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Sep 26, 2016 - arms of healthcare, and data could be used effectively . ... how medical big data can be analyzed, and what are the challenges for medical ...
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Promise and Challenges of High-Voltage SiC Bipolar Power Devices Tsunenobu Kimoto *, Kyosuke Yamada, Hiroki Niwa and Jun Suda Department of Electronic Science and Engineering, Kyoto University, Kyoto 615 8510, Japan; [email protected] (K.Y.); [email protected] (H.N.); [email protected] (J.S.) * Correspondence: [email protected]; Tel.: +81-75-383-2300 Academic Editors: Alberto Castellazzi and Andrea Irace Received: 11 October 2016; Accepted: 28 October 2016; Published: 3 November 2016
Abstract: Although various silicon carbide (SiC) power devices with very high blocking voltages over 10 kV have been demonstrated, basic issues associated with the device operation are still not well understood. In this paper, the promise and limitations of high-voltage SiC bipolar devices are presented, taking account of the injection-level dependence of carrier lifetimes. It is shown that the major limitation of SiC bipolar devices originates from band-to-band recombination, which becomes significant at a high-injection level. A trial of unipolar/bipolar hybrid operation to reduce power loss is introduced, and an 11 kV SiC hybrid (merged pin-Schottky) diodes is experimentally demonstrated. The fabricated diodes with an epitaxial anode exhibit much better forward characteristics than diodes with an implanted anode. The temperature dependence of forward characteristics is discussed. Keywords: silicon carbide; power device; carrier lifetime; conductivity modulation; merged pin-Schottky (MPS) diodes
1. Introduction Silicon carbide (SiC) has received increasing attention as a wide bandgap semiconductor well suited for high-voltage power devices. The promise of SiC power devices stems from its superior physical properties and rapid progress in growth and device technologies [1–5]. SiC (4H polytype) unipolar devices such as metal-oxide-semiconductor field effect transistors (MOSFETs) and Schottky barrier diodes (SBDs) with blocking voltages of 600–1700 V have been commercialized, demonstrating substantial reduction of power loss in various power conversion systems. Higher-voltage (3.3–6.5 kV) SiC power MOSFETs have currently been developed [6–8], and 15 kV SiC MOSFETs have also been demonstrated . These very high-voltage SiC MOSFETs can be fabricated with process technology similar to that used for 1 kV-class SiC MOSFETs, though some modifications are required in the cell design and termination structure. However, the specific on-resistance (Ron ) of power MOSFETs significantly increases with increasing the blocking voltage (V B ), following the relationship of Ron ~V B 2.2–2.5 . The on-resistance further increases at elevated temperature due to the mobility drop. For ultrahigh-voltage (>10 kV) power devices, SiC bipolar devices are promising, because the resistance of the thick voltage-blocking layer can remarkably be reduced by the conductivity modulation effect, and the on-resistance exhibits a very small temperature dependence . A drawback of bipolar devices is, of course, the slower switching speed and thereby the increased switching loss. Thus, the choice of unipolar/bipolar devices must be carefully determined taking account of the operation condition required for individual applications . Another weak point of SiC bipolar devices is the large built-in potential caused by the wide bandgap of SiC (3.26 eV). Thus, a forward voltage drop of about 2.8 V is required before significant current flows in SiC bipolar devices except for bipolar junction transistors. This large built-in voltage (knee voltage) naturally results in a relatively high forward voltage drop at low current density. Energies 2016, 9, 908; doi:10.3390/en9110908
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In recent years, ultrahigh-voltage SiC pin diodes [11–13], thyristors , bipolar junction transistors , and insulated-gate bipolar transistors (IGBTs) [16–18] have been reported. Using 10 kV SiC MOSFETs or 15 kV SiC IGBTs, several power converters such as boost converters and modular-leg converters have been fabricated, demonstrating good power efficiencies [19–22]. The major technological challenges for development of ultrahigh-voltage SiC bipolar devices include fast epitaxial growth, reduction of extended defects, control of carrier lifetimes, surface passivation, and packages. The most critical concern about the reliability of SiC bipolar devices is degradation caused by expansion of single-Shockley stacking faults, which nucleate from basal plane dislocations when excess carriers are injected and recombine [23,24]. Elimination of basal plane dislocations in SiC wafers is one of the most important challenges for commercialization of SiC bipolar power devices. Furthermore, development of ultrahigh-voltage packages is another challenge. Although some packages and modules with special insulating materials have been reported for such high-voltage devices [25,26], the ultrahigh-voltage package technology is still not mature. Though the performances of these devices and converters reported are promising, the ideal characteristics limited by the intrinsic material properties of SiC have not been well studied. In the first part of this paper, the forward characteristics of SiC pin diodes having different thickness of i-layer (voltage-blocking layer) are simulated, and the major intrinsic limitations are discussed. Although 10–30 kV-class SiC pin diodes exhibit good characteristics when the carrier lifetime is long, the forward voltage drop significantly increases when the breakdown voltage exceeds 50–60 kV. It is shown that this limitation originates from the band-to-band recombination in SiC, being an inherent limitation of SiC. In the second part, a trial of unipolar/bipolar hybrid operation with SiC pin diodes is described. The characteristics of merged pin-Schottky (MPS) diodes having epitaxial junction and implanted junction are compared. The epitaxial MPS diodes show good hybrid operation as expected and a high breakdown voltage of 11.3 kV is demonstrated by adopting an appropriate junction termination. 2. Results 2.1. Simulation of Forward Characteristics of Ultrahigh-Voltage SiC Pin Diodes Figure 1 shows the structure of a SiC pin diode simulated in this study. The anode is 5 µm-thick, doped to 5 × 1020 cm−3 , and the cathode (n+ -layer beneath the i-layer) is 10 µm-thick, doped to 5 × 1018 cm–3 . These structures were optimized to achieve highest carrier-injection efficiencies (for example, see Figure S1). Although a higher doping concentration is generally desirable to enhance the injection efficiency, one should consider bandgap narrowing . When the doping concentration becomes too high, a bandgap narrowing phenomenon occurs for the anode or cathode, which remarkably decreases the injection efficiency. Furthermore, when the thickness of the anode or cathode is not enough, the carrier recombination at the anode/cathode contact becomes significant and this recombination increases the minority-carrier current inside the anode/cathode, leading to the decreased injection efficiency . Therefore, the anode and cathode structures were fixed and the thickness of i-layer was varied in this study. The i-layer was a lightly-doped n-type epilayer with a donor concentration of 1 × 1014 cm−3 , which is technically achievable with the current technology. Since the switching frequency in ultrahigh-voltage power converters is usually low, the major component of power loss in semiconductor devices is the on-state loss. Thus, the forward characteristics of SiC pin diodes were mainly investigated in this study. In recent years, elimination of carrier-lifetime killers has been reported [29,30] and the carrier lifetime of n-type SiC has substantially been improved from about 1–2 µs (as-grown) to 30 µs or even longer [31,32]. In this study, the forward characteristics were simulated by changing the carrier lifetime in a wide range.
Figure 1. Schematic structure of a SiC pin diode simulated in this study. study. Figure 1. Schematic structure of a SiC pin diode simulated in this study.
Breakdown BreakdownVoltage Voltage(kV) (kV)
Figure 222depicts depictsthe the i-layer thickness dependence of breakdown breakdown voltage for SiC pin having diodes Figure i-layer thickness dependence of breakdown voltage for SiCfor pinSiC diodes Figure depicts the i-layer thickness dependence of voltage pin diodes having the structure shown in Figure 1. The breakdown voltage almost linearly increases with the having the structure shown in 1. Figure 1. The breakdown increases with the the structure shown in Figure The breakdown voltagevoltage almost almost linearlylinearly increases with the i-layer i-layer thickness, thickness, because the doping doping concentration concentration of the theisi-layer i-layer is low low and the the diodes are of of i-layer the of is and are thickness, becausebecause the doping concentration of the i-layer low and the diodes arediodes of complete complete punchthrough structure. However, the breakdown voltage shows a slight saturation trend complete punchthrough the breakdown voltage shows a slight saturation trend punchthrough structure. structure. However,However, the breakdown voltage shows a slight saturation trend when the when the thickness exceeds 300 µm, where the electric field strength at the end of the i-layer (space when the thickness exceeds 300 µm, thefield electric field strength at the endi-layer of the(space i-layercharge (space thickness exceeds 300 µm, where thewhere electric strength at the end of the charge region) region) becomes becomes significantly lower than that at the theinterface. junction Roughly interface.speaking, Roughlythe speaking, charge significantly lower at junction interface. Roughly speaking, region) becomes significantly lower than thatthan at thethat junction diodes the diodes diodes withthickness an i-layer i-layerofthickness thickness of 100, 300, and 500 µmand are 58 15,kV-class, 40, and and 58 58 kV-class, respectively. respectively. the with an 100, µm are 15, 40, kV-class, with an i-layer 100, 300, of and 500300, µmand are 500 15, 40, respectively.
Nd < < 1x10 1x1013 cm cm-3 N d
60 60 40 40 20 20 0 0
Nd = = 1x10 1x1014 cm cm-3 N d SiC pin pin diodes diodes SiC
100 200 200 300 300 400 400 500 500 600 600 100 Thickness of of i-layer i-layer (μm) (μm) Thickness
Dependence ofofbreakdown voltage forfor SiC pin diodes on i-layer thickness. The solidsolid and Figure 2. Dependence Dependenceof breakdownvoltage voltagefor SiC diodes i-layer thickness. Figure 2. breakdown SiC pinpin diodes on on i-layer thickness. The The solid and 14 −13 3 cm 14of −3× −3, cm1−3 and <1 cm × 10 dashed lines lines denote the results for the for i-layer doping concentration of 1 × 1014 and dashed denote the results the i-layer doping concentration 10 and 13 dashed lines denote the results for the i-layer doping concentration of 1 × 10 cm and <1 × 10 cm−3, 13 − 3 <1 × 10 cm , respectively. respectively. respectively.
As aaa typical typicalexample, example,the theforward forwardcharacteristics characteristicsofof ofSiC SiCpin pin diodes having an i-layer thickness As diodes having anan i-layer thickness of typical example, the forward characteristics SiC pin diodes having i-layer thickness of 200 µm (28 kV class) are presented in Figure 3, where the carrier lifetime in the i-layer was 200 µmµm (28 kV are presented in Figure 3, where the carrier in the i-layer changed of 200 (28 class) kV class) are presented in Figure 3, where the lifetime carrier lifetime in thewas i-layer was changed from µs (an to 100 100 µs (an (an analytical analytical expression for the the characteristics characteristics of aaand pin brief diodediscussion and brief brief from 1 µsfrom to 10011 µs analytical expressionexpression for the characteristics of a pin diode changed to µs for of pin diode and discussion are given in Note S1). As As expected, the characteristics characteristics are remarkably improved the by are given inare Note S1). in AsNote expected, theexpected, characteristics are remarkably improved by increasing discussion given S1). the are remarkably improved by increasing the carrier carrier lifetime, whichwith is consistent consistent withonthe the studies on on lower-voltage lower-voltage SiCreported pin diodes diodes carrier lifetime, whichlifetime, is consistent the studies lower-voltage SiC pin diodes in increasing the which is with studies SiC pin reported in literature [33,34]. Note that the carrier lifetimes longer than 10 µs do not contribute very literature [33,34]. Note that the carrier lifetimes than 10 µsthan do not contribute very much to reported in literature [33,34]. Note that the carrierlonger lifetimes longer 10 µs do not contribute very much to reduction of the forward voltage drop. For example, the forward voltage drop at a current reduction of the forward Fordrop. example, the forward voltage drop at adrop current much to reduction of the voltage forwarddrop. voltage For example, the forward voltage at a density current 2 is reduced from 3.93 V to 3.20 V by increasing the carrier lifetime from 1 µs to density of 100 100 A/cm 2 is 2 is of 100 A/cm reduced from 3.93 V 3.93 to 3.20 V 3.20 by increasing the carrier lifetime from 1from µs to1 10 density of A/cm reduced from V to V by increasing the carrier lifetime µsµs. to 10 µs. µs. However, However, improvement of the the forward forward voltage drop is is marginal marginal when theislifetime lifetime is further further However, improvement of the forward voltage drop is marginal when thewhen lifetime further increased 10 improvement of voltage drop the is increased to 20 µs µs or or longer. longer. to 20 µs orto longer. increased 20
Figure3. 3.Forward Forwardcharacteristics characteristics of SiC pin diodes having an i-layer i-layer thickness ofµm 200(28 µm (28 kV Figure ofof SiC pinpin diodes having an i-layer thickness of 200of kV(28 class). Figure 3. Forward characteristics SiC diodes having an thickness 200 µm kV The carrier lifetime in the i-layer was changed from 1 µs to 100 µs. class). The carrier lifetime in the i-layer was changed from 1 µs to 100 µs. class). The carrier lifetime in the i-layer was changed from 1 µs to 100 µs.
Figure4 44shows shows the forward characteristics of SiC pinhaving diodes havingi-layer different i-layer Figure shows forward characteristics pin diodes having different i-layer Figure thethe forward characteristics of SiC of pinSiC diodes different thicknesses thicknesses from 100 µmto (15 kVµm class) to 600 600 µmHere (65 kV kV class). Here thewas carrier lifetime was fixed at thicknesses 100 µm (15 kV class) to µm (65 the carrier lifetime was fixed at from 100 µmfrom (15 kV class) 600 (65 kV class). theclass). carrierHere lifetime fixed at 50 µs. The diode 50 µs. The diode with an i-layer thickness of 100 µm exhibits nearly ideal characteristics; The 50 µs. diode with of an100 i-layer thickness of 100 µm exhibits nearly characteristics; The with an The i-layer thickness µm exhibits nearly ideal characteristics; Theideal differential on-resistance 22 and differential on-resistance is as as low low as 0.76 0.76 mΩcm and the V forward voltage dropthe is 3.02 3.02 V at at 100 100 2 . Since differential on-resistance is as mΩcm the forward voltage drop is V is as low as 0.76 mΩcm2 and the forward voltage drop is 3.02 at 100 A/cm on-resistance 2 2 A/cm2.. Since the on-resistance on-resistance include substrate resistance of 0.08 0.08 mΩcm2,, the the actual differential 2 , the actual A/cm the aa substrate resistance of mΩcm differential include aSince substrate resistance of include 0.08 mΩcm differential on-resistance of actual the thick i-layer is 2. However, the on-resistance 2 on-resistance of the thick i-layer is estimated to be about 0.68 mΩcm 2 on-resistance the thick i-layer .isHowever, estimatedthe toon-resistance be about 0.68and mΩcm . However, the on-resistance estimated to beofabout 0.68 mΩcm forward voltage drop considerably 22 and and forward forward voltage drop considerably increasefor to the 12.2diode mΩcm and a4.62 4.62 V, respectively, for the 2 andconsiderably and voltage increase to 12.2 mΩcm respectively, for the increase to 12.2 mΩcmdrop 4.62 V, respectively, having 600V, µm-thick i-layer, even diode having a 600 µm-thick i-layer, even though the lifetime is as long as 50 µs. diode having a 600 µm-thick i-layer, even though the lifetime is as long as 50 µs. though the lifetime is as long as 50 µs. SiC pin pin diodes diodes (carrier (carrier lifetime lifetime in in i-layer i-layer == 50 50 μs) μs) SiC
Forward Voltage Voltage (V) (V) Forward Figure 4. 4. Forward Forward characteristics characteristics of of SiC SiC pin pin diodes diodes having having different different i-layer i-layer thicknesses thicknesses from from 100 100 µm µm Figure Figure 4. Forward characteristics of SiC pin diodes having different i-layer thicknesses from 100 µm (15 kV kV class) class) to to 600 600 µm µm (65 (65 kV kV class). class). Here Here the the carrier carrier lifetime lifetime was was fixed fixed at at 50 50 µs. µs. (15 (15 kV class) to 600 µm (65 kV class). Here the carrier lifetime was fixed at 50 µs.
Figure 55 demonstrates demonstrates the the i-layer i-layer thickness thickness dependence dependence of of the the forward forward voltage voltage drop drop at at aa Figure Figure 5 demonstrates the i-layer thickness dependence of the forward voltage drop at a current 2 2 current density density of of 100 100 A/cm A/cm for for SiC SiC pin pin diodes, diodes, where where the the carrier carrier lifetime lifetime in in the the i-layer i-layer was was varied varied current 2 for SiC pin density of 100 A/cm whereofthe lifetime in the i-layer was varied from 1S2). µs from 11 µs µs to 200 200 µs (the (the lifetime lifetime diodes, dependence thecarrier forward voltage drop can be be seen in Figure Figure from to µs dependence of the forward voltage drop can seen in S2). to 200 µs (the lifetime dependence of the forward voltage drop can be seen in Figure S2). The right The right right axis axis indicates indicates the the density density of of on-state on-state loss loss at at 100 100 A/cm A/cm22.. For For aa given given carrier carrier lifetime, lifetime, the the The 2 . For axis indicates the density of on-state loss at 100 A/cm a given carrier lifetime, the forward forward voltage voltage drop drop monotonically monotonically increases increases with with increasing increasing the the i-layer i-layer thickness. thickness. As As in in the the case case forward voltage drop monotonically increasesvoltage with increasing the i-layer thickness. when As in the case of Figure 3, of Figure 3, reduction of forward drop is not very significant the carrier lifetime of Figure 3, reduction of forward voltage drop is not very significant when the carrier lifetime reduction of forward voltage drop is not very significant when the carrier lifetime becomes very becomes very very long long (>50 (>50 µs). µs). To To clarify clarify the the conductivity conductivity modulation modulation of of high-voltage high-voltage SiC SiC pin pin diodes, diodes, becomes the profiles profiles of of excess excess carrier carrier concentration concentration inside inside the the i-layer i-layer at at aa current current density density of of 100 100 A/cm A/cm22 are are the
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Forward ForwardVoltage VoltageDrop Drop(V) (V)
600 600 SiC pin pin diodes diodes SiC 2 = 100 100 A/cm A/cm2 JJ =
300 300 600 600
Thickness of of i-layer i-layer (( μm) μm) Thickness
2 Power PowerLoss LossDensity Density(W/cm (W/cm 2))
long (>50 µs). To clarify the conductivity modulation of high-voltage SiC pin diodes, the profiles of 2 are plotted plottedcarrier in Figure Figure 6. Here Here the the diode with aaat600 600 µm-thick i-layer was considered and in theFigure carrier excess concentration inside the i-layer a current density of 100 A/cm 6. plotted in 6. diode with µm-thick i-layer was considered and the carrier lifetime was varied from 5 µs to 200 µs. The excess carrier concentration is significantly higher than Here thewas diode withfrom a 6005µm-thick considered and the carrier was varied lifetime varied µs to 200i-layer µs. Thewas excess carrier concentration is lifetime significantly higherfrom than 14 cm−3) even for the case of a relatively the background carrier concentration in the the i-layer i-layer (1 ×× 10 1014 −3) even 5the µsbackground to 200 µs. The excess carrier concentration is significantly than carrier carrier concentration in (1 cmhigher for the the background case of a relatively −3 )increasing short lifetime lifetime in (5 the µs),i-layer and it it (1 increases with thecase carrier lifetime. However, the excess excess carrier concentration × 1014 cm even for the of a lifetime. relativelyHowever, short lifetime (5 µs),carrier and it short (5 µs), and increases with increasing the carrier the concentration does not show further increase very much when the carrier lifetime exceeds 50 µs, increases with increasing the carrier lifetime. However, the excess concentration does not50 show concentration does not show further increase very much when carrier the carrier lifetime exceeds µs, which is isincrease the main main reason why improvement of the the forward forward characteristics isthe almost saturated for further very muchwhy when the carrier lifetime exceedscharacteristics 50 µs, which isis mainsaturated reason why which the reason improvement of almost for very long carrier lifetime as shown in Figure 5. This result identifies the major limitation of SiC pin improvement of the forward characteristics is almost for verythe long carrier lifetimeof asSiC shown very long carrier lifetime as shown in Figure 5. Thissaturated result identifies major limitation pin diodes (and other bipolar devices) inmajor termslimitation of the the on-state on-state loss and its its (and origin is discussed discussed in Section Section in Figure 5. This result identifies thein of SiC loss pin diodes other bipolar devices) in diodes (and other bipolar devices) terms of and origin is in 3. terms of the on-state loss and its origin is discussed in Section 3. 3.
2 for SiC pin Figure 5. 5. Dependence Dependenceofof ofthe the forward voltage drop at current density of 100 2A/cm A/cm 2 for forward voltage drop at a at current density of 100of A/cm for SiC pinSiC diodes Figure 5. Dependence the forward voltage drop aa current density 100 pin diodes on the i-layer thickness. Here the carrier lifetime in the i-layer was varied from 1 µs to to 200 µs. on the i-layer Here theHere carrier the i-layer was varied from 1 µsfrom to 2001 µs. The right diodes on thethickness. i-layer thickness. thelifetime carrier in lifetime in the i-layer was varied µs 200 µs. 2 2 The indicates right axis axisthe indicates the density ofloss on-state loss at 100 100 A/cm2.. axis densitythe of density on-stateof at 100loss A/cm . A/cm The right indicates on-state at
Distance from from p-i p-i Junction Junction (( μm) μm) Distance
2 Figure 6. 6. Profiles Profiles of of excess excess carrier carrier concentration concentration inside inside the i-layer i-layer at aa current current density density of of 100 100 A/cm A/cm 2 for2 Figure Figure 6. Profiles of excess carrier concentration inside thethe i-layer at aatcurrent density of 100 A/cm for aa SiC SiC pin pin diode. diode. The The i-layer i-layer thickness thickness is 600 600 µm µm and and the the carrier lifetime lifetime was was varied varied from from 55 µs µs to to afor SiC pin diode. The i-layer thickness is 600isµm and the carriercarrier lifetime was varied from 5 µs to 200 µs. 200 µs. 200 µs.
2.2. Characteristics of Ultrahigh-Voltage SiC Merged Pin-Schottky (MPS) Diodes 2.2. Characteristics Characteristics of of Ultrahigh-Voltage Ultrahigh-Voltage SiC SiC Merged-Pin-Schottky Merged-Pin-Schottky (MPS) (MPS) Diodes Diodes 2.2. As shown in Figure 4, a relatively large knee voltage of about 2.8 V is a major drawback of SiC pin As shown shown in in Figure Figure 4, 4, aa relatively relatively large large knee knee voltage voltage of of about about 2.8 2.8 V V is is aa major major drawback drawback of of SiC SiC As diodes (and other bipolar devices except bipolar junction transistors). The high knee voltage originates pin diodes diodes (and (and other other bipolar bipolar devices devices except except bipolar bipolar junction junction transistors). transistors). The The high high knee knee voltage voltage pin originates from the large built-in potential of a pn junction in SiC and is an inherent characteristic, originates from the large built-in potential of a pn junction in SiC and is an inherent characteristic, which is is caused caused by by the the wide wide bandgap bandgap (3.26 (3.26 eV). eV). Since Since the the forward forward voltage voltage drop drop of of SiC SiC pin pin diodes diodes is is which
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from the large built-in potential of a pn junction in SiC and is an inherent characteristic, which is never smaller than 2.8 V at room temperature, the on-state loss of SiC pin diodes can be larger than caused by the wide bandgap (3.26 eV). Since the forward voltage drop of SiC pin diodes is never that of SiC Schottky barrier diodes in the case of partial load (relatively low current density). To smaller than 2.8 V at room temperature, the on-state loss of SiC pin diodes can be larger than that of overcome this issue, merged-pin-Schottky (MPS) diodes have been proposed in Si  and then this SiC Schottky barrier diodes in the case of partial load (relatively low current density). To overcome this device concept has been employed to reduce the leakage current caused by tunneling as well as to issue, merged pin-Schottky (MPS) diodes have been proposed in Si  and then this device concept enhance the surge current capability of SiC Schottky barrier diodes (SBD) [36,37]. In this study, has been employed to reduce current caused tunneling well as to enhance the or surge high-voltage SiC MPS diodes,the theleakage junction of which wasby fabricated byaseither epitaxial growth ion current capability of SiC Schottky barrier diodes (SBD) [36,37]. In this study, high-voltage SiC implantation, were fabricated and tested to reduce the forward voltage drop at relativelyMPS low diodes, junction of which was fabricated by either epitaxial growth ion implantation, current the density. Figure 7 illustrates the schematic structures of SiC MPSordiodes fabricated inwere this fabricated and tested to reduce the forward voltage drop at relatively low current density. Figure 7 study. The main junction was formed by either (a) epitaxial growth or (b) Al+ ion implantation. Due illustrates the schematic structures of SiC MPS diodes fabricated in this study. The main junction was to the difference in junction formation, the “epitaxial” MPS diode has a mesa structure (Figure 7a), formed by “implanted” either (a) epitaxial growthdoes or (b) Al+ ionstructure implantation. Due to The the difference junction while the MPS diode a planar (Figure 7b). details of in fabrication formation, the “epitaxial” MPS diode has a mesa structure (Figure 7a), while the “implanted” MPS process are described in Section 4. diode does a planar structure (Figure 7b). The details of fabrication process are described in Section 4.
JBS region anode contact
Figure 7. 7. Schematic Schematic structures structures of of SiC SiC MPS MPS diodes diodes fabricated fabricated in in this this study. study. The The main main junction junction was was Figure + ion implantation. The widths of the pin and JBS formed by either (a) epitaxial growth or (b) Al + formed by either (a) epitaxial growth or (b) Al ion implantation. The widths of the pin and JBS (Junction-Barrier Schottky) Schottky) regions regions were were both both 150 150 µm µm and and of of aa stripe-like stripe-like geometry. geometry. The The active active area area (Junction-Barrier −3 2 was 3.6 3.6 × × 10 was 10−3cm cm. 2 .
Figure 8 depicts the forward characteristics for two types of SiC MPS diodes (epitaxial or Figure 8 depicts the forward characteristics for two types of SiC MPS diodes (epitaxial or implanted anode) fabricated in this study. In both diodes, the current starts to flow at a forward implanted anode) fabricated in this study. In both diodes, the current starts to flow at a forward voltage of 0.9 V, which corresponds to the knee voltage of Ti/SiC Schottky barrier diodes. From the voltage of 0.9 V, which corresponds to the knee voltage of Ti/SiC Schottky barrier diodes. From the semilogarithmic plots of forward characteristics, the barrier height of Ti Schottky was estimated as semilogarithmic plots of forward characteristics, the barrier height of Ti Schottky was estimated 1.12 eV, which is consistent with a previous report . The differential on-resistance in this as 1.12 eV, which is consistent with a previous report . The differential on-resistance in this unipolar operation was approximately 85–100 mΩcm22 irrespective of the formation process of the unipolar operation was approximately 85–100 mΩcm irrespective of the formation process of the main junction (anode). This on-resistance is slightly higher than but reasonably agrees with the drift main junction (anode). This on-resistance is slightly higher than but reasonably agrees with the resistance calculated from the structure of a lightly-doped n-type epilayer. The epitaxial MPS diode drift resistance calculated from the structure of a lightly-doped n-type epilayer. The epitaxial MPS exhibited the second knee voltage at about 3.2 V, which means the onset of bipolar operation diode exhibited the second knee voltage at about 3.2 V, which means the onset of bipolar operation caused by minority-carrier injection. Above this second knee voltage, the differential on-resistance caused by minority-carrier injection. Above this second knee voltage, the differential on-resistance was remarkably reduced by the conductivity modulation effect and it reaches 14.9 2mΩcm2 at a was remarkably reduced by the conductivity modulation effect and it reaches 14.9 mΩcm at a current current density of 100 A/cm2. On the other hand, the implanted MPS diode did not show a clear density of 100 A/cm2 . On the other hand, the implanted MPS diode did not show a clear “second “second knee voltage” and the current density gradually increased and the differential knee voltage” and the current density gradually increased and the differential on-resistance slowly2 on-resistance slowly decreased with the current. As a result, the forward voltage drop at 100 A/cm decreased with the current. As a result, the forward voltage drop at 100 A/cm2 was much smaller for was much smaller for the epitaxial MPS diode (4.65 V) than for the implanted MPS diode (7.81 V). the epitaxial MPS diode (4.65 V) than for the implanted MPS diode (7.81 V). Therefore, SiC epitaxial Therefore, SiC epitaxial MPS diodes are promising because the forward voltage drop can be MPS diodes are promising because the forward voltage drop can be remarkably reduced at relatively remarkably reduced at relatively low current density (“partial load” condition) compared with SiC low current density (“partial load” condition) compared with SiC pin diodes and the voltage drop at pin diodes and the voltage drop at high current density is much lower than that of a SiC Schottky high current density is much lower than that of a SiC Schottky barrier diode with the same breakdown barrier diode with the same breakdown voltage, though the forward voltage at high current density is slightly higher than that of a pure SiC pin diode (For comparison, see Figure S3).
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SiC MPS MPS diodes, diodes, 25 25 ooC C SiC
2 Current CurrentDensity Density(A/cm (A/cm ))
voltage, though the forward voltage at high current density is slightly higher than that of a pure SiC Energies 2016, 9, 908 908 of 15 15 pin diode (For comparison, see Figure S3). Energies 2016, 9, 77 of
Epitaxial Anode Anode Epitaxial
100 100 50 50 000 0
Implanted Anode Anode Implanted 14 -3 6x1014 cm cm-3,, 95 95 μm μm Nd == 6x10 N d Ti-MPS; P=S=150 μm Ti-MPS; P=S=150 μm
22 44 66 88 Voltage (V) Forward Voltage (V) Voltage (V) (V) Forward Voltage
Figure 8. 8. Forward Forward characteristics characteristics for for two two types types (epitaxial (epitaxial or or implanted implanted anode) anode) of of SiC SiC MPS MPS diodes diodes Figure Forward characteristics for two types (epitaxial or implanted anode) of SiC Figure 8. MPS diodes fabricated in this study. The widths of the pin-region (P) and JBS region (S) were both 150 µm. fabricated in this study. The widths of the pin-region (P) and JBS region (S) were both 150 µm. fabricated in this study. The widths of the pin-region (P) and JBS region (S) were both 150 µm.
Figure 99 shows shows the the forward forward characteristics characteristics of of (a) (a) epitaxial epitaxial and and (b) (b) implanted implanted MPS MPS diodes diodes at at Figure different temperatures temperatures from from 25 25 to to 300 300 °C. In In both both diodes, diodes, the the current density density in in the the unipolar unipolar ◦ C. In different °C. temperatures from 25 to 300 both diodes, the current current density in the unipolar operation operation at a given forward voltage (below 2.5 V) decreased at elevated temperature, which is operation a givenvoltage forward voltage 2.5 V) decreased elevated temperature, which to is at a given at forward (below 2.5(below V) decreased at elevatedattemperature, which is similar similar to the characteristics of Schottky barrier diodes. In the bipolar operation, the knee voltage similar to the characteristics Schottky barrier In the bipolar operation, the knee voltage the characteristics of Schottkyofbarrier diodes. In diodes. the bipolar operation, the knee voltage gradually gradually decreased with elevating the temperature, leading to higher current density at high high gradually withthe elevating the temperature, leadingcurrent to higher current density at decreased decreased with elevating temperature, leading to higher density at high temperature. temperature. This phenomenon is naturally expected because the built-in potential of a pn junction temperature. This phenomenon is naturally expected because the built-in potential a pn junction This phenomenon is naturally expected because the built-in potential of a pn junctionofdeclines when declines when the temperature is elevated. The increase of current density in the bipolar operation declines when the temperature elevated. The increase ofin current density in the bipolar operation the temperature is elevated. Theisincrease of current density the bipolar operation by increasing the by increasing increasing the the temperature temperature is is more significant significant for for the the implanted implanted MPS MPS diode diode compared compared with with the the by temperature is more significant formore the implanted MPS diode compared with the epitaxial MPS diode, epitaxial MPS diode, and the characteristics of the implanted MPS diode gradually approach those epitaxial MPS diode, and characteristics the implanted diodethose gradually and the characteristics of the implanted MPSofdiode graduallyMPS approach of theapproach epitaxialthose MPS of the epitaxial MPS diode at high temperature. However, the epitaxial MPS diode showed better of the at epitaxial MPS diode at high temperature. However, the showed epitaxialbetter MPS diode showedeven better diode high temperature. However, the epitaxial MPS diode performance at performance even at at 300 300 °C. °C. More More quantitative analyses analyses are are described described in the next section. performance 300 ◦ C. More even quantitative analysesquantitative are described in the next section. in the next section.
2 Current CurrentDensity Density(A/cm (A/cm2))
SiC MPS MPS diode diode SiC
2 Current CurrentDensity Density(A/cm (A/cm ))
150 150 100 100
Epitaxial anode anode Epitaxial 25ooC C 25 o 100ooC C 100 200ooC C 200 300oC C 300
50 50 000 0
22 44 66 88 Forward Voltage (V) Forward Voltage (V) (a) (a)
SiC MPS MPS diode diode SiC
100 100 50 50 000 0
Implanted anode Implanted anode 25ooC C 25 o 100ooC C 100 200oC C 200 o oC 300 300 C
22 44 66 88 Forward Voltage (V) Forward Voltage (V)
Figure 9. 9. Forward Forward characteristics characteristics of (a) (a) epitaxial and and (b) implanted implanted SiC MPS MPS diodes at at different different Figure Figure 9. Forward characteristics of of (a) epitaxial epitaxial and (b) (b) implanted SiC SiC MPS diodes diodes at different temperatures from 25 to 300 °C. temperatures from 25 25 to to 300 300 ◦°C. temperatures from C.
The reverse reverse characteristic characteristic of of aa fabricated fabricated SiC SiC MPS MPS diode diode is is shown shown in in Figure Figure 10, 10, where where the the The current-voltage curve curve of of an an epitaxial epitaxial MPS MPS diode diode was was measured measured by by aa high-voltage high-voltage DC DC sweep. sweep. The The current-voltage diode exhibited a very high breakdown voltage of 11.3 kV, which is about 90% of the ideal diode exhibited a very high breakdown voltage of 11.3 kV, which is about 90% of the ideal parallel-plane breakdown breakdown voltage voltage determined determined by by the the epilayer epilayer structure. structure. As As far far as as the the blocking blocking parallel-plane performance is is concerned, concerned, there there is is not not aa significant significant difference difference between between the the epitaxial epitaxial and and implanted implanted performance
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Current Density (mA/cm2)
The reverse characteristic of a fabricated SiC MPS diode is shown in Figure 10, where the current-voltage curve of an epitaxial MPS diode was measured by a high-voltage DC sweep. The diode exhibited a very high breakdown voltage of 11.3 kV, which is about 90% of the ideal parallel-plane breakdown determined by the epilayer structure. As far as the blocking performance is Energies 2016, 9,voltage 908 8 of 15 concerned, there is not a significant difference between the epitaxial and implanted MPS diodes, MPS diodes, because theoccurs breakdown in thefor JTE forfabricated the diodesinfabricated thisi-layer study because the breakdown in theoccurs JTE region theregion diodes this studyin(The (The i-layer structure and theare JTEalmost regionidentical are almost for both the diodes). structure and the JTE region for identical both the diodes).
SiC MPS diode, 25 oC
i-layer: 95 µm, 6×1014 cm-3 Type A (6x10 14 cm-3, 95 μm) 11.3 Hexagon Pattern, P = S =V100 B =μm
Figure Figure 10. 10. Reverse Reverse characteristic characteristic of of aa SiC SiC epitaxial epitaxial MPS MPS diode fabricated in this study.
3. 3. Discussion Discussion 3.1. Limitation of Ultrahigh-Voltage SiC Bipolar Devices As shown shown in inFigures Figures5 5and and6, 6, major limitation of ultrahigh-voltage SiC diodes pin diodes the thethe major limitation of ultrahigh-voltage SiC pin is theislarge large forward drop and high differential on-resistance i-layer thickness becomes forward voltagevoltage drop and high differential on-resistance when thewhen i-layerthe thickness becomes very thick veryexample, thick (for600 example, µm (65The kV class)). forward characteristics are not improved even if (for µm (65 600 kV class)). forwardThe characteristics are not improved even if an extremely an extremely long carrier 100 µs Therefore, is assumed.recombination Therefore, recombination paths of excess long carrier lifetime over lifetime 100 µs isover assumed. paths of excess carriers are carriers analyzed andindiscussed in this subsection. It noted shouldthat be noted that the carrier lifetime analyzedare and discussed this subsection. It should be the carrier lifetime employed employed in the simulation so-called Shockley-Read-Hall (SRH)(τlifetime (τSRH) determined by in the simulation is so-calledis Shockley-Read-Hall (SRH) lifetime by indirect SRH ) determined indirect recombination via deep In a semiconductor, (direct) and Auger recombination via deep levels. In a levels. semiconductor, band-to-band band-to-band (direct) and Auger recombinations recombinations alsothese take recombination place, and these recombination be especially taken intowhen account also take place, and processes must be processes taken into must account the especially when the excess is carrier concentration is highof. Thus, theconcentration decay of excess carrier excess carrier concentration high . Thus, the decay excess carrier in SiC was concentration in SiC was theoretically analyzed. theoretically analyzed. Recombination Recombination of of excess carriers carriers in n-type semiconductor semiconductor can be expressed by the following differential equation :
Δn 2 2 2 d∆ndΔn = ∆n 2 2n Δn + Δn )Δn 2 C ( p + 2 p2 Δn + Δn )Δn 2 + Δn)Δn − Cn (nn(0n20 + 0 +pp 0 0 ∆n + ∆n )− 0 p ( p0 0+ 2p0 ∆n + ∆n ) ∆n(1) =t − − τ − B−(Bn(0n+ + 2n ∆np− C (1) 00+ ∆n ) ∆n − C d dt τSRHSRH here, Δn, thethe excess carrier concentration, the the equilibrium electron concentration (1 × ∆n, nn00, ,and andp0pare excess carrier concentration, equilibrium electron concentration 0 are −3 in the 1014 case),case), and and the the equilibrium hole concentration, (1 × cm 1014 cm−3 inpresent the present equilibrium hole concentration,respectively. respectively.Regarding Regarding the carrier recombination, recombination,SRH SRHrecombination recombination(τ(τ SRH band-to-band(direct) (direct) recombination (coefficient: ),),band-to-band recombination (coefficient: B), SRH B), Auger recombination (coefficients: p) were considered. The SRH lifetime was assumed andand Auger recombination (coefficients: Cn ,CCnp, )Cwere considered. The SRH lifetime was assumed to to be independent of the excess carrier concentration. Table 1 summarizes these parameters be independent of the excess carrier concentration. Table 1 summarizes these parameters employed employed in this analysis . Both the SRH lifetime the carrier initial concentration excess carrier were concentration in this analysis . Both the SRH lifetime and the initialand excess varied in were varied wide and of thethe time decay of theconcentration excess carrierwas concentration wide ranges,in and theranges, time decay excess carrier calculated. was calculated. Table 1. Carrier recombination coefficients used in the calculation .
Parameter B Cn Cp
Value 1.5 × 10−12 5.0 × 10−31 2.0 × 10−31
Unit cm3/s cm6/s cm6/s
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Table 1. Carrier recombination coefficients used in the calculation . Parameter
cm3 /s cm6 /s cm6 /s
1.5 × 5.0 × 10−31 2.0 × 10−31
B Cn Cp
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Δn: excess carrier concentration (initial)
Δn = 1x10
τSRH = 100 μs
Δn = 1x10
1014 1013 0
Δn = 1x10
Δn = 1x10
Δn = 1x10
Effective Lifetime ( μs)
Excess Carrier Concentration (cm )
Figure11a 11ashows showsthe thedecay decay curves excess carrier concentration in n-type SiC having Figure curves of of thethe excess carrier concentration in n-type SiC having a longa long SRH lifetime of 100 µs. A relatively fast decay is observed when the initial excess carrier SRH lifetime of 100 µs. A relatively fast decay is observed when the initial excess carrier concentration concentration is decay high. For each decay curve, slope the differential slopeand wasdefined calculated andeffective definedcarrier as the is high. For each curve, the differential was calculated as the effective carrier lifetime (τ eff). Figure 11b depicts the effective carrier lifetime as a function of time lifetime (τeff ). Figure 11b depicts the effective carrier lifetime as a function of time for several different 14 −3 −3 to 1 × 1018 cm−3. for several initial excess carrier initial excessdifferent carrier concentrations from 1concentrations × 1014 cm−3 tofrom 1 × 110×1810cmcm .
Δn: excess carrier concentration (initial)
τSRH = 100 μs
Δn = 1x10 14 cm-3 Δn = 1x10 15 cm-3
Δn = 1x10 16 cm-3
Δn = 1x10 17 cm-3
Δn = 1x10 18 cm-3
Figure 11. 11. (a) (a) Decay Decay curves curves of of the the excess excess carrier carrier concentration concentration in in n-type n-type SiC SiC having having aa long long SRH SRH Figure 14 −3 cmcmto lifetime of of 100 100 µs µs for for several several different different initial −3 1to× lifetime initial excess excess carrier carrier concentrations concentrations from from11××101014 18 cm −3; (b) 10 Effective carrier lifetime as a function of time for several different initial excess carrier 18 − 3 1 × 10 cm ; (b) Effective carrier lifetime as a function of time for several different initial excess concentrations. carrier concentrations.
Effective Carrier Lifetime (μs)
When the excess carrier concentration is relatively low, 1 × 1015 cm−3, the effective lifetime is When the excess carrier concentration is relatively low, 1 × 1015 cm−3 , the effective lifetime is 90–92 µs, close to the SRH lifetime (100 µs). However, the effective lifetime at the initial decay 90–92 µs, close to the SRH lifetime (100 µs). However, the effective lifetime at the initial decay drops drops drastically with increasing the excess carrier concentration and it reaches 8.5 µs17at 1–3× 1017 drastically with increasing the excess carrier concentration and it reaches 8.5 µs at 1 × 10 cm and cm–3 and 0.6 µs at 1 × 1018 cm–3. Figure 12 plots the effective carrier lifetime as a function of the 0.6 µs at 1 × 1018 cm–3 . Figure 12 plots the effective carrier lifetime as a function of the excess carrier excess carrier concentration calculated in this study. concentration calculated in this study. Results for different SRH lifetimes from 2 µs to 200 µs are shown, and the lifetimes limited purely n0 = 1x10 14 cm-3 by band-to-band (direct) recombination and Auger recombination are indicated by dashed and dotted τAuger lines, respectively. It is well known that direct and Auger recombinations become significant when τdirect 200 μs the carrier concentration is very high. As shown in Figure 12, however, the direct recombination μs 102 100 severely affects the effective carrier lifetime even at a relatively low excess carrier concentration of 16 − 3 50 μs 1 × 10 cm when the SRH lifetime is very long (>50 µs). In such a case, the effective carrier lifetime is mainly limited not by SRH recombination but by direct recombination. This situation is more 20 μs pronounced as the SRH lifetime becomes longer. The ambipolar diffusion constant (Da ) and ambipolar 10 μs 1 diffusion length (La ) of excess 10 carriers are given by the following equations: 5 μs
= a2= μs τSRH D
n+p , n/Dn + p/Dp
Excess Carrier Concentration (cm ) Figure 12. Effective carrier lifetime in SiC as a function of the excess carrier concentration.
Δn = 1x10
Δn = 1x10
Δn = 1x10
Excess Carrier Con
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Δn = 1x10 16 cm-3
Δn = 1x10 17 cm-3
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Δn = 1x10 18 cm-3
L100 a =
Da τeff .
where n and p are the electron concentration (n = n0 + ∆n) and hole concentration (p = p0 + ∆n), (b) respectively. Dn and D(a) p are the diffusion constants of electrons and holes, respectively. Based on Figures 11 and 12,Decay the ambipolar be estimated to be 210 at an excess carrier Figure 11. (a) curves of diffusion the excesslength carrier can concentration in n-type SiC µm having a long SRH 15 cm−3 , 140 µm at 1 × 1016 cm−3 , and decreased to 45 µm at 1 × 1017 cm−3 concentration of 1 × 10 14 −3 lifetime of 100 µs for several different initial excess carrier concentrations from 1 × 10 cm to 1 × 18 cm −3; (b) Effective for a 10 SRH lifetime of 100 carrier µs. Since the excess carrier theinitial i-layer of acarrier pin diode lifetime as a function of concentration time for severalinside different excess 16 is in concentrations. the 10 –1017 cm−3 range at high current density (Figure 6), the ambipolar diffusion length at such current density is almost saturated at about 50–150 µm, even if all the defects can be eliminated. When the ambipolar excess carrier concentration is relatively low, × 1015 cm−3long , the to effective is The estimated diffusion length (about 100 µm) is 1sufficiently highly lifetime modulate 90–92 µs, close i-layer to the SRH lifetime (100 µs).forHowever, the effective at the initial decay a 200 µm-thick  but not enough a 600 µm-thick i-layer.lifetime As discussed above, this drops drastically with increasing the excess carrier concentration andwhich it reaches 8.5 µs atproperty 1 × 1017 limitation originates from band-to-band (direct) recombination in SiC, is an intrinsic –3 18 –3 cmtheand 0.6 µs Thus, at 1 × band-to-band 10 cm . Figure 12 plots theiseffective carrier as a function of the of material. recombination the major factorlifetime which inherently limits the excess carrierof concentration calculated in this study. performance ultrahigh-voltage SiC pin diodes and possibly other bipolar devices.
Effective Carrier Lifetime (μs)
n0 = 1x10 14 cm-3
102 100 μs 50 μs 20 μs
10 μs 5 μs τSRH = 2 μs
Excess Carrier Concentration (cm ) Figure 12. Effective carrier lifetime in SiC as a function of the excess carrier concentration.
Results for different SRH lifetimes from 2 µs to 200 µs are shown, and the lifetimes limited As a future work, it is important to predict the limitation of SiC pin diodes in the wide temperature purely by band-to-band (direct) recombination and Auger recombination are indicated by dashed range, especially at high temperature. To this end, basic studies on the temperature dependence of and dotted lines, respectively. It is well known that direct and Auger recombinations become carrier recombination coefficients (both band-to-band and Auger recombinations) are essential. 3.2. Comparison of SiC MPS Diodes Formed by Epitaxial Growth and Ion Implantation Based on the results shown in Figure 9, the differential on-resistances in the (a) unipolar and (b) bipolar operations were determined at each temperature and are plotted in Figure 13. Here the differential on-resistance was defined at a forward voltage of 2 V for the unipolar operation and at a current density of 100 A/cm2 for the bipolar operation. As shown in Figure 13a, the unipolar on-resistance is almost independent of the type of MPS diodes, namely epitaxial or implanted junctions, though the epitaxial MPS diode exhibited slightly lower on-resistance. Since the i-layer (lightly-doped n-type epilayer) is very thick (95 µm), the impact of implantation-induced damage, which is created inside the implanted p-type anode as well as near the implantation tail , is very small, as far as the majority carrier conduction is concerned. Thus, the differential on-resistance in the unipolar operation should be determined simply by the bulk resistance of the very thick n-type epilayer. This is the reason why the difference in the unipolar on-resistance between epitaxial and implanted MPS diodes was very small. The increase of the unipolar on-resistance at elevated temperature is ascribed to the decrease of electron mobility due to enhanced phonon scattering. The temperature dependence of the unipolar
SiC MPS diode Unipolar resistance (@ 2 V)
300 Epitaxial anode
200 100 0
100 200 300 o Temperature ( C) (a)
Differential RON (mΩcm2)
Differential RON (mΩcm2)
consistent with a report on 4.5 kV SiC pin diodes formed by implantation . When the carrier lifetime in the i-layer is long enough, the differential on-resistance of a pin diode is low and its temperature dependence is small , which is the major reason why the bipolar on-resistance of the epitaxial MPS diode in this study was much lower and showed the small temperature dependence. However, the increase of carrier lifetime at elevated temperature significantly contributes to the Energies 2016, 9, 908 11 of 15 conductivity modulation and to decrease of the differential on-resistance when the carrier lifetime is short, because the conductivity of more and more region inside the i-layer is modulated with increasing the temperature. Thisis trend is more for the bipolar on-resistance of the on-resistance shown in Figure 13a consistent with significant the temperature dependence of electron mobility implanted MPS diodes as shown in lightly-doped n-type SiC . in Figure 13b.
SiC MPS diode 2
Bipolar resistance (@ 100 A/cm )
40 30 Implanted anode
20 10 0
100 200 300 Temperature (oC) (b)
Figure13. 13.Differential Differentialon-resistances on-resistances of fabricated diodes (a) unipolar (b) Figure of fabricated SiC SiC MPSMPS diodes in thein (a)the unipolar and (b) and bipolar bipolar operations at different temperatures. The results for epitaxial and implanted MPS diodes are operations at different temperatures. The results for epitaxial and implanted MPS diodes are shown. shown.
In contrast, a definite difference was observed for the differential on-resistance in the bipolar Since a potential of unipolar/bipolar hybrid operation could be demonstrated with operation. As shown in Figure 13b, the bipolar on-resistance is reduced at high temperature, which high-voltage SiC diodes in this study, monolithic integration of SiC vertical power MOSFET and can be attributed to the increased carrier lifetime and the enhanced injection efficiency of holes from IGBT is an attractive challenge. Since MOSFETs exhibit no knee voltage in the on-state the p-type anode to the lightly-doped n-type epilayer. The latter is naturally expected owing to characteristics, remarkable reduction of on-state loss can be expected by the hybrid operation of the increased ionization of Al acceptors at elevated temperature, which are energetically deep in MOSFET and IGBT. However, the unipolar on-resistance becomes unacceptably high for extremely SiC. Since a high density of deep levels and extended defects are generated inside the p-type anode high-voltage devices and at high temperature. Thus, such hybrid devices must have severe and near the main junction by ion implantation and subsequent annealing, the carrier lifetime is limitations when the breakdown voltage is higher than 20 kV or the operation temperature exceeds substantially shortened in these regions. The short charrier lifetime near the main junction kills the 200 °C. The proposed hybrid MOSFET-IGBT devices will be promising for 8–15 kV switching injection efficiency, leading to considerable decrease of the conductivity modulation effect and thereby devices. to the increased differential on-resistance of the implanted MPS diode. This result is consistent with a report on 4.5 and kV SiC pin diodes formed by implantation . When the carrier lifetime in the i-layer 4. Materials Methods is long enough, the differential on-resistance of a pin diode is low and its temperature dependence For, simulation pin reason diodes,why a commercial simulator, a Synopsis is small which is of theSiC major the bipolardevice on-resistance of the epitaxialTCAD-Sentaurs MPS diode in device , was used. The standard set of physical properties of 4H-SiC embedded theincrease simulator this study was much lower and showed the small temperature dependence. However,inthe of was employed. In the simulation, incomplete ionization of dopants (nitrogen donors and aluminum carrier lifetime at elevated temperature significantly contributes to the conductivity modulation and to acceptors) a bandgap on-resistance narrowing effect heavily-doped taken account. of In decrease of and the differential whenfor the carrier lifetimematerials is short, were because the into conductivity simulation of SiC pin diodes, accurate for the with carrier statistics the andtemperature. basic transport properties more and more region inside the i-layermodels is modulated increasing This trend is of SiC are included. However, the injection-level dependence of SRH carrier lifetime and more significant for the bipolar on-resistance of the implanted MPS diodes as shown in Figure 13b. carrier-carrier scattering are not known in SiC. Therefore, we assumed an injection-level Since a potential of unipolar/bipolar hybrid operation could be demonstrated with high-voltage independent SRH lifetime and integration neglected carrier-carrier scattering. These induce SiC diodes in this study, monolithic of SiC vertical power MOSFET andassumptions IGBT is an attractive certain inaccuracy in the present simulation. Establishment of these physical models is an important challenge. Since MOSFETs exhibit no knee voltage in the on-state characteristics, remarkable reduction subject of study in be theexpected future. by the hybrid operation of MOSFET and IGBT. However, the unipolar of on-state loss can on-resistance becomes unacceptably high for extremely high-voltage devices and at high temperature. Thus, such hybrid devices must have severe limitations when the breakdown voltage is higher than 20 kV or the operation temperature exceeds 200 ◦ C. The proposed hybrid MOSFET-IGBT devices will be promising for 8–15 kV switching devices.
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4. Materials and Methods For simulation of SiC pin diodes, a commercial device simulator, a Synopsis TCAD-Sentaurs device , was used. The standard set of physical properties of 4H-SiC embedded in the simulator was employed. In the simulation, incomplete ionization of dopants (nitrogen donors and aluminum acceptors) and a bandgap narrowing effect for heavily-doped materials were taken into account. In simulation of SiC pin diodes, accurate models for the carrier statistics and basic transport properties of SiC are included. However, the injection-level dependence of SRH carrier lifetime and carrier-carrier scattering are not known in SiC. Therefore, we assumed an injection-level independent SRH lifetime and neglected carrier-carrier scattering. These assumptions induce certain inaccuracy in the present simulation. Establishment of these physical models is an important subject of study in the future. In the simulation, a heavily-doped n-type SiC substrate was assumed beneath the active pin diode. The thickness and resistivity of the substrate were 50 µm and 0.016 Ωcm, respectively, where the substrate thinning was considered because the active region (mainly i-layer) is very thick. The contact resistivity for the anode and cathode was neglected for simplicity. For calculating the ideal breakdown voltage, a latest set of impact ionization coefficients  was employed. The diode temperature was fixed at room temperature (300 K) in all the simulations. In fabrication of SiC MPS diodes, a 95 µm-thick n-type epilayer grown on a heavily doped n-type substrate was used as a starting material. The n-type epilayer was intentionally doped with nitrogen to 6 × 1014 cm−3 . Before formation of the p-type anode, thermal oxidation at 1400 ◦ C for 48 h was performed to enhance the carrier lifetime [30,32]. The low-injection carrier lifetime can be increased from 2 µs to about 30 µs by this process. After removal of the formed oxide, the anode region was formed by either epitaxial growth or Al+ implantation. In the case of epitaxial growth, the anode consists of two p-type layers, 2.0 µm-thick layer doped to 1 × 1019 cm−3 and 0.2 µm-thick layer doped to 1 × 1020 cm−3 (top). After epitaxial growth of the p-type anode, sloped mesas were formed for device isolation by fluorine-based reactive ion etching using SiO2 as an etching mask . In the case of implanted junctions, a 0.8 µm-deep box profile with the dopant concentration of 2 × 1018 cm−3 was formed by Al+ implantation (implant energy: 270–700 keV). Higher-dose Al+ implantation was carried out to form a 0.2 µm-thick heavily-doped surface layer (dopant concentration: 2 × 1020 cm−3 ) for improving the contact resistivity. Along the periphery of a diode, a 500 µm-long space-modulated junction termination extension (JTE) structure was formed by Al+ implantation . All the implanted dopants were activated by annealing at 1650 ◦ C for 20 min with a carbon cap . The Ohmic and Schottky contacts on the anode were Ti/Al sintered at 1000 ◦ C and Ti sintered at 300 ◦ C, respectively. The specific contact resistance of the Ti/Al ohmic contact was determined from a transfer length method. The contact resistance was 1.4 × 10−5 Ωcm2 on the epitaxial anode and 2.2 × 10−4 Ωcm2 on the implanted anode. The cathode contact was Ni sintered at 1000 ◦ C. Note that the Schottky area was actually of junction-barrier Schottky (JBS) structure, which was formed by Al+ implantation (stripe geometry), to reduce the leakage current. The surface was passivated with a thermally grown oxide and a 7 µm-thick polyimide. The widths of the pin-diode and Schottky-areas were both 150 µm, and the active area excluding the JTE region was 3.6 × 10−3 cm2 . This anode geometry and widths were optimized to suppress a “snapback” phenomenon , which is often observed in unipolar/bipolar hybrid operation. The forward characteristics of fabricated diodes were tested with a semiconductor parameter analyzer (4200, Keithley, Solon, OH, USA) without packaging. The high-voltage reverse characteristics were measured with a custom-made high-voltage DC tester (Keithley, Solon, OH, USA) by immersing a diode in Fluorinart to avoid air sparking. The diode temperature was changed by using a heating stage. 5. Conclusions Performance and potential limitation of SiC bipolar diodes were investigated by simulation and experiments. SiC pin diodes and other bipolar devices are especially attractive in the blocking voltage range of 10–30 kV, owing to the conductivity modulation effect. When the i-layer becomes
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very thick to further increase the blocking voltage, however, the conductivity of such a thick i-layer is not fully modulated because of a limited carrier lifetime, leading to higher differential on-resistance. The intrinsic limitation of effective carrier lifetime in SiC originates from band-to-band recombination at the high-injection level. This limitation especially affects the device performance when the blocking voltage exceeds 50–60 kV. One weak point of SiC pin diodes and other bipolar devices except bipolar junction transistors is the high knee voltage in the on-state. To overcome this issue, unipolar/bipolar hybrid operation is promising, because unipolar devices can carry the current at lower voltage drop when the current density is relatively low. As the first step, an 11 kV SiC MPS diode with an epitaxial anode was experimentally demonstrated. The proposed hybrid devices are attractive in the blocking voltage range of 8–15 kV. However, the advantage of these hybrid devices is almost lost when the blocking voltage is higher than 20 kV especially in the high-temperature operation, where the unipolar on-resistance becomes unacceptably high. Supplementary Materials: The following are available online at www.mdpi.com/1996-1073/9/11/908/s1, Figure S1: Forward voltage drop at 100 A/cm2 vs. doping concentration in the p-anode simulated with and without the bandgap narrowing effect in SiC pin diodes. The i-layer thickness is 200 µm. The optimum doping concentration can be determined as 5 × 1020 cm−3 , taking account of the bandgap narrowing effect, Figure S2: Forward voltage drop at 100 A/cm2 vs. carrier lifetime in the i-layer simulated for SiC pin diodes having different i-layer thicknesses. In each case, the forward voltage drop shows saturation when the carrier lifetime is long enough. The saturated voltage drop significantly increases with increasing the i-layer thickness, Figure S3: Forward characteristics of the epitaxial MPS diode, pure pin diode, and pure JBS diode fabricated in this study. The proposed MPS diode exhibited a lower voltage drop than the pin diode at low current density and did than the JBS diode at high current density, Figure S4: Forward characteristics of SiC pin diodes having an i-layer thickness of 200 µm (28 kV class). The carrier lifetime in the i-layer was changed from 1 µs to 100 µs. Acknowledgments: This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Next-generation power electronics/Consistent R&D of next-generation SiC power electronics” (funding agency: NEDO). Author Contributions: T.K. conceived and designed the simulation and experiments. K.Y. performed the simulation and analyzed the data. H.N. carried out the simulation, fabrication of diodes, and measurements. J.S. extensively supported the simulation and contributed to the data analyses. T.K. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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