S. Cordes, H. Preis, L. Lorenz Infineon Technologies AG St.-Martinstr. 76 81541 München
Introduction : The key component for power Electronic applications – the power switch - is still a semiconductor with high development potential. Starting with switching frequencies up to 10kHz the dominant power-losses occur during conduction. Thereforethe InsulatedGate-Bipolar-Transistor (IGBT) was developed. Compared to the MOS-Field-Effect-Transistor (MOSFET) the IGBT has lower conduction losses but higher switching losses. From the first IGBT generation IGBT-1 the next generation IGBT-2 was developed in a way that enhanced short circuit withstand time – up to 10µs – became possible. For the IGBT-2 generation the conduction and switchinglosses are equally shared at a switching frequency off about 15kHz. To achieve the best performance for each application there has to be a separate IGBT concept for applications with low switching frequency and for applications with high switching frequency. One of the application specific IGBT is Infineons latest IGBT development - the IGBT³. Compared to the IGBT-2 this IGBT shows a massivereduction in the conduction losses while the switching losses stay the same. On the other hand for the applications with high switching frequencies the IGBT is optimized in a way to reduce the switching losses. In future Infineon will offer three Types of discrete IGBT’s : • The 600V High Speed IGBT is recommended for the high frequency power switching applications with switching frequencies between 40kHzand 100kHz. Possible applications are welding machine and switch mode power supply. • The Fast IGBT in 600V and 1200V version is the best choice for general purpose Applications - with switching frequencies below 40kHz. The upcoming 1200V IGBT³ will be the ideal power switch for the motor-drive Applications – with switching frequencies below 30kHz.
PT IGBT : The IGBT combines in it all theadvantages of the bipolar Transistor and MOSFET. Fig. 1 shows the cross section of the PT IGBT.
Fig. 1 : Cross section of the PT IGBT. The additional p zone of the IGBT backside – the Collector – is the only difference compared to the MOSFET. Due to the presence of this layer, holes are injected into the highlyresistive n layer and a carrier overflow is created. This increase in conductivity of the n layer allows to reduce the on-state saturation voltage VCEsat of the IGBT. For a MOSFET the relative high on state resistance RDSon is mainly influenced by a low doped center region, which is essential for the voltage blocking capability. The additional p layer of the IGBT causes a carrier overflow in the centerregion reducing the “RDSon”. In spite of the threshold voltage which is created by the pn junction at the collector side, a 1000V IGBT has an “RDSon”, which is reduce by a factor of 5
compared to a MOSFET with similar blocking characteristics and identical Chip area. During switch off the IGBT the carriers stored in the n layer have to be removed in order to block the collector-emittervoltage VCE. The holes are extracted through the collector while the electrons have to be extracted through the Emitter. This process takes time and results in the current tail which causes high switch off losses compared to the MOSFET. To adjust the switch off behavior the carrier life time has to be adjusted. For this “life time killing” process, diffusion with platinum or irradiation in order tocreate disorder in the silicon lattice, has to be done. Due to this process the temperature coefficient of the Collector Emitter voltage VCE is negative. Referred to the off-state the IGBT has to support an electric field in order to block the Collector Emitter voltage. This electric field starts at the gate structure, punches trough the + n layer and is stopped in the n Buffer layer as can be...