4H-Sic Drift Step Recovery Diode with Super Junction for Hard Recovery

materials

Article 4H-SiC Drift Step Recovery Diode with Super Junction for Hard Recovery

Xiaoxue Yan , Lin Liang *, Xinyuan Huang, Heqing Zhong and Zewei Yang

State Key Laboratory of Advanced Electromagnetic Engineering and Technology, School of Electrical and Electronic Engineering, Huazhong University of Science and Technology, Wuhan 430074, China; [email protected] (X.Y.); [email protected] (X.H.); [email protected] (H.Z.); [email protected] (Z.Y.) * Correspondence: [email protected]

Abstract: Silicon carbide (SiC) drift step recovery diode (DSRD) is a kind of opening-type pulsed power device with wide bandgap material. The super junction (SJ) structure is introduced in the SiC DSRD for the first time in this paper, in order to increase the hardness of the recovery process, and improve the blocking capability at the same time. The device model of the SJ SiC DSRD is established and its breakdown principle is verified. The effects of various structure parameters including the concentration, the thickness, and the width of the SJ layer on the electrical characteristics of the SJ SiC DSRD are discussed. The characteristics of the SJ SiC DSRD and the conventional SiC DSRD are compared. The results show that the breakdown voltage of the SJ SiC DSRD is 28% higher than that of the conventional SiC DSRD, and the dv/dt output by the circuit based on SJ SiC DSRD is 31% higher than that of conventional SiC DSRD. It is verified that the SJ SiC DSRD can achieve higher voltage, higher cut-off current and harder recovery characteristics than the conventional SiC DSRD, so as to output a higher dv/dt voltage on the load. Keywords: silicon carbide (SiC); drift step recovery diode (DSRD); super junction (SJ); hard recovery; Citation: Yan, X.; Liang, L.; Huang, pulsed power switch; diode X.; Zhong, H.; Yang, Z. 4H-SiC Drift Step Recovery Diode with Super Junction for Hard Recovery. Materials 2021, 14, 684. https://doi.org/ 1. Introduction 10.3390/ma14030684 In the early 1980s [1], the Ioffe Physical Technical Institute proposed a nanosecond switch drift step recovery diode (DSRD). It is a kind of pulse power switch based on the Academic Editor: Sergey Kukushkin plasma principle, which can operate in the inductive energy storage circuit. The DSRD- Received: 31 December 2020 based pulse generator is widely used in ground penetrating radar [2], electromagnetic Accepted: 30 January 2021 Published: 2 February 2021 pulse radar [3,4], accelerator [5], igniter [6], ultra-high speed broadband beam deflection [7], etc. In order to improve its operating voltage and current and switching speed, DSRDs

Publisher’s Note: MDPI stays neutral based on new materials such as silicon carbide (SiC) and gallium arsenide (GaAs) were with regard to jurisdictional claims in prepared. GaAs can produce DSRD with a faster switching speed than SiC because its published maps and institutional affil- electron mobility is about 10 times that of SiC. The reference [8] reported a GaAs DSRD iations. with a blocking voltage of 150 V and a switching speed of 100 ps. However, SiC has more advantages than GaAs at higher voltage levels, because its bandgap is more than 2 times that of GaAs, and its critical breakdown electric field is 7–8 times that of GaAs. DSRD based on SiC was proposed in 2002 [9]. In terms of electrical characteristics, the switching speed of SiC DSRD can reach 2–4 times of Si DSRD due to its high critical breakdown Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. electric field and fast saturated electron drift velocity, and the breakdown voltage can reach This article is an open access article 10 times of Si DSRD under the same drift layer thickness. −3 2 distributed under the terms and In 2003 [10], I. V. Grekhov et al. reported a SiC DSRD with an area of 2.2 × 10 cm , conditions of the Creative Commons which can output a voltage with a rise time of about 4 ns and a pulse amplitude of 400 V. −3 2 Attribution (CC BY) license (https:// In 2012 [11], P. A. Ivanov et al. fabricated devices with an area of 3.9 × 10 cm which can creativecommons.org/licenses/by/ output a voltage with a rise time of 2 ns. In 2015 [12], A.V. Afanasyev et al. reported that a −2 2 4.0/). SiC DSRD with a breakdown voltage of 1 kV and an active area of 1 × 10 cm , which

Materials 2021, 14, 684. https://doi.org/10.3390/ma14030684 https://www.mdpi.com/journal/materials Materials 2021, 14, 684 2 of 10

can output a pulse voltage with an amplitude of 1125 V and a rise time of fewer than 2 ns. In 2020 [13], Ruize Sun et al. output a voltage pulse with a leading edge rise time of 1.8 ns (20–90%) on the load. The die area of SiC DSRD needs to be further increased to adapt to higher current applications. However, the increase of area leads to the increase of junction capacitance, which leads to the decrease of switching speed. Therefore, it is very important to obtain a faster switching speed and a larger operating current in the same area without reducing breakdown voltage. In order to solve the above problem, in addition to choosing materials with better physical properties, such as SiC, the device structure can also be improved. Super junction (SJ) technology is a kind of technology that uses alternating p-type doping region and n-type doping region structure to realize charge compensation and act as voltage withstand layer, so as to obtain a low specific on-resistance and high breakdown voltage at the same time. In 1984 [14], D. J. Coe proposed the method of using alternate pn junction instead of low doped drift layer as voltage withstand layer in laterally diffused metal– oxide–semiconductor field-effect transistor (LDMOSFET) for the first time. In 1991 [15], Xingbi Chen proposed the idea of using multiple pn junction structures as drift layers in longitudinal power devices (especially longitudinal MOSFETs), which is called the Composite Buffer layer. In 1997 [16], T. Fujihira formally proposed the concept of SJ structure, which is a summary of the above ideas. SJ is often used in unipolar devices, such as MOSFET [15,17,18], schottky barrier diode (SBD) [19], and junction field-effect transistor (JFET) [20], to coordinate the contradiction between on-state resistance and breakdown voltage. However, at the same time, it is found that the recovery characteristics of MOSFET bulk diodes become worse after using SJ, that is, a higher current peak value appears during reverse recovery [21]. However, this phenomenon may be beneficial to the dynamic characteristics of DSRD. On one hand, as a kind of current-breaker, a larger reverse current peak on DSRD means that its working current (i.e., cut-off current) can be larger. On the other hand, in the pulse generator based on DSRD, the load is often in parallel with DSRD. The shutdown process is essentially a process of current switching from DSRD to load. Ideally, the greater the reverse current of DSRD in the shutdown process, the greater the current on the load and the greater the load voltage (Vpeak). In addition, it is worth noting that the rising speed of the voltage on the load largely depends on the current turn-off speed of DSRD, that is, the faster the reverse current of DSRD drops, the shorter the rising time (tr) of the load voltage and the greater the leading edge rise rate (dv/dt). In this paper, the SJ structure is introduced in the SiC DSRD for the first time to increase the hardness of the recovery process, and improve the blocking capability at the same time. The device model of the SJ SiC DSRD is established by TCAD. Based on this model, the rectangular electric field distribution of SJ SiC DSRD during the reverse breakdown is analyzed. The dynamic characteristics of the SJ SiC DSRD and the conventional SiC DSRD under different structure parameters and different external circuit conditions are compared.

2. Methods and Results DSRD is a kind of current breaker based on the drift step recovery effect. There are two common structures: p+-p-n+ and p+-p-n-n+. For high-voltage DSRD, p+-p-n-n+ has more advantages than p+-p-n+ because it does not need mesa deep etching and the saturated drift velocity of n-type drift layer is higher [22]. In this paper, the p+-p-n-n+ four-layer structure is selected as the basic structure of 4H-SiC DSRD, as shown in Figure1a, in which the moderately doped p layer is used as the charge storage layer, and the lightly doped n layer is used as the voltage withstand layer. SJ is a structure composed of p-type semiconductor and n-type semiconductor alternately to replace the original voltage withstand region. A longitudinal structure of SiC DSRD with SJ structure is shown in Figure1b. The structure models of conventional SiC DSRD and SJ SiC DSRD are established by simulation, and the parameters of SiC DSRD and SJ SiC DSRD are identical, except for the structure of the drift layer: the concentration of p+ emitter layer is 1 × 1019 cm−3, and the thickness is 1 µm; the Materials 2021, 14, x FOR PEER REVIEW 3 of 12

Materials 2021, 14, 684 3 of 10 simulation, and the parameters of SiC DSRD and SJ SiC DSRD are identical, except for the structure of the drift layer: the concentration of p+ emitter layer is 1 × 1019 cm−3, and the concentrationthickness is 1 of μ p-basem; the layerconcentration is 8 × 1016 ofcm p-base−3, and layer the is thickness 8 × 1016 cm is 2−3µ, m;and the the concentration thickness is 2 + + 18 −3 18 −3 ofμ nm;substrate the concentration layer is 5 of× n10 substratecm , and layer the is thickness5 × 10 cm is 350, andµ m;the the thickness concentration is 350 μ (m;Nnb the) ofconcentration drift region (referring (Nnb) of drift to n region base layer (referri of conventionalng to n base SiClayer DSRD of conventional or p-pillar andSiC n-pillarDSRD or 16 −3 16 −3 ofp-pillar SJ SiC DSRD)and n-pillar is 1 × of10 SJ SiCcm DSRD), and is the 1 × thickness 10 cm , ( Tandnb) the is 10 thicknessµm. The (T widthsnb) is 10 ( Wμm.SJ) The of p-pillarwidths and (WSJ n-pillar) of p-pillar of DSRD and aren-pillar equal, ofwhich DSRD are are 2 equal,µm. Each which layer are is 2 uniformly μm. Each doped.layer is Theuniformly front side doped. is the anodeThe front and side the backis the side anode is the an cathode.d the back It isside also is necessary the cathode. to design It is also a terminationnecessary to structure design ina termination the actual preparation, structure in and the siliconactual dioxidepreparation, or silicon and nitridesilicon candioxide be usedor silicon as the nitride terminal can passivation be used as layer the terminal [23]. passivation layer [23].

Anode Anode

p+ 1 μm, 1× 1019 cm− 3 p+

p 2 μm, 8× 1016 cm− 3 p

10 μm, 1× 1016 cm− 3 n p n n0 p n

WSJ WSJ

18− 3 + 350 μm, 5× 10 cm + n n

Cathode Cathode

(a) (b)

FigureFigure 1. 1.Proﬁles Profiles of of (a ()a conventional) conventional SiC SiC DSRD DSRD and and (b ()b SJ) SJ SiC SiC DSRD. DSRD.

InIn order order to to analyzeanalyze thethe breakdown mechanism mechanism of of SJ SJ SiC SiC DSRD, DSRD, the the electric electric field field dis- distributiontribution of of conventional conventional SiC SiC DSRD DSRD and and SJ SJ SiC SiC DSRD DSRD during during reverse reverse breakdown breakdown was was ex- explored.plored. According According to to the the reference reference [24], [24 ],band bandgapgap narrowing narrowing in inheavily heavily doped doped emitters emitters and andincomplete incomplete ionization ionization of impurities of impurities have have a great a great influence influence on onthethe switching switching characteristics character- isticsof SiC of SiCDSRD. DSRD. Therefore, Therefore, the theabove above physical physical effects effects are are considered considered in inthe the simulation. simulation. In Inaddition, addition, high high field field saturation, saturation, Auger Auger recombination recombination and and Shockley-Read-Hall Shockley-Read-Hall recombina- recombi- nationtion [25] [25 ]are are considered considered in in the the model, model, too. too. Th Thee electrical electrical parameters parameters and and characteristics characteristics of ofconventional conventional SiC SiC DSRD DSRD and and SJ SiC DSRDDSRD areare analyzedanalyzed by by numerically numerically solving solving Poisson Poisson equation,equation, continuity continuity equation equation and and transport transport equation. equation. AsAs shown shown in in Figure Figure2, 2, the the electric electric field field distribution distribution of of the the two two devices devices under under reverse reverse breakdownbreakdown is is compared, compared, wherewhere thethe SJ SiC DSRD DSRD is is the the electric electric field field distribution distribution along along the theinterface interface of p-pillar of p-pillar and and n-pill n-pillar.ar. When When the external the external reverse reverse voltage voltage is applied, is applied, the p-pillar the p-pillarand n-pillar and n-pillar regions regions in the in voltage the voltage withstand withstand layer layer are depleted, are depleted, and and the thenegative negative and andpositive positive charges charges are generated are generated by the by ionization the ionization of the of acceptor the acceptor and donor. and donor. The positive The positivecharges charges in the n-pillar in then-pillar generate generate the electric the electricfield terminating field terminating in the p-pillar in the along p-pillar the along trans- theverse transverse direction. direction. With the With increase the increase of reverse of reversevoltage, voltage, the transverse the transverse pn junction pn junction and lon- andgitudinal longitudinal pn junction pn junction interact, interact, and the andelectric the field electric broadens field broadens in the transverse in the transverse and longi- andtudinal longitudinal directions, directions, so that the so total that theelectric total fi electriceld can fieldchange can from change the fromtraditional the traditional triangular triangulardistribution distribution (non through) (non through)or trapezoidal or trapezoidal distribution distribution (through) (through) to rectangular to rectangular distribu- distribution, thus increasing the breakdown voltage of the device. The reverse breakdown tion, thus increasing the breakdown voltage of the device. The reverse breakdown voltage voltage of the SJ SiC DSRD in Figure2 is 1606 V, which is 28% higher than that of the conventional SiC DSRD (1257 V).

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of the SJ SiC DSRD in Figure 2 is 1606 V, which is 28% higher than that of the conventional Materials 2021, 14, 684 4 of 10 SiCof the DSRD SJ SiC (1257 DSRD V). in Figure 2 is 1606 V, which is 28% higher than that of the conventional SiC DSRD (1257 V).

Figure 2. Electric field distributions in drift layer of conventional SiC DSRD and SJ SiC DSRD. (Tnb of 10 μm, Nnb of 1 × 1016 cm−3). FigureFigure 2.2. ElectricElectric ﬁeldfield distributions distributions in in drift drift layer layer of of conventional conventional SiC SiC DSRD DSRD and and SJ SiCSJ SiC DSRD. DSRD. (Tnb (Tofnb 16 16 −3−3 10of µ10m, μNm,nb Nofnb 1of× 1 10× 10 cm cm ).). Theoretically, the area of the n-pillar in the voltage withstand layer of SiC DSRD be- comesTheoretically,Theoretically, half of the original thethe area area of th the aftere n-pillar n-pillar the in in introduction the the voltage voltage of withstand SJ. withstand Under layerthe layer premise of ofSiC SiC DSRD of DSRD equal be- becomesdopingcomes halfconcentration, half of of the the original original the carrier area area after afterconcentration the the in introductiontroduction is reduced of of SJ. by UnderUnde half, rso thethe it premisehaspremise a larger ofof equal equalpeak dopingcurrentdoping concentration,andconcentration, a faster current thethe carriercarrier drop concentrationconcentrationrate in the reverse isis reducedreduced recovery byby half, half,process. soso itit In hashas order aa largerlarger to verify peakpeak currentthecurrent above andand inference, a a faster faster current acurrent pulse drop testdrop ratecircuit rate in in thebased the reverse reverseon DSRD recovery recovery is specially process. process. established In order In order to in verify to simula- verify the abovetion,the above as inference, shown inference, in a Figure pulse a pulse test3. The circuittest detailed circuit based based operat on DSRD oning DSRD principle is specially is specially of the established circuitestablished can in simulation,be in referred simula- astotion, shown[26]. as shown in Figure in Figure3. The detailed3. The detailed operating operat principleing principle of the circuit of the can circuit be referred can be referred to [ 26]. to [26]. +Vee +Vff

+Vee +Vff

L1 R1 DSRD

D L2 L3 C2 Δt C1 D L2 L3 C2 Rload Δt C1 Rload

Figure 3.3. Pulse test circuit basedbased onon SiCSiC DSRD.DSRD. Figure 3. Pulse test circuit based on SiC DSRD. InIn the simulation, the the active active area area of of both both devices devices is is 4 4× ×1010−2 cm−2 2cm. The2. Thecircuit circuit parameters param- etersare setIn are as the setV eesimulation, as= 40V eeV,= V 40ff the= V,30 active VV,ff L=1 area= 30 75 V, nH,ofL 1both =L2 75= devices 65 nH, nH,L 2 isL=3 4 = 65× 45 10 nH, nH,−2 cmL C32.1 =The= 45100 circuit nH, pF, CC 1parameters2 = 1 100 μF, pF, Δt µ 1 load C=are 250= set ns, 1 as FR, V∆ =eet 20 == 50 40Ω ns,,V, R RV1ff == 5030 20 ΩΩV,., LRAs1load = shown 75= nH, 50 Ω inL.2 As Figure= 65 shown nH, 4a, L in 3the = Figure 45 current nH,4a, C the1waveforms = 100 current pF, C waveforms of2 = the1 μ F,con- Δt ofventional= 50 the ns, conventional R 1SiC = 20 DSRD Ω, R SiCload and = DSRD 50the Ω SJ.and As SiC shown the DSRD SJ SiC in are Figure DSRD compared. 4a, are the compared. Itcurrent can be waveforms It seen can bethat seen the of thatthereverse con- the reversepeakventional current peak SiC of current DSRD SJ SiC ofandDSRD SJ the SiC is SJ larger DSRD SiC DSRDthan is larger that are of thancompared. conventional that of It conventional can SiC be DSRD, seen SiC thatand DSRD, the reversereverse and thecurrentpeak reverse current of SJ current ofSiC SJ DSRD SiC of SJDSRD SiCdecreases DSRDis larger faster decreases than than that faster thatof conventional of than conventional that of SiC conventional DSRD, SiC DSRD. and SiCthe Figure reverse DSRD. 4b Figureshowscurrent4 theb of shows comparisonSJ SiC the DSRD comparison of decreases load voltage of faster load waveforms voltagethan that waveforms betweenof conventional conventional between SiC conventional DSRD. SiC DSRD Figure SiCand 4b DSRDSJshows SiC DSRD. andthe comparison SJ It SiC can DSRD. be seenof Itload canthat voltage bethe seen SJ SiCwaveforms that DSRD the SJ can between SiC output DSRD conventional pulse can outputvoltage SiC pulse with DSRD a voltage faster- and withrisingSJ SiC a speed faster-risingDSRD. and It can higher speedbe seen peak and that value higher the than SJ peak SiC th eDSRD value conventional thancan output the SiC conventional pulseDSRD. voltage The SiC parameters with DSRD. a faster- The are V parameterssummarizedrising speed are andas summarized shown higher in peak Table as value shown 1, where than in Table th BVe 1conventional is, where the breakdown BV is SiC the DSRD. breakdown voltage, The V peakparameters voltage, is the peakpeak are is the peak voltage, tr is the rise time of load voltage leading edge, which usually refers to voltage,summarized tr is theas shownrise time in ofTable load 1, voltage where BVleadin is theg edge, breakdown which usually voltage, refers Vpeak tois the peaktime the time from 10% of Vpeak to 90% of Vpeak, dv/dt is average voltage change rate in tr as voltage, tr is the rise time of load voltage leading edge, which usually refers to the time shown in Equation (1). = ( − ) dv/dt 0.9 0.1 Vpeak/tr (1)

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Materials 2021, 14, 684 from 10% of Vpeak to 90% of Vpeak, dv/dt is average voltage change rate in tr as shown in5 of 10 Equation (1). =− dv dt(0.9 0.1) Vpeak t r (1) It can be seen that the tr of SJ SiC DSRD is 18.5% less than that of conventional SiC DSRD, andIt can the be dv/dt seen isthat 29% the higher tr of SJ thanSiC DSRD thatof is conventional18.5% less than SiC that DSRD. of conventional SiC DSRD, and the dv/dt is 29% higher than that of conventional SiC DSRD.

(a) (b)

FigureFigure 4. Comparison 4. Comparison of (ofa) (a current) current waveforms waveforms and and ((bb)) loadload output voltage voltage waveforms waveforms of ofconventional conventional SiC SiC DSRD DSRD and andSJ SJ 16 −3 SiC DSRD. (Tnb of 10 μm, Nnb of 1 × 1016 cm −) 3 SiC DSRD. (Tnb of 10 µm, Nnb of 1 × 10 cm ) Table 1. Comparison of parameter characteristics between conventional SiC DSRD and SJ SiC TableDSRD. 1. Comparison of parameter characteristics between conventional SiC DSRD and SJ SiC DSRD.

DeviceDevice BV BV (V) (V) Vpeak Vpeak (V) (V) tr (ns) tr (ns) dv/dt dv/dt (V/ns) (V/ns) ConventionalConventional SiC 12571257 610 610 2.7 2.7 181 181 SiCDSRD DSRD SJ SiC DSRD 16061606 (28% (28% higher) 643 2.2 (18.5% faster) 234 (29% higher) SJ SiC DSRD 643 2.2 (18.5% faster) 234 (29% higher) higher) 3. Discussion 3. Discussion 3.1. Influence of Structure Parameters on Blocking Characteristics 3.1.By Influence introducing of Structure an electric Parameters field on in Blocking the transverse Characteristics direction, the SJ structure affects the distributionBy introducing of the longitudinal an electric field electric in the field, transv anderse finally direction, changes the SJ the structure breakdown affects voltage.the Thedistribution voltage is of essentially the longitudinal affected electric by Wfield,SJ, Tandnb andfinallyN nbchanges. As shown the breakdown in Figure voltage.5, it is the BVThe curve voltage formed is essentially by connecting affected the by discreteWSJ, Tnb and simulation Nnb. As shown values in obtained Figure 5, under it is the different BV WcurveSJ. With formed the increaseby connecting of the theW SJdiscrete, the BV simulation decreases values rapidly. obtained The reasonunder different is that withWSJ. the Materials 2021, 14, x FOR PEER REVIEW 6 of 12 increaseWith the of increaseWSJ, the of adjustmentthe WSJ, the BV ability decreases of the rapidly. transverse The reason electric is fieldthat with to the the total increase electric fieldof W isSJ weakened,, the adjustment andthe ability total of electric the transverse field distribution electric field is noto longerthe total rectangular. electric field is weakened, and the total electric field distribution is no longer rectangular.

16 −3 16 −3 FigureFigure 5. 5. BVBV of of SJ SJSiC SiC DSRD DSRD with with different different WSJ.W (TSJnb.( ofT 10nb μofm, 10 Nµnbm, of N1 ×nb 10of 1cm× 10). cm ).

As shown in Figure 6, the breakdown voltages of conventional SiC DSRD and SJ SiC DSRD are compared under different drift layer concentrations and thicknesses. Note that in order to facilitate observation and analysis, the breakdown voltages are connected into curves. As can be seen from the figure that the BV of both devices decreases with the increase of Nnb, but the BV of SJ SiC DSRD always keeps the trend of being larger than or equal to that of conventional SiC DSRD. Taking the concentrations of 5 × 1015 cm−3 and 1 × 1016 cm−3 as examples, as shown in Figure 7a,b, the electric field distribution of conventional SiC DSRD and SJ SiC DSRD with drift layer concentrations of 5 × 1015 cm−3 and 1 × 1016 cm−3 are compared. The area formed by the electric field curve and the horizontal axis represents the breakdown voltage. It can be seen that for the SJ SiC DSRD, the breakdown voltages at the concentration of 5 × 1015 cm−3 and 1 × 1016 cm−3 are almost the same; for the conventional SiC DSRD, when the concentration changes from 5 × 1015 cm−3 to 1 × 1016 cm−3, the electric field distribution tends from the trapezoidal distribution with larger punch- through to the trapezoidal distribution with smaller punch-through, that is, the breakdown voltage decreases rapidly. In conclusion, when the concentration is 1 × 1016 cm−3, the increase of breakdown voltage is better than that when the concentration is 5 × 1015 cm−3. In addition, it can be seen from Figure 6 that the breakdown voltage of SJ SiC DSRD gradually increases with the increase of the thickness of the drift layer at the same doping concentration; at the same time, in the appropriate doping concentration range, the greater the thickness of the drift layer, the higher the degree of improvement of the breakdown voltage of the SJ SiC DSRD.

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As shown in Figure6, the breakdown voltages of conventional SiC DSRD and SJ SiC DSRD are compared under different drift layer concentrations and thicknesses. Note that in order to facilitate observation and analysis, the breakdown voltages are connected into curves. As can be seen from the figure that the BV of both devices decreases with the increase of Nnb, but the BV of SJ SiC DSRD always keeps the trend of being larger than or equal to that of conventional SiC DSRD. Taking the concentrations of 5 × 1015 cm−3 and 1 × 1016 cm−3 as examples, as shown in Figure7a,b, the electric field distribution of conventional SiC DSRD and SJ SiC DSRD with drift layer concentrations of 5 × 1015 cm−3 and 1 × 1016 cm−3 are compared. The area formed by the electric field curve and the horizontal axis represents the breakdown voltage. It can be seen that for the SJ SiC DSRD, the breakdown voltages at the concentration of 5 × 1015 cm−3 and 1 × 1016 cm−3 are almost the same; for the conventional SiC DSRD, when the concentration changes from 5 × 1015 cm−3 to 1 × 1016 cm−3, the electric field distribution tends from the trapezoidal distribution with larger punch-through to the trapezoidal distribution with smaller punch-through, that is, the breakdown voltage decreases rapidly. In conclusion, when the concentration is 1 × 1016 cm−3, the increase of breakdown voltage is better than that when the concentration is 5 × 1015 cm−3. In addition, it can be seen from Figure6 that the breakdown voltage of SJ SiC DSRD gradually increases with the increase of the thickness of the drift layer at the same doping concentration; at the same time, in the appropriate doping concentration

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Figure 6 BV of conventional SiC DSRD and SJ SiC DSRD under different Nnb and Tnb. Figure 6. BV of conventional SiC DSRD and SJ SiC DSRD under different Nnb and Tnb. Figure 6 BV of conventional SiC DSRD and SJ SiC DSRD under different Nnb and Tnb.

(a) (b) Figure 7. Comparison of electric field(a )distributions of conventional SiC DSRD and SJ SiC DSRD with(b) drift layer concentrations of (a) 5 × 1015 cm−3 and (b) 1 × 1016 cm−3 respectively. Figure 7. ComparisonFigure 7. Comparison of electric of ﬁeldelectric distributions field distributions of conventionalof conventional SiCSiC DSRD DSRD and and SJ SiC SJ SiCDSRD DSRD with drift with layer drift concen- layer concentra- trations of (a) 5 × 1015 cm−3 and (b) 1 × 1016 cm−3 respectively. − − tions of (a) 5 × 1015 cm 3 and3.2. (b Influence) 1 × 10 of16 Structurecm 3 respectively. Parameters on Dynamic Characteristics For3.2. SiC Influence DSRD, oneof Structure of the most Parameters important on Dynamic applications Characteristics is to output a fast-rising high voltage pulseFor on SiC load DSRD, for the one pulse of the generator. most important In this process, applications the most is to important output a fast-rising param- high eters arevoltage the rise pulse time on tr load(usually for thebetween pulse generator.10% and 90% In this of the process, pulse thevoltage most amplitude)important param- and theeters peak are value the riseVpeak time. In sometr (usually applications, between small 10% andtr and 90% large of the Vpeak pulse are voltagerequired amplitude) to achieveand high the dv/dt. peak The value quality Vpeak of. Inthe some load pulseapplications, is directly small affected tr and by largethe switching Vpeak are char- required to acteristicsachieve of DSRD. high dv/dt.According The qualityto the operating of the load principle pulse is directlyof DSRD, affected the faster by the the switching reverse char- currentacteristics of DSRD ofdecreases, DSRD. According the shorter to the the rise operating time of principle the output of DSRD,pulse voltage the faster on thethe reverse load; thecurrent larger ofthe DSRD reverse decreases, current peak the valueshorter of theDSRD, rise thetime larger of the the output output pulse voltage voltage peak on the value onload; the theload. larger Based the on reverse the pulse current generator peak valuecircuit of shown DSRD, in the Figure larger 3, the we output focus on voltage the peak influencevalue of different on the load. structure Based parameters on the pulse on generator the output circuit pulse shown of conventional in Figure 3,SiC we DSRD focus on the and SJ SiCinfluence DSRD. of different structure parameters on the output pulse of conventional SiC DSRD Theand current SJ SiC waveforms DSRD. of conventional SiC DSRD and SJ SiC DSRD with Tnb of 6 μm, 8 μm, 10 μmThe and current 12 μm waveforms are shown ofin conventional Figure 8. It can SiC be DSRD seen andthat SJ the SiC peak DSRD value with of T thenb of 6 μm, reverse 8current μm, 10 of μ mthe and SJ SiC 12 μDSRDm are is shown larger inthan Figure that 8.of Itthe can conventional be seen that SiC the DSRD, peak value and of the the decreasereverse speed current of the of reversethe SJ SiC current DSRD is fasteris larger than than that that of conventionalof the conventional SiC DSRD. SiC DSRD,The and the decrease speed of the reverse current is faster than that of conventional SiC DSRD. The

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3.2. Inﬂuence of Structure Parameters on Dynamic Characteristics For SiC DSRD, one of the most important applications is to output a fast-rising high voltage pulse on load for the pulse generator. In this process, the most important parameters are the rise time tr (usually between 10% and 90% of the pulse voltage amplitude) and the peak value Vpeak. In some applications, small tr and large Vpeak are required to achieve high dv/dt. The quality of the load pulse is directly affected by the switching characteristics of DSRD. According to the operating principle of DSRD, the faster the reverse current of DSRD decreases, the shorter the rise time of the output pulse voltage on the load; the larger the reverse current peak value of DSRD, the larger the output voltage peak value on the load. Based on the pulse generator circuit shown in Figure3, we focus on the inﬂuence of different structure parameters on the output pulse of conventional SiC DSRD and SJ SiC DSRD. The current waveforms of conventional SiC DSRD and SJ SiC DSRD with Tnb of 6 µm, Materials 2021, 14, x FOR PEER REVIEW8 µm, 10 µm and 12 µm are shown in Figure8. It can be seen that the peak value of the8 of 12 reverse current of the SJ SiC DSRD is larger than that of the conventional SiC DSRD, and the decrease speed of the reverse current is faster than that of conventional SiC DSRD. The Vpeak, tr and dv/dt of the output voltage pulse on the load in the pulse circuit of the two Vpeak, tr and dv/dt of the output voltage pulse on the load in the pulse circuit of the two devices are shown in Table2, where Tnb is the thickness of the drift layer. It can be seen devices are shown in Table 2, where Tnb is the thickness of the drift layer. It can be seen that the Vpeak of SJ SiC DSRD is generally larger and tr is smaller than that of conventional that the Vpeak of SJ SiC DSRD is generally larger and tr is smaller than that of conventional SiC DSRD. For dv/dt, the improved range of SJ SiC DSRD can reach 31% compared with SiC DSRD. For dv/dt, the improved range of SJ SiC DSRD can reach 31% compared with conventional SiC DSRD. conventional SiC DSRD.

Figure 8. Current waveforms of conventional SiC DSRD and SJ SiC DSRD with Tnb of 6 μm, 8 μm, Figure 8. Current waveforms of conventional SiC DSRD and SJ SiC DSRD with Tnb of 6 µm, 8 µm, 10 μm, and 12 μm. (Tnb of 7 × 1015 cm−3, the same circuit conditions) 15 −3 10 µm, and 12 µm. (Tnb of 7 × 10 cm , the same circuit conditions) Table 2. Comparison of output pulse parameters of conventional SiC DSRD and SJ SiC DSRD with µ µ Table 2. Comparison of outputTnbpulse of 6 μ parametersm, 8 μm, 10 ofμm, conventional and 12 μm. SiC DSRD and SJ SiC DSRD with Tnb of 6 m, 8 m, 10 µm, and 12 µm. Vpeak (V) tr (ns) dv/dt (V/ns) Vpeak (V)Conven- tr (ns)Conven- dv/dt (V/ns)Conven- Tnb (μm) SJ SiC SJ SiC SJ SiC Tnb (µm) Conventional SJ SiC tionalConventional SiC SJ SiC tional ConventionalSiC tionalSJ SiC SiC DSRD DSRD DSRD SiC DSRD DSRD SiCDSRD DSRD DSRD DSRDSiC DSRD DSRDDSRD 6 604 603 2.7 2.5 179 193 (8% higher)193 (8% 6 604 603 2.7 2.5 179 8 629 635 2.5 2.3 201 221 (10% higher)higher) 10 636 654 2.5 2.1 204 249 (22% higher) 221 (10% 12 635 6628 629 2.5 635 2.02.5 2032.3 265201 (31% higher) higher) 249 (22% Using10 the same 636 method, the654 inﬂuence of2.5N on the2.1 dynamic characteristics204 of the nb higher) conventional SiC DSRD and the SJ SiC DSRD are discussed. The current waveforms 265 (31% 12 635 662 2.5 2.0 203 higher)

Using the same method, the influence of Nnb on the dynamic characteristics of the conventional SiC DSRD and the SJ SiC DSRD are discussed. The current waveforms of them with Nnb of 7 × 1015 cm−3, 9 × 1015 cm−3 and 1 × 1016 cm−3 are shown in Figure 9. It can be seen that the SJ SiC DSRD has larger reverse peak currents and harder recovery characteristics. The output voltage parameters of the two devices under these three parameters are summarized in Table 3. It can be seen that the SJ SiC DSRD can output voltage pulses with a higher peak, shorter rise time, and larger dv/dt than the conventional SiC DSRD. When the Nnb is 1 × 1016 cm−3, the increase of dv/dt can be as high as 29%. Combined with the above breakdown analysis, when Nnb is 1 × 1016 cm−3, the breakdown voltage of SJ SiC DSRD is 1606 V, which is 28% higher than that of conventional SiC DSRD 1257 V.

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15 −3 15 −3 16 −3 of them with Nnb of 7 × 10 cm , 9 × 10 cm and 1 × 10 cm are shown in Figure9. It can be seen that the SJ SiC DSRD has larger reverse peak currents and harder recovery characteristics. The output voltage parameters of the two devices under these three parameters are summarized in Table3. It can be seen that the SJ SiC DSRD can output voltage pulses with a higher peak, shorter rise time, and larger dv/dt than the conventional 16 −3 SiC DSRD. When the Nnb is 1 × 10 cm , the increase of dv/dt can be as high as 29%. × 16 −3 Materials 2021, 14, x FOR PEER REVIEWCombined with the above breakdown analysis, when Nnb is 1 10 cm , the breakdown9 of 12 voltage of SJ SiC DSRD is 1606 V, which is 28% higher than that of conventional SiC DSRD 1257 V.

15 −3 Figure 9. Current waveformswaveformsof ofconventional conventional SiC SiC DSRD DSRD and and SJ SJ SiC SiC DSRD DSRD with withN Nnbof of 7 ×7 ×10 1015 cm cm−3, 15 −3 16 −3 nb 9 × 10 15 cm −, and3 1 × 10 cm16 . (T−nb3 of 10 μm, the same circuit conditions) 9 × 10 cm , and 1 × 10 cm .(Tnb of 10 µm, the same circuit conditions)

Table 3. Comparison of output pulse parameters of conventional SiC DSRD and SJ SiC15 DSRD−3 with Table 3. Comparison of output pulse parameters of conventional SiC DSRD and SJ SiC DSRD with Nnb of 7 × 10 cm , Nnb of 7 × 1015 cm−3, 9 × 1015 cm−3, and 1 × 1016 cm−3. (Tnb of 10 μm, the same circuit conditions). 15 −3 16 −3 9 × 10 cm , and 1 × 10 cm .(Tnb of 10 µm, the same circuit conditions). Vpeak (V) tr (ns) dv/dt (V/ns) V (V) t (ns) dv/dt (V/ns) peak Conven- r Conven- Conven- −3 Nnb (cm−3) SJ SiC SJ SiC SJ SiC Nnb (cm ) Conventional SJ SiC tionalConventional SiC SJ SiCtional SiCConventional tional SiCSJ SiC DSRD DSRD DSRD SiC DSRD DSRD DSRDSiC DSRD DSRD DSRD SiC DSRD DSRDDSRD 15 7 × 10 636 654 2.6 2.2 196 238 (21% higher)238 (21% × 15 7 × 1015 636 654 2.6 2.2 196 9 10 619 648 2.6 2.3 190 225 (18% higher)higher) 1 × 1016 609 643 2.7 2.2 181 234 (29% higher) 225 (18% 9 × 1015 619 648 2.6 2.3 190 higher) 3.3. Inﬂuence of External Circuit Parameters on Dynamic Characteristics 234 (29% 1 × 1016 609 643 2.7 2.2 181 In addition, it is well known that the Vpeak and dv/dt of the pulse voltage thathigher) DSRD can output on the load are also related to the forward pumping time of the external circuit. Therefore,3.3. Influence the of dynamic External characteristicsCircuit Parameters of conventional on Dynamic Characteristics SiC DSRD and SJ SiC DSRD under different forward pumping time are also simulated. In the simulation, the parameters of the devicesIn addition, are the it sameis well except known that that one the uses Vpeak a separateand dv/dt n-type of the region pulse voltage and the that other DSRD uses acan SJ output composed on the of load p-type are and also n-type related alternately to the forward as their pumping drift layer. time of As the shown external in Table circuit.4, whenTherefore,∆t (forward the dynamic pumping characteristics time) is 30 of ns, co 40nventional ns, 50 ns, SiC 60 ns,DSRD 80 ns, and 100 SJ ns,SiC 120ns, DSRD 140 under ns, 160different ns, 180 forward ns and 200pumping ns, the time output are voltagealso simulated. parameters In the of conventional simulation, the SiC parameters DSRD and SJof the devices are the same except that one uses a separate n-type region and the other uses SiC DSRD are compared. It can be seen that the Vpeak and dv/dt of SJ SiC DSRD are larger, a SJ composed of p-type and n-type alternately as their drift layer. As shown in Table 4, and the tr is smaller than those of conventional SiC DSRD at any ∆t. Speciﬁcally, the dv/dt canwhen be Δ increasedt (forward by pumping 12–23%. time) is 30 ns, 40 ns, 50 ns, 60 ns, 80 ns, 100 ns, 120ns, 140 ns, 160 ns, 180 ns and 200 ns, the output voltage parameters of conventional SiC DSRD and SJ SiC DSRD are compared. It can be seen that the Vpeak and dv/dt of SJ SiC DSRD are larger, and the tr is smaller than those of conventional SiC DSRD at any Δt. Specifically, the dv/dt can be increased by 12–23%.

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Table 4. Output voltage comparison of conventional SiC DSRD and SJ SiC DSRD when ∆t is 30 ns, 40 ns, 50 ns, 60 ns, 80 ns, 15 −3 100 ns, 120 ns, 140 ns, 160 ns, 180 ns, and 200 ns respectively. (Tnb of 10 µm, Nnb of 7 × 10 cm ).

Vpeak (V) tr (ns) dv/dt (V/ns) ∆t (ns) Conventional SJ SiC Conventional SJ SiC Conventional SJ SiC SiC DSRD DSRD SiC DSRD DSRD SiC DSRD DSRD 30 367 383 3.1 2.8 95 109 (15% higher) 40 502 522 2.6 2.3 154 182 (18% higher) 50 636 654 2.5 2.1 204 249 (22% higher) 60 766 782 2.4 2.1 255 298 (17% higher) 80 1015 1029 2.3 2.0 353 412 (17% higher) 100 1247 1266 2.2 2.0 453 506 (12% higher) 120 1350 1488 2.2 2.1 491 567 (15% higher) 140 1442 1562 2.2 2.0 524 625 (19% higher) 160 1473 1628 2.2 2.0 535 651 (22% higher) 180 1507 1628 2.2 2.0 548 651 (19% higher) 200 1534 1723 2.3 2.1 534 656 (23% higher)

4. Conclusions The SJ structure is introduced in the SiC DSRD for the ﬁrst time in this paper. Static simulations show that the breakdown voltage of SJ SiC DSRD is affected by the doping concentration, thickness and width of the SJ. The breakdown voltage increases with the decrease of SJ doping concentration, the increase of SJ thickness, and the decrease of SJ width. What’s more, SJ SiC DSRD can withstand higher reverse breakdown voltage than conventional SiC DSRD under the same conditions. When the drift region thickness is 10 µm and the concentration is 1 × 1016 cm−3, the breakdown voltage of SJ SiC DSRD is 28% higher than that of conventional SiC DSRD. Dynamic simulations show that SJ SiC DSRD can cut off a larger reverse current and has a shorter reverse current fall time than conventional SiC DSRD. In other words, SJ can provide a higher operating current and improve the recovery hardness of SiC DSRD at the same time. Because of the above advantages, the pulse circuit based on SJ SiC DSRD can output voltage pulses with larger amplitude and steeper rising front compared with the pulse circuit based on conventional SiC DSRD. When the doping concentration of the drift region is 7 × 1015 cm−3 and the thickness is 12 µm, the dv/dt of SJ SiC DSRD is 31% higher than that of conventional SiC DSRD.

Author Contributions: Conceptualization, X.Y. and L.L.; methodology, X.Y.; software, X.Y. and X.H.; validation, X.Y. and X.H.; formal analysis, X.Y.; investigation, H.Z. and Z.Y.; data curation, X.Y. and X.H.; writing—original draft preparation, X.Y.; writing—review and editing, L.L., X.H. and Z.Y.; supervision, L.L.; project administration, L.L.; funding acquisition, L.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Natural Science Foundation of China, grant number 51877092. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Detailed data are available on request from the corresponding author. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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