The insulated gate bipolar transistors (IGBTs) combines an easily driven MOS gate and low conduction loss, and is quickly displacing power bipolar transistors as the device of choice for high current and high voltage applications. The balance in tradeoffs between switching speed, conduction loss, and ruggedness is now being finely tuned so that IGBTs are encroaching upon the high frequency, high efficiency domain of power MOSFETs. In fact, the industry trend is for IGBTs to replace power MOSFETs except in very low current applications. Part 1 helps you understand the tradeoffs and helps with IGBT device selection, application and a relatively painless overview of IGBT technology. Part 2 provides an example walkthrough of IGBT datasheet information.
How to select an IGBT
This section is intentionally placed before the technical discourse. Answers to the following set of burning questions will help determine which IGBT is appropriate for a particular application. The differences between Non Punch-Through (NPT) and Punch-Through (PT) devices as well as terms and graphs will be explained later.
1. What is the operating voltage? The highest voltage the IGBT has to block should be no more than 80% of the VCES rating.
2. Is it hard or soft switched? A PT device is better suited for soft switching due to reduced tail current, however a NPT device will also work.
3. What is the current that will flow through the device? The first two numbers in the part number give a rough indication of the usable current. For hard switching applications, the usable frequency versus current graph is helpful in determining whether a device will fit the application. Differences between datasheet test conditions and the application should be taken into account, and an example of how to do this will be given later. For soft switching applications, the IC2 rating could be used as a starting point.
4. What is the desired switching speed? If the answer is "the higher, the better", then a PT device is the best choice. Again, the usable frequency versus current graph can help answer this question for hard switching applications.
5. Is short circuit withstand capability required? For applications such as motor drives, the answer is yes, and the switching frequency also tends to be relatively low. An NPT device would be required. Switch mode power supplies often don't require short circuit capability.
An N-channel IGBT is basically an N-channel power MOSFET constructed on a p-type substrate, as illustrated by the generic IGBT cross section in Figure 1. (PT IGBTs have an additional n+ layer as well as will be explained.) Consequently, operation of an IGBT is very similar to a power MOSFET. A positive voltage applied from the emitter to gate terminals causes electrons to be drawn toward the gate terminal in the body region. If the gate-emitter voltage is at or above what is called the threshold voltage, enough electrons are drawn toward the gate to form a conductive channel across the body region, allowing current to flow from the collector to the emitter. (To be precise, it allows electrons to flow from the emitter to the collector.) This flow of electrons draws positive ions, or holes, from the p-type substrate into the drift region toward the emitter. This leads to a couple of simplified equivalent circuits for an IGBT as shown in Figure 2.
Figure 1 N-Channel IGBT Cross Section
Figure 2 IGBT Simplified Equivalent Circuits
The first circuit shows an N-channel power MOSFET driving a wide base PNP bipolar transistor in a Darlington configuration. The second circuit simply shows a diode in series with the drain of an N-channel power MOSFET. At first glance, it would seem that the on state voltage across the IGBT would be one diode drop higher than for the N-channel power MOSFET by itself. It is true in fact that the on state voltage across an IGBT is always at least one diode drop. However, compared to a power MOSFET of the same die size and operating at the same temperature and current, an IGBT can have significantly lower on state voltage. The reason for this is that a MOSFET is a majority carrier device only. In other words, in an Nchannel MOSFET only electrons flow. As mentioned before, the p-type substrate in an N-channel IGBT injects holes into the drift region. Therefore, current flow in an IGBT is composed of both electrons and holes. This injection of holes (minority carriers) significantly reduces the effective resistance to current flow in the drift region. Stated otherwise, hole injection significantly increases the conductivity, or the conductivity is modulated. The resulting reduction in on state voltage is the main advantage of IGBTs over power MOSFETs.
Nothing comes for free of course, and the price for lower on state voltage is slower switching speed, especially at turn-off. The reason for this is that during turn-off the electron flow can be stopped rather abruptly, just as in a power MOSFET, by reducing the gate-emitter voltage below the threshold voltage. However, holes are left in the drift region, and there is no way to remove them except by voltage gradient and recombination. The IGBT exhibits a tail current during turn-off until all the holes are swept out or recombined. The rate of recombination can be controlled, which is the purpose of the n+ buffer layer shown in Figure 1. This buffer layer quickly absorbs trapped holes during turn-off. Not all IGBTs incorporate an n+ buffer layer; those that do are called punch-through (PT), those that do not are called non punch-through (NPT). PT IGBTs are sometimes referred to as asymmetrical, and NPT as symmetrical.
The other price for lower on state voltage is the possibility of latchup if the IGBT is operated well outside the datasheet ratings. Latchup is a failure mode where the IGBT can no longer be turned off by the gate. Latchup can be induced in any IGBT through misuse. Thus the latchup failure mechanism in IGBTs warrants some explanation.