IGBT (Insulated Gate Bipolar Transistor) is a power semiconductor device widely used in fields such as rail transportation, smart grids, industrial energy conservation, electric vehicles, and new energy equipment. It features energy-saving, easy installation, easy maintenance, and stable heat dissipation. It serves as a core device for energy conversion and transmission. In simple terms, IGBT can be considered a combination of a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) and a BJT (Bipolar Junction Transistor). It integrates the gate-controlled properties of a MOSFET (high input impedance) with the ability to handle high currents like a BJT (voltage-controlled bipolar device). So, what does the internal structure of such a combination look like?

I. Detailed Explanation of the IGBT Module

Let's take the FF1400R17IP4 IGBT module as an example. The external appearance and equivalent circuit of the module are shown in Figure 1. The module has dimensions of 25 cm x 8.9 cm x 3.8 cm. The module contains two IGBTs, which we commonly refer to as a half-bridge module. The rated voltage and current for each IGBT are 1.7 kV and 1.4 kA, respectively.

Figure 1. FF1400R17IP4

Parts 8, 9, 10, 11, and 12 are power terminals that need to be connected to the power circuit. Parts 1, 2, 3, 4, and 5 are auxiliary control terminals that need to be connected to the gate drive circuit. Parts 6 and 7 are NTC thermistors used for temperature detection or over-temperature protection. After gaining a basic understanding of its structure, what can we do with this black module structure? Let's take an example from our daily lives: electric buses, which most people are familiar with. Three such black modules can be used as a three-phase motor driver. If equipped with a battery, it can drive an electric bus. Of course, this module is also used in many other applications.

Figure 2. IGBT in an electric bus

II. Internal Structure of the IGBT

After gaining a preliminary understanding of the external structure and application of the IGBT module, let's dive into the main topic and explore the internals of this high-tech black module. Figure 3 shows the inside of an IGBT module after removing the black outer casing. It's worth noting that the most common materials inside the IGBT module are copper and aluminum.

Figure 3. Internal structure of the IGBT

Figure 4 is a cross-sectional view of the IGBT module. After removing the black casing and external connection terminals, the IGBT module mainly contains a heat dissipation substrate, a DBC substrate, and silicon chips (including IGBT chips and diode chips), while the rest primarily consist of solder layers and interconnection wires used to connect the IGBT chips, diode chips, power terminals, control terminals, and DBC (Direct Bond Copper). Below is a brief introduction to each part.

Figure 4. Cross-sectional view of the IGBT

Heat Dissipation Substrate
The bottom of the IGBT module is the heat dissipation substrate, whose main purpose is to quickly dissipate the heat generated during the IGBT switching process. Since copper has better thermal conductivity, the substrate is usually made of copper with a thickness of 3-8 mm. Of course, substrates made of other materials, such as aluminum silicon carbide (AlSiC), are also used, each with its own advantages and disadvantages.

DBC
DBC (Direct Bond Copper) is a ceramic surface metallization technology, consisting of three layers. The middle layer is a ceramic insulation layer, with copper layers on both sides, as shown in Figure 5(a). Simply put, it involves covering both sides of the insulation material with a layer of copper, etching a current-carrying pattern on the front side, while the back must be directly welded to the heat dissipation substrate.

Figure 5. DBC Base and PCB

The main function of the DBC is to ensure electrical insulation and good thermal conductivity between the silicon chip and the heat dissipation substrate, while also providing sufficient current transmission capability. The DBC substrate is similar to a two-layer PCB circuit board. The insulation material in the middle of the PCB is typically FR4, while the commonly used ceramic insulation materials for DBC are alumina (Al2O3) and aluminum nitride (AlN). The IGBT module analyzed in this article contains six DBCs, each containing four IGBT chips and two diode chips. Two of the IGBT chips and one diode chip serve as the upper switches, while the others serve as the lower switches, as shown in Figure 6.

Figure 6. DBC Diagram and Equivalent Circuit

IGBT Chip
The IGBT chip model used inside the module is IGCT136T170. The datasheet can be downloaded from Infineon's official website. Figure 7 shows the top view and basic parameters of the IGBT chip. The gate and emitter of the IGBT are located on the top of the chip (front side), and the collector is on the bottom (back side). The chip thickness is 200 micrometers. When the IGBT is powered on, the current flows from bottom to top, so this structure of IGBT can also be referred to as a vertical device.

Figure 7. IGBT Chip Diagram

If a vertical cut is made on the 200-micrometer-thick chip, the internal structure shown in Figure 8 can be obtained, which is composed of P-type and N-type semiconductors with different doping. Figure 8 is a well-known equivalent circuit of an IGBT, typically understood as a MOS-controlled PNP transistor. When first learning about power electronics, you might find this diagram a bit unfamiliar. Why not draw the collector on top and the emitter at the bottom? Until you understand that the current in an IGBT flows from bottom to top, this becomes easy to explain.

Figure 8. IGBT Chip Structure and Equivalent Circuit

Let's now take a brief look at the electrical parameters of this IGBT chip. At 100°C, this chip can handle a direct current of 117.5 A. As shown in Figure 4, the IGBT device inside the module contains a total of 12 IGBT chips, so the total current is 117.5 * 12 = 1412 A, which is consistent with the 1400 A rated current in the IGBT module datasheet.
To ensure current sharing between IGBT chips, an 11.5 Ω resistor is integrated into the gate of each chip. Considering current sharing between DBCs, the two chips on each DBC share a common gate resistor externally, as shown in Figure 10. When measured with a multimeter, the resistance is about 4.13 Ω. You can calculate it in conjunction with Figure 9 to see if it matches the 1.6 Ω value in the IGBT module datasheet. For more detailed IGBT chip parameters, please refer to the official datasheet.

Figure 9. IGBT Equivalent Circuit

Diode Chip
Figure 10 shows the top view of the diode chip, with the anode on the front and the cathode on the back. The current direction of the diode is from top to bottom, opposite to the current direction of the IGBT. The diode chip's rated current is 235 A, with each IGBT consisting of six diodes connected in parallel, resulting in a total current of up to 1410 A, which is consistent with the 1400 A rating in the module datasheet. The diode chip thickness, like that of the IGBT, is also 200 micrometers. For more detailed diode chip parameters, please refer to the official datasheet.

Figure 10. Diode Diagram

It’s impressive how such a thin semiconductor material can handle thousands of volts and hundreds of amps of current switching. This is why high-power semiconductor devices are so expensive. IGBT chips, diode chips, and DBC’s upper copper layer interconnections are typically achieved through bonding wires. The commonly used bonding wires are aluminum wires and copper wires. The aluminum wire bonding process is mature and cost-effective, but its electrical and thermodynamic performance is poor, and the mismatch in expansion coefficients affects the lifespan of IGBTs. The copper wire bonding process offers excellent electrical and thermodynamic performance, high reliability, and is suitable for high power density and high-efficiency heat dissipation modules.

III. Internal Current Flow of IGBT

After gaining a basic understanding of the internal structure of the IGBT module, let's revisit and interconnect all the components mentioned above to see how the current flows inside the IGBT module. Here, we use the upper IGBT in one of the DBCs as an example to explain the current flow. The red color represents the current direction of the upper IGBT (S1 and S2), and the blue color represents the current direction of diode D1. Figure 11(b) is the left cross-sectional view and current direction diagram of module 11(b).

Figure 11(a) IGBT Current

Figure 11(b) IGBT Current

IV. How to Disassemble an IGBT Module?

Some of you may be curious about how to disassemble an IGBT module. It's quite simple: first, remove the external terminals and power terminals, then use a vise to remove the black casing, and finally use a vise to remove the heat dissipation substrate. If you're interested, you can try it yourself.

In this article, we've discussed the structure of the IGBT module. This analysis is still superficial; IGBT technology is highly specialized, involving a lot of theory and complex processes. Hopefully, the content of this article has been somewhat helpful in gaining a basic understanding of IGBT modules.