IGBT Performance Reaches New Levels

IGBTs (Integrated Gate Bipolar Transistors) have now been around for 30 years or so and have played a very useful role in their capacity as a key component in power switches, particularly at high voltages. However, the use of IGBTs has been eclipsed somewhat by power MOSFETs for applications where the switching frequency is at the high end of the power spectrum – greater than 100 kHz. However, that has now changed. IGBT research and development has come up with improvements in IGBT design which mean that these components are now able to handle the frequency range and temperatures which are being demanded by more and more applications.

The new IGBT design is an extremely thin IGBT, fabricated on a wafer structure, which is reported to have a blocking voltage of 650 volts. It is designed for DC to DC conversions of up to 200 kHz. These ultra rapid operating IGBTs are now more than a match for their competitors in the high end semiconductor market.

The superiority of SJ MOSFETS challenged by new version IGBTs

Up to the recent development phase of these ultra thin wafer IGBTs, superjunction (SJ) MOSFETS have been the semiconductor of choice for those applications requiring tolerance to both high temperature and high switching frequency. SJ MOSFETS have actually had a better performance record for these sorts of applications than conventional power MOSFETS as well as IGBTs which have lagged behind in performance. The new IGBT design is touted as matching the performance level of SJ MOSFETS in terms of switching capacity but the main advantage is that the manufacturing process is simpler, making these components much more competitively priced compared to their competitors.

Another comparison between the new IGBTs and SJ MOSFETS gives the IGBTs a clear lead in terms of their Tjmax, which is 175oC, compared to that of the SJ MOSFETS at 150oC.

IGBT structure improvements

The key improvement of these IGBTs is their fabrication process. They are fabricated on an extremely thin wafer of around 70μm using a punch through structure. This design permits the components to incorporate a collector which is only lightly doped. This means that they have less stored charge and consequently much improved switching performance – as has already been mentioned. They are rated up to 200 kHz, which is a doubling of the performance of their predecessors.

One of the inherent problems inherent in older style IGBTs was the fact that electron irradiation or metal doping was used to enhance switching speed. The design has the in-built problem that as the operating temperature increases, the current tends to leak. This limitation has meant that IGBT have had a limited usage role when the Tjmax has been above anything like 150oC. The improvements in design have meant that temperatures up to 175oC and beyond are now tolerated.

At this temperature the leakage of current is very much reduced compared to older style GBT designs. The new style IGBTs have a noticeably higher cell density which has a knock on effect on various other component properties. The lower voltage drop also accompanies a smaller gate capacitance. This combination means that the minimal internal gate resistance still means that the components are capable of the reported doubling of switching speed.

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