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Posted on 23 July 2019

The Bi-mode Insulated Gate Transistor (BIGT)

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 A High Voltage Switch for Next Generation Megawatt Applications

An advanced high voltage reverse conducting IGBT concept referred to as the Bi-mode Insulated Gate Transistor (BIGT) is currently being developed. The BIGT can operate at the same current densities in both IGBT and diode modes by utilizing the same available silicon volume as it targets to fully replace the traditional IGBT/Diode two-chip approach with a BIGT single chip.

By Munaf Rahimo, Arnost Kopta and Ulrich Schlapbach, ABB Switzerland Ltd, Semiconductors

 

The past two decades has seen major improvements in silicon power semiconductors in terms of reduced losses and wider Safe-Operating- Area (SOA) performance. The main drive behind these advances is the continuous need for higher power densities in new power system designs. Therefore, devices such as the IGBT and the freewheeling diode continue to provide Megawatt applications such as traction, industrial drives and transmission and distribution with optimum components which have enabled with each improved generation a clear leap in power levels.

The IGBT in particular with its inherent advantages including a controlled low power driving requirement and short circuit self limiting capability has experienced many performance breakthroughs, which has resulted in its wide employment in many high power applications. The most recent low loss and high SOA improvements were mainly due to the introduction of the Soft-Punch-Through (SPT) or Field- Stop (FS) thinner silicon concepts combined with advanced planar or trench emitter structures. Similar advances were also achieved for the anti-parallel freewheeling diode to match the continuously improving IGBT. Today, high power IGBT modules have voltage/current ratings ranging from 1700V/3600A to 6500V/750A.

High Power Semiconductor Devices and the Future Options

Currently, it could appear that the development of high voltage silicon power devices has reached a limit with regards to further reductions in the device total losses. The state-of-the-art SPT/FS structures are close to the so-called “Silicon Design Limits” and the emitter plasma enhancement will only provide smaller steps with fine optimization of the IGBT cell designs. In addition, the extremely robust modern IGBT designs already provide the necessary SOA performance with adequate margins. Hence, one can conclude that the possibility for achieving major leaps in increased power densities for silicon is becoming more restricted. Although, increasing the device total area monolithically or equally through device paralleling can provide the higher power solution for some applications, this approach will remain selective due to its negative impact on the cost, size and complexity of the overall system. Therefore, the quest for more advance device concepts will remain as the trend continues for next generation Megawatt systems with increased efficiencies.

The assumed technological barrier or silicon limit has led to the recent trend towards increased operating temperatures compared to the traditional 125°C maximum junction temperatures operation limit. The low losses and high SOA of modern IGBTs and diodes has enabled this step to be taken while also focusing on reduced leakage currents and improved package reliability as the major limiting factors. An increase of around 10-15% in total output current capability can be predicted with this approach.

Furthermore, there are continuous developments in Wide Band-Gap (WBG) materials such as SiC and GaN for power semiconductors due to its ten fold thinner base region structures having substantial loss reduction potentials and the high operating temperature capability when compared to silicon. With the clear progress achieved for ultra fast unipolar power diodes rated up to 1700V and many switch concepts demonstrated recently, WBG power devices are now being regarded as the next major performance leap. Nevertheless, the current cost of such devices and some technological and performance aspects yet to be fully resolved especially for higher voltage/current devices will continue to seriously delay the introduction of WBG components in Megawatt applications. This while also taking into account that silicon power devices could still provide another breakthrough in performance.

The Limits of Silicon Power Devices

To further explore the potential of silicon based power devices in general, and IGBTs/diodes in particular, the performance/design limits must be clearly defined in order to determine a future development trend. Today, by carrying out a standard performance study of stateof- the-art high power IGBT modules, one can clearly see that the main limiting factor in terms of maximum output current capability (i.e. power density) is the diode in both inverter and rectifier mode operation. In addition, the diode also presents another major restriction with regard to its surge current capability. Both limits are clearly a result of the limited diode area available in a given package footprint design which has a typical IGBT to diode area ratio of around 2:1. This limit in diode current capability was fundamentally established after the introduction of modern low-loss IGBT designs. As mentioned earlier, the simple solution of increasing the diode area is not a preferred one and in any case remains restricted by the package stan- dard footprint design. This leads to the conclusion that the development effort must target an improved diode performance to match at least the current IGBT designs. In other words, there is currently no need for improved switch generations unless the diode experiences a major revolution in terms of reduced losses and thus, higher power capabilities. As discussed above, while ignoring cost and material issues, WBG based diodes could provide the required performance due to the low switching losses, but the high conduction losses of high voltage WBG unipolar and bipolar diodes compared to current silicon diodes will restrict their application to relatively high frequencies for Megawatt applications. Other aspects to consider are the soft reverse recovery performance of WBG diodes in general under high currents and high voltages which have not yet been thoroughly evaluated.

The above analysis leads to the following conclusion; in order to increase the power density of high voltage IGBT modules while also solving the real limiting factors due to the diode performance, an IGBT and diode integration solution is needed, or what has been normally referred to as a Reverse Conducting RC-IGBT. The practical realization of a single-chip technology will provide an ideal solution for next generation high voltage applications demanding compact systems with higher power levels, which is proving to be beyond the capability of the standard two-chip approach.

The Bi-Mode Insulated Gate Transistor (BIGT)

Similar to power MOSFETs, the traditional goal for a reverse conducting device having an integral diode is to obtain higher power for a given footprint package area by eliminating the need for a separate anti-parallel diode. This approach has been demonstrated experimentally in recent years for medium voltage IGBTs (600V-1200V) mainly operating at low currents and/or soft switching conditions in special applications. On the other hand, we at ABB have been investigating the RC-IGBT concept by demonstrating its feasibility for high voltage chips under heavy paralleling. The realization of the RC-IGBT concept has always faced a difficult challenge due to a number of technological design and process barriers such as (a) the conflicting requirement for plasma enhancement for the IGBT and diode (b) matching the silicon and buffer design parameters for both the IGBT and diode with regard to device softness (c) the inherited on-state snap-back phenomenon associated with RC-IGBTs and finally (d) the layout and alignment design of the shorted collector for minimizing non-uniform charge distributions during device operation.

Further development work to resolve the above issues and improve the device characteristics has resulted in a clear breakthrough in performance by adopting an advanced collector backside P/N layout design, fine doping profiles and controlled lifetime reduction for enabling optimal operation in both IGBT mode and diode mode. The new device concept is referred to as the Bi-mode Insulated Gate Transistor (BIGT). The results obtained show that the BIGT exhibits low losses in both modes of operation with no typical snap-back behavior in the transistor on-state mode when compared to a standard RC-IGBT, while also maintaining high levels of SOA performance. The BIGT offers in addition a number of device performance advantages as described below. The BIGT technology consists of a hybrid structure integrating an IGBT and an RC-IGBT into a single chip as illustrated in figure (1). A circuit symbol is also proposed for the BIGT.

The BIGT basic structure

The main target of this combination is to eliminate snap-back behavior at low temperatures in the BIGT transistor on-state mode by ensuring that hole injection occurs at low voltages and currents from the P+ collector region in the IGBT section of the BIGT. The BIGT provides an optimum solution especially for thin devices with SPT type buffer designs where the snap-back phenomenon is pronounced in RC-IGBTs. The backside layout design and dimensioning provides smooth transition into full chip IGBT conduction while maximizing the RC-IGBT area for diode conduction. Therefore, the BIGT concept has resulted in a better trade-off between the above mentioned parameters compared to the standard RC-IGBT design.

To optimize the BIGT for low dynamic and switching losses, the other main challenge was to enable low diode mode recovery losses while not having a considerable effect on the transistor mode on-state losses. A three step approach is utilized to achieve this target. The first step is the fine control of the doping profiles of the emitter p-well cells and collector P+/N+ regions. As shown in figure (1:top), the Enhanced Planar (EP) cell design does not include any highly doped P+ well regions and provides the BIGT with a fine pattern p-well profile for obtaining low injection efficiency for better diode performance while maintaining the typical low IGBT losses associated with EP designs. The second optimization step employs a Local p-well Lifetime (LpL) control technique utilizing a well-defined particle implantation which further reduces the diode recovery losses without degrading the transistor losses and blocking characteristics. Further reduction in the reverse recovery losses is achieved with a uniform local lifetime control employing proton irradiation. A further optional 10% reduction of in recovery losses can be obtained with a MOS Control Diode function as demonstrated in the module results presented in the following sections.

The BIGT technology has mainly been developed for high voltage devices and the work presented here was carried out on a 3300V/62.5A BIGT (active area = 1cm2). The on-state characteristics of the BIGT in transistor and diode modes are shown in figure (2) at 25°C and 125°C. For safe paralleling of chips, the curves show a strong positive temperature coefficient even at very low currents in both modes of operation due to the optimum emitter injection efficiency and lifetime control employed in the BIGT structure.

3.3kV-62.5A BIGT on-state in transistor and diode mode

A remarkable performance feature of the BIGT is that it provides soft turn-off behavior under all operating conditions in both transistor and diode modes. The optimized collector P+ doping profiles will ensure that during the turn-off tail in both modes, the passing electrons towards the N+ regions will induce a large potential across the collector PN junction forcing a controlled level of hole injection into the base region. The main advantage of this method lies in the fact that normally the diode silicon specification does not match the IGBT silicon for obtaining soft recovery performance. Thus, such conflicting requirements could result in diode mode snappy behavior in an integrated structure. However, in a BIGT, soft behavior is granted under all conditions for both the transistor and diode modes even under extreme conditions as shown in figure (3) and (4) respectively.

3.3kV-62.5A BIGT in transistor mode turn-off softness test

3.3kV-62.5A BIGT in diode mode reverse recovery softness test

3300V BIGT Module Results

High current 3.3kV BIGT (140 x 130)mm modules were fabricated and tested under conditions similar to those applied to state-of-the-art IGBT modules. The BIGT module contained 24 BIGT chips for the estimated current rating of 1500A. The nominal transistor mode switching characteristics of the BIGT modules are shown in figure (5) along with the associated switching losses at 125°C. The BIGT diode mode reverse recovery performance is mirrored in the turn-on waveforms. The freewheeling reverse recovery losses were approximately 2.3J. The SOA performance of the BIGT in transistor and diode mode at a high DC link voltage is shown in figure (6). Both modes show rugged characteristics similar to the current IGBT and diode modules. Furthermore, the BIGT also provides improved short circuit and softness performance compared to state-of-the-art IGBTs.

3.3kV-1500A BIGT (140x130)mm module nominal turn-on

3.3kV-1500A BIGT (140x130)mm module SOA transistor

The BIGT advantage is clearly demonstrated here since this module can practically replace a similarly rated larger (140 x 190)mm module which normally contains 24 IGBTs and 12 diodes. The larger standard IGBT module has the further disadvantage of employing much less diode area which is normally a limiting factor in rectifier mode of operation and surge current capability. On the other hand, when the larger (140x190)mm module employs only BIGT chips i.e. 36 devices, its rating can potentially reach up to 2250A. Figure (7) shows the targeted scaled output current performance for the BIGT compared to today`s EP-IGBT module at 125°C in both Inverter and Rectifier modes. The curves show that the diode performance is a limiting factor for the standard module approach while for the BIGT module the transistor mode defines the limit. The curves show approximately a 30% increase in output current capability up to 2 kHz with the BIGT technology.

3.3kV (140x190)mm BIGT module performance chart

Finally, the expected thermal cycling load pattern in a BIGT module is shown in figure (8). Due to the fact that no inactive periods are present per IGBT/Diode compared to the standard approach, a lower temperature difference and more efficient thermal utilization of the module will result in better thermal cycling capability and eventually improved reliability performance.

Predicted BIGT thermal load cycling in PWM application

 

 

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