A Simulation Study on Novel Field Stop IGBTs
Using Superjunction
有肉的言情小说Kwang-Hoon Oh,Jaegil Lee,Kyu-Hyun Lee,Young Chul Kim,and Chongman Yun
Abstract—Performing device simulation,a novel insulated gate bipolar transistor(IGBT)that employs the superjunction as well asfield stop(FS),has been investigated.For a planar1200-V IGBT,the novel superjunction FS IGBT demonstrates the re-markable device performance such as the ON-state voltage drop of 1.6V and switching-off energy of20µJ/A at the collector current density of100A/cm2,which is considered as the best tradeoff performance in its class.In addition,the impact of various design parameters on device performance has been explored,and a com-prehensive analysis for understanding of the operating mechanism is prented,which will be of help for realizing the SJFS IGBTs with optimum design.
Index Terms—Field stop(FS),insulated gate bipolar transistor (IGBT),nonpunchthrough(NPT),superjunction(SJ).
I.I NTRODUCTION
斜线
积极的财政政策
I N RECENT years,the nonpunchthrough(NPT)structures
are the mainstream for the high-voltage insulated gate bipo-lar transistors(IGBTs)since they show the superior static and dynamic characteristics as well as ruggedness.In addition,the NPT IGBTs do not require the lifetime control compared to the punchthrough(PT)IGBTs[1]–[3].
In the meantime,in order to take full advantage of the thin wafer technology as well as the principle of PT,thefield stop (FS)concept has also been applied to the NPT IGBT structures, which realizes the same blocking capability with reduced wafer thickness by virtue of the trapezoidal electric-field distribution during the OFF state,as illustrated in Fig.1[4].The IGBTs with the FS layer(FSL)show better tradeoff performance against the typical NPT IGBTs,and thus are commercially available. Hence,the FS IGBT structure is considered as one of the most advanced designs,and numerous extended FS concepts have been investigated[5]–[8].However,even for the FS IGBTs, high resistivity substrate,as well asfinite drift-layer thickness, is inevitably required,which ts a limit to further improvement of device performance.
Using the concept of charge balance[9],which is generally called the superjunction,it is shown that the concentration of the drift layer can be incread while prerving the same blocking voltage(BV).
This principle was already practical for the power MOSFET devices.As a result,the superjunction(SJ)
Manuscript received June29,2005;revid November9,2005.The review of this paper was arranged by Editor M.A.Shibib.毫升与立方厘米
The authors are with the Fairchild Semiconductor,Kyunggi-Do420-711, Korea(e-mail:kr).
Digital Object Identifier
10.1109/TED.2006.870278
Fig.1.(a)Typical NPT IGBT and(b)FS IGBT for analysis.
MOSFET devices show dramatically reduced ON resistance
compared to the conventional MOSFET devices[10].Since
the principle of the charge balance offered a new paradigm
for the power miconductor devices,pervasive investigations
into majority carrier SJ devices have been performed[11]–[13].
Meanwhile,Fujihira and Miyasaka also explored the minority
乙脑传播途径
carrier superjunction devices and furthermore showed their
feasibility[14].
However,the application of the superjunction theory to the
IGBT devices has not been fully investigated so far.Therefore,
the understanding of the behavior of the superjunction FS
IGBT is instructive to validate its feasibility,and furthermore
to estimate device characteristics.
This work aims to confirm the feasibility of the superjunction
FS IGBTs,and to demonstrate their characteristics using device
simulation.Along with the device feasibility,the main design
considerations for the SJ FS IGBTs as well as their implications
will also be discusd.
II.D EVICE S IMULATION
The primary effect of the superjunction IGBT with the FSL
is that the drift layer can be designed with reduced thick-
ness as well as low resistivity compared to the conventional
NPT IGBTs.To investigate the impact of the new device
concept on the IGBT performance,device simulation with
TWB[15]has been performed for1200-V planar IGBTs
at27◦C.In addition,dynamic performance of the devices
was estimated using switching-off characteristics under induc-
tive load.
0018-9383/$20.00©2006IEEE
q
(1)
where d FSL is the thickness of the FSL,N FSL is the con-centration of the FSL,and E c is the critical electricfield for silicon.Unlike the buffer concentration of the PT structure,
Fig.5.SJFS IGBT for device simulation.
B.Superjunction FS IGBT
The benefit of the superjunctionfield stop(SJFS)IGBT stems from its unique structure,thin highly doped drift layer having multiple pillars.In Fig.5,an SJFS IGBT is shown for analysis.For the ideal charge-balance structure,perfectlyflat electric-field profile should be obtained.In this ca,BV is simply given by
BV=E c·t pillar(2) where t pillar is the length of the pillar in the drift layer as indicated in Fig.5.According to(2),for1200V IGBTs,t pillar of∼70µm ems sufficient.At this condition,in order to fully deplete the drift layer under avalanche breakdown,the number of total charges per unit area Q[cm−2]should also be less thanεSi·E c/q,as discusd in[16].This results in Q
of∼1.3×1012cm−2,but the lower value of Q is preferable since the nsitivity of charge imbalance to BV can be reduced. Depending on the widths of n-type pillar W n and p-type pillar W p,the doping concentration is determined for a given Q value. For direct comparison,the GO and GW are t to the same values as ud in the NPT and FS IGBT simulations,and the pillar widths W p and W n are chon to be identical to the GO and GW,respectively.Unlike the superjunction MOSFET devices,the p-type anode region exists at the bottom of the IGBT structures.Therefore,the moderately doped FSL is re-quired to vanish the electricfield.Otherwi,the electricfield penetrates into the anode and thus the blocking characteristics can be degraded.As discusd previously,the appropriate value of Q FSL is3×1012cm−2and is also ud for simulation of the SJFS IGBT.
In Fig.6,the tradeoff performances as well as BV are investigated,varying the thickness of the device t SJFS,t pillar, and Q values.As shown in Fig.6(a),BV is dependent on the value of Q since the concentration of each pillar determined by Q and accordingly E c also changes with Q.Meanwhile,it s
hould be noted that BV also depends on the current gain of the IGBT,which impos a constraint on the collector doping concentration.As is apparent,higher BV can be
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obtained Fig.6.(a)Q dependence of the BV and(b)tradeoff performance with various t pillar and t SJFS.
with the increa in t SJFS for the same t pillar.For t SJFS= 100µm,the decreasing rate of BV with Q is rapider than the ca of t SJFS=90µm.In addition,the BV dependence on t pillar is not consistent for the cas with different t SJFS. This implies that even with thefixed t SJFS,the t pillar and Q values should be carefully adjusted for adequate blocking characteristics.Also,the simulated corresponding tradeoff per-formances are shown in Fig.6(b).The tradeoff performance improves as t pillar extends and t SJFS reduces.At the same V cesat,E offdramatically decreas with an increa in t pillar for both t SJFS cas.Therefore,it can be said that the longer t pillar and the shorter t SJFS are more advantageous for the design of the SJFS IGBT as long as the blocking characteristics are satisfied.
To understand the impact of the pillar structure,as well as the anode conditions on the switching performance,transient simulation under inductive load was also done for the SJFS IGBTs.The simulated switching-off waveforms are shown in Fig.7(a)and(b).Like other NPT or FS IGBTs,E offdecreas as the collector doping concentration N c decreas.The col-lector emitter voltage V ce waveforms also become steeper,and
Fig.7.Simulated switching-off waveforms with varying N c under inductive load.(a)t pillar=65µm and(b)t pillar=80µm.
the associated voltage overshoots increa with the decrea
in N c.Also,for the same N c,the SJFS IGBT with the longer
清晰的成语
t pillar shows a faster switching-off performance,which ems a unique operating principle of the SJFS IGBT.Increa in the
switching-off speed for the SJFS IGBT with t pillar=80µm finally induces the oscillatory voltage and current waveforms, where dV ce/dt is up to52.4kV/µs.Despite the high dV ce/dt, the dynamic latch up or gate rebias is not obrved.To fur-ther investigate the dynamic behavior of the SJFS IGBT,the currentflowline and the minority carrier distribution at the peak V ce during the switching-off are inspected,where N c= 1.7×1019cm−3.
As shown in Fig.8(a),the switching-off currentflows along
the p-type pillar,which acts like a drain for the hole carriers,
and this feature is distinctive from the superjunction MOS-
FET devices,where the p-type pillar does not function as a
current path.
However,during the switching-off,the minority hole carriers
still remain in the area between the p-type pillar and FSL while
gradually disappear via recombination process.In Fig.8(b),the
hole-carrier profiles corresponding to the two different pillars in
the turn-off transition are shown.The hole-carrier profile shows
that the amount of stored hole carriers reduces with t pillar
and Fig.8.(a)Currentflowlines and(b)hole-carrier profiles during switching-off at the peak V ce and I c=100A/cm2for the devices with t pillar=65and 80µm,where t SJFS=100µm and N c=1.7×1019cm−3.
in turn,E offdecreas with t pillar as apparent in the tradeoff curves in Fig.6(b).Therefore,it can be understood that the longer pillar is more effective in eliminating the stored hole carriers,leading to fast switching-off time.
III.D ISCUSSION
As demonstrated in Fig.6,the predicted tradeoff perfor-mances of the SJFS IGBT em very promising.For instance, the FS IGBT with t FS=120µm in Fig.4has E offof80µJ/A when V cesat=1.6V,but the SJFS IGBT shows much smaller switching-off energy at the same V cesat condition.Therefore, it is verified that the high-speed IGBT with low conduction
Fig.9.Trade-off curves for the SJFS IGBTs,FS IGBTs(ρd=70Ω-cm), and NPT IGBT with different drift-layer thickness.The solid line indicates the region where BV>1200V.
loss can be achievable using the superjunction even without additional lifetime-control process.As can be en in Fig.6,all E offvalues with t SJFS=100µm are placed over50µJ/A.This implies that for fast switching IGBTs,the current gain needs to be reduced through the adjustment of the collector doping conditions.
By decreasing the collector doping concentration N c from 1.7×1019to5×1018cm−3,various tradeoff
curves are gen-erated,as shown in Fig.9.The switching-off characteristics are much improved,and the lowest value is even less than 20µJ/A.In addition,like the typical NPT IGBTs,tradeoff performance advances with the decrea in t SJFS.Meanwhile, the reduced current gain,due to the lightly doped collector augments BV and in turn BV over1200V,is obtained for the devices with t SJFS=90µm.This means that,in opti-mizing the SJFS IGBT,the anode process condition is one of the key design considerations including t pillar,t SJFS,Q, and Q FSL.The simulated SJFS IGBT demonstrates V cesat of1.6V and E offof20µJ/A at the collector current of 100A/cm2,which is considered as the best tradeoff perfor-mance in its class.
Along with the remarkable tradeoff performance,very high latch-up immunity is also expected for the novel IGBTs since during the normal operation the p-type pillar functions as a hole-current path,which enables less hole current toflow underneath the n+emitter region,and in turn increas the latch-up current level.The simulation results show the crucial difference in the hole-current path for the SJFS and typical FS IGBTs,as shown in Fig.10.
The peak value of electron current density for the SJFS IGBT is less,either showing more evenly distributed hole current. This can also be of help for reducing the temperature ri under high-current operation.Accordingly,the lower peak electron current as well as rather uniform hole-current distributi
on for the SJFS IGBT will also be effective for improving the short-circuit capability.
With the short-circuit peak current of about1160A/cm2, device simulation shows the different short-circuit
withstand Fig.10.(a)Currentflowlines for the SJFS IGBT and FS IGBT,(b)the hole and electron current density at y=1µm,when I c=100A/cm2,and(c)I c= 200A/cm2,where t SJFS=t FS=100µm.工作邮件结尾礼貌用语
time of13.8and16.5µs for the FS IGBT and SJFS IGBT, respectively,as shown in Fig.11.In the ca of the FS IGBT,the peak lattice temperature,with progress of short-circuit event, increas more rapidly resulting in early failure since the short-circuit current distribution is more localized.
In Fig.12(a),the short-circuit currentflowline for the two IGBTs is shown when the peak short-circuit currentsflow.A part of the short-circuit currents alsoflow along the p-type pillar for the SJFS IGBT,which eventually exhibit the rather smooth temperature distribution with a low peak value as in Fig.12(b).This intrinsic difference in the operating mechanism of the SJFS IGBT results in high ruggedness,compared to the typical FS IGBT.
