Novel SiC Junction Barrier Schottky Diode Structure for Efficiency Improvement of EV Inverter

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1 EVS28 KINTEX, Korea, May 3-6, 2015 Novel SiC Junction Barrier Schottky iode Structure for Efficiency Improvement of EV Inverter ae Hwan Chun, Jong Seok Lee, Young Kyun Jung, Kyoung Kook Hong, Jung Hee Park, Tae Won Lim Research & evelopment ivision, Hyundai Motors, , Sam-dong, Uiwang-si, Gyeonggi-do, Korea, Abstract The future of electric vehicles(ev) depends on technological advancements of power electronic systems. Power semiconductor devices especially play a key role in power electronic systems. Most of power semiconductor devices of electric vehicles today have been enabled by silicon(si). However recently, power semiconductor devices based on silicon carbide(sic) are gotten attention as important components of the inverters/converters of EV. The SiC power semiconductor devices have lower on-resistance and higher breakdown voltage compared with Si devices. In addition, they ensure high reliability in harsh automotive environments, such as high temperatures. SiC Schottky barrier diodes(sb) had been proposed for low conduction loss and fast switching speed compared with Si PiN diodes. However, the disadvantage of SiC SB is its high off-state leakage current. SiC Junction Barrier Schottky(JBS) diodes had been appeared to overcome the drawback of SB, but the presence of P+ regions leads to a reduction in on-state current. In this paper, buried P+ JBS diode structure is proposed for increasing on-state current while maintaining low off-state leakage current. Compared to the conventional JBS diodes, N type epi-layer of novel JBS diode consists of two layers. Each separated N type epi-layer has a role of blocking off-state leakage current and on-state resistance reduction. In detail, P+ regions are located in low doped first N type epi-layer and block off-state leakage current. Another highly doped second N type epi-layer causes a reduction of on-state resistance. The proposed JBS diode is applicable to EV for minimizing the power consumption, which is generated in inverters/converters. As a result, it can contribute to volume reduction of cooling system by decreasing heat generation. In conclusion, the proposed JBS diode can contribute to improving fuel efficiency through effective management of a limited battery power. Keywords: Inverter/Converter, Silicon Carbide, Schottky Barrier iode, Junction Barrier Schottky iode EVS28 International Electric Vehicle Symposium and Exhibition 1

2 1 Introduction 1.1 SiC power device for EV Semiconductor devices play a key role in power electronic systems. Today, most of these applications are enabled by silicon. However, wide bandgap semiconductor materials, such as SiC and gallium nitride(gan), have superior properties compared to Si for using at power switching operation. Especially, SiC is an attractive wide bandgap semiconductor material with many noteworthy properties such as high saturation velocity, high breakdown electric field, high hardness, and high chemical stability in a variety of environments. It is one of the talented candidates to achieve power devices operating at high-temperature, high-frequency, with highpower, and high-voltage within harsh environment [1]. Therefore, SiC based power devices are expected to replace Si in EV for better energy efficiencies (see Fig. 2). 1.2 Theory about JBS diode The SB structure (see Fig. 3 ) has been proposed with better properties of lower conduction loss and faster switching speed. Because of that, SB gets attention as an important component for inverters and converters of EV. However, most concerned problem of SB is high off-state leakage current. To overcome the drawback current, the JBS diodes had been suggested (see Fig. 3 ). The depletion region, generated from PN junction of the JBS diode blocks off-state leakage current. Unlike the intention, the presence of P+ regions in the structure leads to a reduction in on-state current. In this paper, novel SiC JBS diode is proposed for increasing on-state current without increasing offstate leakage current compared with conventional SiC JBS diode. Power module Inverter/Converter Motor Figure 3: Schematic cross-section of SB and JBS diode The N epi-layer of JBS diode is the key to determine the off-state leakage current and on-state resistance. By increasing N, doping concentration of N epi-layer, on-state resistance is reduced (see Eq. (1)). Figure 1: Power module of EV L Ron, sp (1) qμsicn On the other hand, off-state leakage current increases due to the decreasing of the depletion layer (see Eq. (2)) and breakdown voltage is decreases (see Eq. (3)) [1]~[3]. 2εSiCVbi W (2) n N qn 1 N A BV(4 H SiC) N (3) 3 / 4 Figure 2: Market forecast for SiC based power module (Source : Yole eveloppement SA. SiC Market 2013 ) Conclusively, on-state and off-state characteristics are in a trade-off relationship. EVS28 International Electric Vehicle Symposium and Exhibition 2

3 2 evice Structure and Fabrication 2.1 esign approach Our improved concept of buried P+ JBS diode is proposed to overcome trade-off relationship between on-state resistance and off-state leakage current characteristics. Compared to a conventional JBS diode, the doping concentration of N epi-layer consists of the other two layers. Both separated N epi-layers have roles of blocking off-state leakage current and on-state resistance reduction. In detail, P+ regions are located in low doped first N epi-layer and block off-state leakage current. Second N epi-layer may reduce on-state resistance by higher doping concentration than first N epi-layer (see Fig. 4). (determined from Alpha-Step IQ, KLA-Tencor). This thickness is sufficient to prevent unintentional doping during implantation process. Second N epilayer is composed by same condition of implantation process. Front-side metal deposited 100nm thickness of titanium for Schottky contact metal and 400nm aluminum on second N epi-layer and back side metal deposited of 500nm nickel and 300nm silver for ohmic contact metal. The metal layers on the front and back sides have been sputtered, and subsequently annealed in nitrogen forming gas at 450 C for 30min, thereafter, to form alloy between SiC with metal with rapid thermal process (RTP). This step has been expected to form a nickel silicide layer which is able to enhance the ohmic contact. Figure 5: Top view of buried P+ JBS diode chip and SOT-227 package The cross section inspection of proposal diode was observed with Low Voltage-Scanning Electron Microscopy(LV-SEM) in figure 6. Low voltage condition was adopted to appear the ion implantation region in SEM image. (c) Figure 4: Schematic cross-section of buried P+ JBS diode, Schematic top view of first N- epi-layer and (c) Schematic top view of second N epi-layer 2.2 evice fabrication 2500nm thickness of SiC N epi-layer is deposited on 4 inch 4H-SiC wafer. A concentration rate of 500nm thickness second N epi-layer has higher doping than first epi-layer. The buried P+ regions have been selectively implanted with aluminum by using a silicondioxide(sio2) layer as hard mask. The mask thickness was adjusted to approximately 2000nm Figure 6: Cross-sectional LV-SEM images of buried P+ JBS diode 3 Results and iscussion 3.1 evice characteristics The measurement results of the buried P+ JBS diode are represented in Fig. 7 which shows onstate and off-state electrical characteristics. The forward current is 28A at 1.5V and its density EVS28 International Electric Vehicle Symposium and Exhibition 3

4 becomes 175A/cm 2. The off-state leakage current is under 10nA at 170V and the density becomes 0.063uA/cm 2. The switching waveform of buried P+ JBS diode shows the reverse recovery time with 33.4ns. diode (94uA/cm 2 ) and 105 times smaller compared with conventional SiC diode (6.3uA/cm 2 ). This result indicates that the buried P+ JBS diode is acting effectively in the view of blocking off-state leakage current. However, the forward current density of the proposed JBS diode is similar with Si diode (166A/cm 2 ) and cut in half compared with the value of commercial SiC diode (340A/cm 2 ). Figure 8: Comparison of reverse leakage current density and forward current density Figure 7: Measured Reverse, Forward I-V curves and (c) Turn-off characteristic Fig. 8 indicates a comparison with commercialized Si, SiC Schottky diodes and our improved structure. The off-state leakage current density of the proposed JBS diode is about 1567 times smaller compared with conventional Si (c) 3.2 iscussion and future work In our study, the off-state low leakage current characteristic of the buried P+ JBS diode is verified its availability by the actual manufacturing and test result. On the other hand, the on-state electrical characteristic is somewhat insufficient compared with other SiC Schottky diodes, because this structure is focused on ensuring the excellent off-state electrical characteristic only. Even though, EVS28 International Electric Vehicle Symposium and Exhibition 4

5 the trade-off relationship is overwhelmed between on-state and off-state electrical characteristics, it is very obvious fact that the buried P+ JBS diode structure has very low offstate leakage current. Our next challenge is the buried P+ JBS diode with higher forward current density. These following methods will be adopted for the next manufacturing in order to achieve higher forward current density. Increasing doping concentration of first N epi-layer ecreasing thickness of first N epi-layer Increasing distance between buried P+ regions All of the methods are assured the increase both of forward and leakage current density. Nevertheless, the increase of leakage current can be negligible, because of that the off-state leakage current characteristic of the buried P+ JBS diode is remarkable. In order to review an actual applicability of the three methods for increasing forward current, a method of increasing distance between buried P+ regions are actually performed. In this process, small chips are fabricated and the experimental results are in Table. 1 Both results of manufacturing and measurement is confirmed that the buried P+ JBS diode has a lower off-state leakage current density than commercialized Schottky diodes. In addition, our results proved that there is a huge possibility of buried P+ JBS diode with low leakage current as well as high forward current. The proposed JBS diode is applicable to EV, as a result it can minimize the power consumption, which is inevitably generated in inverter/converter for various electric control systems including motor control. In conclusion, the buried P+ JBS diode can contribute to improving fuel efficiency through effective management of a limited battery power. Moreover, it can also contribute to volume reduction of cooling system in electric vehicles by decreasing heat generation due to minimize power consumption of inverter/converter. References [1] B. J. BALIGA, Silicon Carbide Power evices, World Scientific Publishing Company, [2] B. J. BALIGA, Power Semiconductor evices, PWS Publishing Company, [3] Sima imitrijev, Principle of Semiconductor evices, Oxford University Press, [4] Jung Hee Park, Improved Structure of Siliconcarbide JBS diode using epitaxial growth layer on buried P regions, JSAE Annual Congress, Table 1: Change of electrical characteristics according to the distance between buried P+ regions istance (um) Forward current (A) Leakage current (ma) Through this experiment, we confirm an increase in forward current and leakage current by increase the distance between buried P+ regions. It is an experiment to identify trend of electrical characteristic change. We are confident that it can ensure an optimized design value on the basis of more additional experiments. 4 Conclusion The buried P+ JBS diode is fabricated using epitaxial re-growth process on SiC wafer. Theoretically, we expect that P+ regions are inserted in first N epi-layer can block off effectively the leakage current and highly doped second N epi-layer can improve forward current characteristic. EVS28 International Electric Vehicle Symposium and Exhibition 5