Application-Specific Power Semiconductors Part 1 - Attack of the IGBT

Application-specific IGBTs are taking a major change in direction like any other power devices and adopting distinct bifurcated approaches. The first pertains to innovations improving the silicon and process technologies to overcome the limitations of the current technology and the other approach is towards packaging and the driving of these devices. These IGBTs are targeted for induction heating (IH), motion control, uninterruptible power supplies (UPS), welding, steel cutting, switched mode power supplies (SMPS) and renewable energy (wind power & solar inverter) market,etc. The demand for electricity is increasing and at the same time, the cost of power generation is also going up. There is increasing pressure from governmental agencies to reduce the emission of harmful gases. This is forcing equipment designers to increase efficiency and performance. Governmental agencies will set new minimum efficiency limits. Just one IGBT technology is not suitable for all of the above applications. Each application needs to use its own application-specific IGBT. This is forcing device designers to design application-specific IGBTs. Each application needs its own unique topology variation. In all these topologies, device parameters play a vital role to improve circuit efficiency and performance. Fairchild Semiconductor provides application-specific IGBTs for many types applications.

The insulated gate bipolar transistor (IGBT), also known as the conductivity-modulated field-effect transistor, is one of the most commonly available advanced switching power devices in the market today. This IGBT structure is very similar to that of MOSFET. This is because a MOSFET IGBT is also a voltage-controlled device. One difference between these two devices is the starting material. The starting material for a MOSFET is N+, where as for an IGBT it is P+. Generally the MOSFET has high resistive n-epi region so conduction losses are high. For an IGBT, on the other hand, the n-epi region is placed on P+ substrate forming a p-n junction where conductivity modulation takes place and conduction losses are reduced. This is shown as variable resistance R_DRIFT shown in the first figure.

The R_DRIFT is also known as modulation resistance. An IGBT is available right from a 300V voltage rating to several kilo-volts. The IGBT has a high forward conduction current density and very low drive since it is a voltage-controlled device like a MOSFET. It has significantly superior characteristics for low and medium switching frequency and some of the Fairchild Semiconductor 300V & 600V IGBTs can be used in applications up to 100 kHz. Generally, a 600V IGBT has lower conduction losses compared to a 600V MOSFET at high current operating condition. The IGBT is a minority carrier device compared to a MOSFET, which is a majority carrier device. The recombination of minority carriers at turn-off accounts for current tail and, in turn, increases turn-off losses. Due to this, there is a tradeoff between IGBT and MOSFET applications. Generally at low frequency the turn-off losses become less of an issue so it is better to choose a device that has lower conduction losses for applications such as motor drives, UPS, welding and low frequency PFC applications, etc. IGBT is ideal for these applications.