Structure and working principle of insulated gate bipolar transistor

Structure and working principle of insulated gate bipolar transistor

Bipolar current drive devices such as GTR and GTO have strong withstand voltage and current capacity, but the switching speed is slow, the required drive power is large, and the drive circuit is complicated. However, unipolar voltage drive devices such as power MOSFETs have fast switching speeds, high input impedance, good thermal stability, low drive power required and simple drive circuits. A composite device formed by appropriately combining these two types of devices to complement each other is usually called a Bi-MOS device. Insulated-Gate Bipolar Transistor (IGBT or IGT) developed and produced in the late 1980s belongs to this type of composite device. It combines the advantages of GTR and MOSFET, so it has the advantages of high input impedance, voltage driving type, low driving power, low conduction loss and switching loss caused by saturation voltage drop, large current and voltage capacity, and wide safe working area. The withstand voltage range of IGBT is 600~6500V, and the maximum current of IGBT module can reach 3600A.

Bipolar current drive devices such as GTR and GTO have strong withstand voltage and current capacity, but the switching speed is slow, the required drive power is large, and the drive circuit is complicated. However, unipolar voltage drive devices such as power MOSFETs have fast switching speeds, high input impedance, good thermal stability, low drive power required and simple drive circuits. A composite device formed by combining these two types of devices with each other’s strengths and weaknesses, usually referred to as Bi-MOS devices. At present, IGBTs have gradually replaced the original GTR and GTO markets and become the dominant devices in medium and high-power power electronic equipment.

The structure and working principle of IGBT

IGBT is a three-terminal device, and the three poles are gate G, collector C and emitter E. Figure 1a shows the basic structure of an IGBT composed of an N-channel VDMOSFET and a bipolar transistor. IGBT has one more layer of P+ injection area than VDMOSFET, thus forming a large area P+N junction J1. Therefore, when the IGBT is turned on, the P+ injection region emits minority carriers to the N- drift region, thereby realizing the modulation of the conductivity of the drift region, so that the IGBT has a strong current flow capacity, and solving the contradiction between high withstand voltage and low on-resistance that cannot be solved in the power MOSFET.

Structure and working principle of insulated gate bipolar transistor
Figure 1 – The basic structure, equivalent circuit and graphical symbols of IGBT

The simplified equivalent circuit of the IGBT is shown in Figure 1b. It can be seen from the figure that this is a Darlington structure composed of a bipolar transistor and a MOSFET, which is equivalent to a thick base PNP transistor driven by a MOSFET. In the figure, RN is the modulation resistance in the crystal area. Therefore, the driving principle of an IGBT is basically the same as that of a power MOSFET, and it is a field control device.

IGBT turn-on and turn-off is determined by the voltage uCE between the gate and the emitter. When uCE is positive and greater than the turn-on voltage UCE(th), a channel is formed in the MOSFET, and a base current is provided for the transistor to turn on the IGBT with a small on-state voltage drop. When a reverse voltage or no signal is applied between the gate and the emitter, the channel in the MOSFET disappears and the base current of the PNP transistor is cut off, making the IGBT turn off.

The IGBT formed by the combination of the above-mentioned PNP transistor and N-channel MOSFET is called N-channel IGBT, denoted as N IGBT, and its graphic symbol is shown in Figure 1c. Correspondingly, there is a P-channel IGBT, which is marked as P-IGBT, and its graphic symbol is opposite to the arrow in Figure 1c. As there are many applications of N-channel IGBTs, the following will take them as an example for introduction.