Next Generation HighPerformance BIGT HiPak Modules

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The practical realization of the Bimode Insulated Gate Transistor (BIGT) will provide a potential solution forfuture high voltage applications demanding compact systems with higher power levels. In this article, wegive an outlook into the new technology and the basic performance levels which could be achieved as theBIGT progresses towards the product development stage, Arnost Kopta, Munaf Rahimo and RaffaelSchnell, ABB Switzerland Ltd. Semiconductors

The BIGT is an advanced reverseconducting IGBT device concept whichmainly targets an increase in the powerdensity levels of high voltage IGBTs fornext generation power electronics systems.The new device can operate in bothfreewheeling diode mode and (IGBT)transistor mode by utilizing the sameavailable silicon volume in both modes.Therefore, the BIGT targets to fully replacethe state-of-the-art two-chip IGBT/diodeapproach with a single BIGT chip. This isachieved while also being capable ofimproving on the overall performanceespecially under hard switching conditionswith low losses, soft switchingcharacteristics and high safe operating area(SOA).The BIGT development has resulted in a

clear breakthrough in device performanceby adopting an advanced shorted collectorbackside layout design, optimum dopingprofiles and controlled lifetime reductionfor enabling best possible operation inboth IGBT- and diode mode. In this article,we present the latest electrical andreliability results achieved bydemonstrating the 3.3kV BIGT in twoHiPak module footprints up to Tj=150°C.

The BIGT conceptThe BIGT consists of a hybrid structureintegrating an IGBT and an RC-IGBT into asingle chip as shown in Figure 1. The maintarget of this combination is to eliminatesnap-back behavior at low temperatures inthe BIGT transistor on-state mode byensuring that hole injection occurs at lowvoltages and currents from the P+collector region in the IGBT section of theBIGT. The BIGT concept provides anoptimum solution especially for thindevices with punch-through type bufferdesigns where the snapback phenomenonis pronounced in standard RC-IGBTs.The backside layout design and

dimensioning of the IGBT region isoptimized to provide smooth transition

into full chip conduction as the RC-IGBTsection will also provide holes at highercurrents maximize the RC-IGBT area fordiode conduction and minimize anycurrent non-uniformities especially duringswitching due to the integrated structure.Furthermore, the introduction of the IGBTwill enable the RC-IGBT part layout designto be independently optimized formaximizing the diode area and ensuringthat the BIGT utilizes the whole chip duringtransistor conduction for providing thesame technology curve as for a state-of-the-art IGBT chip. The BIGT concept hasresulted in a better trade-off between theabove mentioned parameters compared tothe standard RC-IGBT design. On the otherhand, to optimize the BIGT for lowdynamic and switching losses, the mainchallenge was to enable low diode mode

recovery losses while not having aconsiderable effect on the transistor modeon-state losses. A three step approach is utilized to

achieve this target. The first step is the finecontrol of the doping profiles of the emitterP-well cells and collector P+/N+ regions.As shown in Figure 1, the Enhanced-Planar(EP) cell technology does not include anyhighly doped P+ well regions and alsoexhibits a compensation effect due to theN-enhancement layer. These two featuresprovide the BIGT with a fine pattern P-wellprofile for obtaining low injection efficiencyfor a better diode performance whilemaintaining the typical low IGBT lossesassociated with EP designs. The secondoptimization step employs a Local P-wellLifetime (LpL) control technique (seeFigure 1 top) utilizing a well-defined

Figure 1: BIGT cross-section

Figure 2: On-statecharacteristics of the3.3kV BIGT substrate

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(140 x 190mm) BIGT modules werefabricated and tested under conditionssimilar to those applied to state-of-the-artIGBT modules. The BIGT advantage is demonstrated

since the HiPak1 module containing 4BIGT substrates for a total of 24 BIGT chipscan practically replace the larger 1500AHiPak2 SPT+ IGBT module which normallycontains 6 substrates having a total of 24IGBTs and 12 diodes. The larger standardIGBT module has the further disadvantageof employing much less diode area whichis normally a limiting factor in rectifiermode of operation and for the surge

current capability. On the other hand, thelarger HiPak2 BIGT module employs a totalof 36 BIGT chips and its rating canpotentially exceed 2000A.Electrical characterization of the 3.3kV

BIGT HiPak modules was carried out and ispresented below, including static anddynamic measurements under nominaland SOA conditions. For the dynamicmeasurements at nominal conditions theDC-link voltage was set to 1800 V, whilefor SOA characterization it was increased to2400 V. The RGon and RGoff values were fixedfor all dynamic tests at 1.0� and 1.5�

respectively. Also, a 220nF gate-emitter

particle implantation which further reducesthe diode recovery without degrading thetransistor losses and blockingcharacteristics. A further reduction in thereverse recovery losses is achieved with auniform local lifetime control employingproton irradiation.

Characteristics of the 3.3kV BIGTThe BIGT technology has mainly beendeveloped for high voltage devices. Thework presented here was carried out on a3300V/62.5A BIGT chip with an activearea of approximately 1cm2. High current3.3kV HiPak1 (140 x 130mm) and HiPak2

Figure 3: 3.3kV BIGT HiPak1 (left: Eoff =2.8J) and SPT+ IGBT HiPak2 (right: Eoff =2.7J) turn-off nominal waveforms

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capacitance Cge was employed.The on-state characteristics of the BIGT

in IGBT and diode modes are shown inFigure 2 at 25°C and 125°C. The resultswere obtained from the HiPak substratetest with a nominal current of 375A. Forboth IGBT and diode modes, an on-stateof 3.5V at 125°C is shown at the nominalcurrent. For safe paralleling of chips, thecurves show a strong positive temperaturecoefficient even at very low currents inboth modes of operation due to theoptimum emitter injection efficiency andlifetime control employed in the BIGTstructure.Figures 3 and 4 show the 3.3kV HiPak1

BIGT and HiPak2 SPT+ IGBT turn-off andturn-on waveforms respectively measured

under nominal conditions (VDC=1800 V,IC=1500 A, Tj=125°C). The respectivemodule switching losses are also indicated.The waveforms demonstrate the normalBIGT switching behavior in both IGBT anddiode modes when compared to a state-of-the-art device. A similar comparison isshown for the nominal reverse recoveryperformance as shown in Figure 5. Thedi/dt during switching exceeds 6kA/µs forall tests.

Softness performanceThe new BIGT technology has inherentlyextremely soft switching behavior in bothIGBT and diode mode of operation. Theoptimized collector P+ doping profileswill ensure that during the turn-off tail in

both modes, the passing electronstowards the N+ regions will induce alarge potential across the PN junctionforcing a controlled level of ChargeExtraction (or hole injection) into thebase. Figure 5 demonstrates this softness

during turn-off for both the BIGT HiPak1and the SPT+ IGBT HiPak2 modules at500A and VDC=2400V and Tj=125°C. Thereverse recovery softness performance ofthe BIGT is also demonstrated in Figure6 at a very low current of 50A andVDC=2400V and Tj=125°C. This feature is of particular importance

for the realization of the BIGT technologysince it overcomes the expected trend ofreduced softness due to the non-

Figure 4: 3.3kV BIGT HiPak1 (left: Eon =2.2J/ Erec =2.3J) and SPT+ IGBT HiPak2 (right: Eon =1.9J Erec =2.2J) turn-on nominal waveforms

Figure 5: 3.3kV BIGT HiPak1 (left) and SPT+ IGBT HiPak2 (right) turn-off softness

Figure 6: 3.3kV BIGT HiPak1 (Left) and SPT+ IGBT HiPak2 (Right) reverse recovery softness

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optimum silicon design of the BIGT fordiode mode operation and the increasediode area provided with the newconcept.

SOA performanceThe SOA switching performance will bepresented here at Tj =150°C todemonstrate the robustness of the BIGTmodules. Figure 7 shows the turn-offwaveforms at 3000A and VDC = 2400V inboth IGBT and diode modes. Similar SOA tests were carried out for

the larger HiPak 2 module at 4500A, VDC = 2400V and 150°C as shown inFigure 8. The BIGT also shows ruggedshort circuit performance with anaverage short circuit current of 6500Awhen measured under these conditions.

Frequency operation and reliabilitytesting3.3kV HiPak1 BIGT modules weresubjected for the first time to a PWMfrequency test in an H-bridgeconfiguration. The test was carried out ata maximum junction temperature of120°C for approximately 30 minutes atthree operational frequencies (500Hz,1000Hz and 1500Hz). At a DC linkvoltage of 2.1kV, peak currents up to800A were achieved during the tests.The frequency test results provide a firstinsight into the feasibility of a single chip

operating in both IGBT and freewheelingdiode modes under hard-switchingconditions.Reliability measurements were also

carried out on BIGT chips. The BIGTssuccessfully passed the standardverification High Temperature ReverseBias (HTRB) stress test at 2640V both at125°C for 168 hours followed by asecond run for the same chips at 150°Cfor a further 168 hours. In addition, theBIGT successfully passed the HighTemperature Gate Bias (HTGB) test atVge = 25V at 150°C for 168 hours and ahumidity (HAST) test at 125°C, Vge =25V, and Vce = 80V with 85% humidityfor 168 hours.Figure 9 shows the expected output

current performance for the BIGT HiPak2module compared to today`s SPT+ IGBT

equivalent module at 125°C in bothInverter and Rectifier modes. It wasassumed that an optimizedchip/package layout design and losstrade-offs were employed in a fullydeveloped BIGT module. The curvesshow that the diode performance is alimiting factor for the standard moduleapproach while for the BIGT module thetransistor mode defines the limit. Thecurves show that with BIGT technology a25% and 38% increase in inverter andrectifier output current capability isachieved respectively. Furthermore, theBIGT technology will pave the way forfuture generations of IGBT baseddesigns to provide higher powerdensities without any limitationsimposed by the diode performance.

Figure 7: 3.3kV BIGT HiPak1 turn-off (left) and reverse recovery (right) SOA at 150°C (peak power = 7.9MW and 3.5MW respectively)

Figure 8: 3.3kV BIGT HiPak2 turn-off (left) and reverse recovery (right) SOA at 150°C (peak power = 12MW and 4.2MW respectively)

Figure 9: Outputpower capability ofthe 3.3kV SPT+ IGBT

against the BIGTHipak2 modules inboth inverter andrectifier modes