By Akhil Nair,
Technical Marketing Manager for IGBTs, ON Semiconductor

Insulated gate bipolar transistor (IGBT) devices are the key elements of high performance power conversion (HPPC) circuits. Power MOSFETs are generally used in medium and low power applications, while IGBTs with voltage ratings exceeding 1 kV are optimized for use in environments where voltage ratings exceeding 1 kV are required. Improving system efficiency and reducing losses using these devices is a major concern for engineers. This topic will be discussed in detail below.

Technologies such as gallium nitride (GaN) or silicon carbide (SiC) are too expensive. The unit price is too high for most applications in terms of cost reduction effects, and it remains a problem. In order to expect a price reduction for widespread use of this technology, economies of scale are required in addition to operational stabilization, so general commercialization of GaN/SiC technology is expected to take 5-8 years. Therefore, this method is not yet judged to have a high possibility of commercialization, and an alternative solution is urgently needed.

Therefore, rather than developing new materials, the semiconductor industry must find ways to improve the performance of existing IGBT technology. The question is, 'What should we do?'

Application Demand

High-power IGBT devices must be developed to support the increasing demand for higher frequencies, reaching at least 20 kHz for high-performance power conversion (HPPC) applications. This performance is expected to be applied in applications such as hybrid/electric vehicles (HEVs), solar infrastructure, uninterruptible power supply (UPS) devices, and HEV charging devices. These applications require significant power efficiency, especially at high frequencies (up to 95% in many cases). This means that the conversion losses of conventional IGBT technology must be reduced, depending on the application requirements.

Solving thermal issues is another important aspect of IGBT technology that should not be overlooked. In today's complex powertrain designs, space is often quite limited. By improving switching performance and system efficiency, the space required for heat sinking mechanisms can be minimized. This allows for greater board space utilization, reduced material costs, and improved reliability.

The only way to improve the performance characteristics of IGBTs while maintaining silicon technology is to take a new approach to IGBT design from various angles. There are two criteria that have the greatest impact on IGBT operation definition and power efficiency. The first is total switching loss (Ets), which is the sum of turn-on (Eon) and turn-off (Eoff) switching losses. IGBT switching losses are particularly critical when the device is operated at high switching frequencies and/or high operating temperatures.

The second is the conduction loss due to the Collector-Emitter Saturation Voltage (VCEsat). There is an inevitable trade-off between switching loss performance and conduction loss performance. Design engineers need to think about the whole rather than looking at the details in isolation, so they refer to the following figure of merit (FoM):

FoM= VCEsat x Ets

Figure of Merit (FoM) provides engineers with a consistent way to benchmark IGBT performance and compare devices manufactured by different vendors.

About IGBT switching losses
As described above, IGBT switching performance is basically composed of two factors: Eon and Eoff.
The Eoff of IGBT can be reduced in two ways:

a, by increasing the speed of the device... (i.e. reducing the transition time from high speed to low speed)
b, by blocking the tail current

Figure 1 below introduces two methods to reduce Eoff using the aforementioned methods. Here, we can see that the gray waveform increases sharply, which reduces Eoff by increasing the speed of the device (ultra-fast di/dt). This is not only bad in terms of EMI, but also causes the generated vibration to increase losses and cause additional losses in the device when the peak voltage exceeds the breakdown voltage. Therefore, an alternative method (green waveform) that allows for reduced losses while maintaining a natural and smooth turn-off is clearly preferable.


Figure 1 : Two approaches to reducing Eoff

Eoff is considered as an important measure of IGBT technology in most cases, but Eon is not given much meaning because it is operated by diodes co-packed together. Despite this fact, when considering the device as a whole and considering its impact on system performance, Eon is a factor that should never be overlooked because it has a significant impact on system loss. As shown in Figure 2, reducing the maximum reverse recovery current (IRRM) and reverse recovery time (TRR) improves the diode reverse recovery performance. Since a TRR value that is too low will result in ringing (a transient oscillation at the output caused by rapid changes in the input) and a very high dIRRM/dT value, the best way to reduce the reverse recovery loss (and by extension the Eon of the device) is to reduce the IRRM.



Figure 2 : Fast switching causing ringing

Limitations of IGBT Manufacturing

In fact, most IGBTs are designed as “vertical” devices, i.e., the emitter and gate terminals are formed on one side of the wafer substrate and the collector terminal on the other side. The FoM of a device is inversely proportional to the thickness of the wafer it is made of. That is, a thinner wafer can be used to achieve a higher FoM for an IGBT. Modern IGBTs are typically developed with wafers <200µm.

Fundamentally, the simplest way to improve the FoM of an IGBT is to reduce its thickness. However, using thin wafers is quite an adventure. Handling wafers as thin as a human hair is a major issue in the manufacturing process, but reducing the thickness to further lower the breakdown voltage is even more important.

Most modern IGBT technologies use field stop technology, which reduces thickness without changing the breakdown voltage. This technology utilizes a buffer layer (field stop layer) between the collector layer and the silicon, which not only allows for a higher breakdown voltage but also helps reduce the Eoff of the IGBT. Although a thick field stop layer is advantageous in terms of device performance, the silicon compression technology that must be used to compress it into one is not so simple.

Next generation IGBT solutions

ON Semiconductor has introduced a series of 1200 V IGBTs with ultra-high FoM using its own UltraField Stop (UFS) trench technology. The reference 1200 V/40 A device with UFS technology boasts 40% lower ETS and 15% lower VCEsat than most of the devices currently dominating the market. This means that these devices are high-performance products that reduce system energy loss and increase power efficiency.

UFS devices are mounted on SIO (silicon on insulator) surface. UFS is also the first product to use 105 µm thick wafers for 1200V IGBTs. Of course, ultra-fine devices require thicker field stop layers. By using thick field stop layers and thin wafers, these devices exhibit excellent FoM.

Another contributor to the performance improvement of these devices is the high-performance platinum-silicon-based diode co-packaged with the IGBT elements, which is specifically optimized to operate in conjunction with the UFS technology. This enables the desired reduction in reverse recovery losses without ringing. Figure 3 shows the power loss difference in a standard half-bridge inverter between the industry standard FS2 technology and ON Semiconductor’s UFS devices. Detailed application conditions for each measurement item are shown in the figure. Demonstrates how power loss is reduced through UFS technology.


Figure 3 : Improved efficiency and power loss using UFS technology

With GaN or SiC adoption and commercialization still a few years away, power IC manufacturers must provide high-voltage switching devices based on silicon processes to the application market, and IGBT technology is expected to be a major driving force. Along with advances in fast-recovery diode technology, new IGBT technologies such as UFS are evolving power efficiency to meet future application demands.