4 mm chip size (active area = 0.79 mm2) were fabricated on 100 µm thick, 3 x 1014 cm-3 doped n- 4H-SiC epilayers. Large-area 6.4 mm x 6.4 mm (active area = 25 mm2) PiN rectifiers were fabricated on 90-95 µm thick, 7-9 x 1014 cm-3 doped n- 4H-SiC epilayers. The epilayer growth was performed on 4° off-axis SiC substrates. The p+ emitter layers were 1.5 µm thick and doped to 1 x 1019 cm-3. Optimized edge termination for the PiN rectifiers was provided by a combination of negative beveled mesa etching and p-type ion-implantation followed by high-temperature annealing for implant activation. Ohmic contacts to the p+ Anode and n+ Cathode layers was formed by Al-based and Ni- based metallization. Thick Al overlayers were deposited on the top and a solderable Au-based metallization was provided on the bottom for die-attaching to Cu baseplates. After on-wafer testing, selected die were assembled in customdesigned packages for detailed high-current, switching and long-term measurements. On-state and blocking voltage characteristics Blocking voltage yields in excess of 56% at 10 kV (leakage current limit = 39 µA) can be noted from histograms and a wafer map of blocking voltages (see Figure 1) measured on a 3” SiC wafer populated with 0.79 mm2 PiN diodes. A sharp onset of avalanche breakdown at 12.9 kV can be clearly observed from representative reverse I-V characteristics measured on a couple of packaged 0.79 mm2 rectifiers (see Figure 2a). The achievement of 12.9 kV blocking voltages corresponds to record-high 129 V/µm and ≈ 90% of the avalanche breakdown limit for the 100 µm thick n- epilayers used for device fabrication. On-state voltage drops as low as 3.75 V at 100 A/cm2 and differential specific on-resistance as low as 3.3 mΩ-cm2 are extracted from on-state I-V characteristics (Figure 2b) of packaged 0.79 mm2 PiN rectifiers indicating a high-level of conductivity modulation of the n-base layer. A negative differential co-efficient of on-state voltage drop is also observed from Figure 2(b), which is due to the reduction of knee voltage at higher temperatures. Figure 1: (Left, a) Histogram and (Right, b) Wafer Map of blocking voltages measured on a 3” SiC wafer populated with 0.79 mm2 PiN rectifiers under a leakage current threshold of 39 µA. Figure 2: (Left, a) Reverse I-V characteristics and (Right, b) Forward I-V characteristics measured on representative 0.79 mm2 PiN rectifiers fabricated on 100 µm thick n- SiC epilayers. High-current, on-state characteristics measured on a representative 25 mm2 PiN rectifier and a module consisting of five, 25 mm2 PiN rectifiers connected in parallel are shown in Figure 3. A slightly higher differential on-resistance of 5.75 mΩ-cm2 is extracted from the I-V characteristics measured on the 25 mm2 rectifier as compared to 3.3 mΩ-cm2 reported for the 0.79 mm2 rectifier. This extra resistance may be due to either (A) Higher n-base doping concentration on the wafers used for fabricating the 25 mm2 rectifier resulting in lower emitter injection efficiency and/or (B) metal spreading resistance on the large-area rectifiers. Carrier Lifetime measurements Open circuit voltage decay (OCVD) measurements were performed on packaged 0.79 mm2 and 25 mm2 PiN rectifiers to extract the high-level carrier lifetime (tHL) in the thick n-base layer at various temperatures. Voltage decay waveforms measured on representative 25 mm2 PiN rectifiers are shown in Figure 4(a). A record-high room-temperature carrier lifetime of 4 µs is extracted from the slope of the linear portion of the voltage decay transients, which surpasses the 3.7 µs carrier lifetime reported by Ivanov et al. [1] on 10 kV SiC PiN diodes. The carrier lifetime increases to 14 µs at 225 °C (Figure 4(b)). A carrier lifetime of 4 µs at 25 °C corresponds to a large ambipolar diffusion length, La = (DatHL)1/2 = 48 µm. La remains relatively constant with temperature, since the increase in carrier lifetime at higher temperatures is countered by a corresponding decrease in carrier mobilities. The temperature dependence of the carrier mobilities was calculated using the procedure detailed in [2] and [3]. The high values of La relative to the n- base thickness is the reason for the high level of observed conductivity modulation in the base layer, as evidenced by the low differential on-resistance obtained from the I-V characteristics shown in Figure 2 and Figure 3. Figure 3: High-current forward bias characteristics measured on (Left, a) A packaged 25 mm2 PiN rectifier and (Right, b) Five 25 mm2 PiN rectifiers co-packaged in parallel. Photographs of the discrete and the multi-device diode module are shown as insets in the respective graphs. Figure 4: (Left, a) OCVD Forward Current and Voltage transients measured at different temperatures on a packaged 25 mm2 PiN rectifier and (Right, b) Extracted high-level carrier lifetime and characteristic diffusion length as a function of temperature. The error bars correspond to the uncertainty in extracting the slope of the linear portion of the voltage decay transients. Switching characteristics A module consisting of five parallel-connected 25 mm2 SiC PiN rectifiers was connected as free-wheeling diode (FWD) with a 1200 V/300 A Si IGBT to evaluate the high-current switching performance of the SiC PiN rectifiers at different temperatures. The well-known double-pulse technique was used to switch 1100 V and 100 A through the Si IGBT/SiC PiN rectifier module at a FWD turn-off dI/dt of 432 A/µs at temperatures up to 225 °C. The current and voltage transients measured during the free-wheeling diode turn-off are shown in Figure 5(a). Due to a positive temperature co-efficient for the injected carrier lifetime in the n-base, the peak reverse recovery current (Irr) is observed to increase from – 48 A at 25 °C to -120 A at 225 °C, while the reverse recovery time (trr) increases from 168 ns at 25 °C to 528 ns at 225 °C (see Figure 5(b)). Figure 5: (Left, a) Voltage and Current transients measured during FWD turn-off of a 1200 V/300 A Si IGBT connected in anti-parallel with a PiN rectifier module consisting of five 25 mm2 PiN rectifiers connected in parallel. (Right, b) Reverse Recovery current and Reverse Recovery Charge extracted from the turn-off current transients at different temperatures. Long-term Stability of on-state characteristics As seen in Figure 6(a), the voltage drop across a packaged 0.79 mm2 PiN rectifier is remarkably stable (within 10 mV) under a constant 100 A/cm2 DC bias, indicating that the SiC epilayers and process used for fabricating these PiN rectifiers is free from bipolar degradation. The current sharing capability of the PiN rectifiers was investigated by passing a DC current of 30 A through two parallel connected 25 mm2 PiN rectifiers mounted on a common heat sink for 90 min. As seen in Figure 6(b), the current is unequally shared as ≈ 17.2 A through the PiN rectifier with the lower Von and ≈ 12.6 A through the PiN rectifier with the slightly higher Von. Initially, the PiN rectifier with the lower specific on-resistance draws increasingly more current, due to the negative temperature co-efficient of Von. The currents through the rectifiers stabilize after ≈ 30 min into the test due to the thermal coupling between the paralleled diodes. Figure 6 (Left, a) Time evolution of Von under a constant DC bias of 100 A/cm2 (0.8 A) applied to a 0.79 mm2 PiN rectifier (Right, b) Current sharing between two 25 mm2 PiN rectifiers with the forward I-V characteristics of the individual PiN rectifiers shown as an inset in the graph. Acknowledgement: The funding support from ARPA-E (under co-operative agreement DEAR0000112), and support of Dr. Rajeev Ram is greatly appreciated. References [1] P. Ivanov et al. Solid-State Electronics 50, 7-8, 1368-1370 (2006). [2] H. Matsuura et al. Journal of Applied Physics 96(5), 2708-2715 (2004). [3] T T Mnatsakanov et al. Semicond. Sci. Tech. 17, 974-977 (2002).