“Insulated gate bipolar transistor (IGBT) is a minority carrier device with high input impedance and large bipolar current carrying capacity. Many designers regard IGBT as a device with MOS input characteristics and bipolar output characteristics. It is a voltage-controlled bipolar device. In order to take advantage of power MOSFET and BJT, IGBT is introduced. It is a functional integration of power MOSFET and BJT device in monolithic form.It combines the best attributes of the two to achieve the best device characteristics[2].

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Insulated gate bipolar transistor (IGBT) is a minority carrier device with high input impedance and large bipolar current carrying capacity. Many designers regard IGBT as a device with MOS input characteristics and bipolar output characteristics. It is a voltage-controlled bipolar device. In order to take advantage of power MOSFET and BJT, IGBT is introduced. It is a functional integration of power MOSFET and BJT device in monolithic form.It combines the best attributes of the two to achieve the best device characteristics[2].

IGBTs are suitable for many applications in the field of power electronics, especially in pulse width modulation (PWM) servos and three-phase drives that require high dynamic range control and low noise. It can also be used in uninterruptible power supply (UPS), switching power supply (SMPS) and other power circuits that require high switching repetition frequency. IGBT improves the dynamic performance and efficiency, and reduces the audible noise level. It is also applicable to resonant mode converter circuits. The optimized IGBT can provide low conduction loss and low switching loss.

The main advantages of IGBT over power MOSFET and BJT are:

Due to conductivity modulation, it has a very low on-state voltage drop and an excellent on-state current density. Therefore, a smaller chip size is possible, and the cost can be reduced.

Due to the input MOS gate structure, the driving power is low and the driving circuit is simple. Compared with current control devices (thyristors, BJTs) in high voltage and high current applications, it is easier to control.

Broad SOA. Compared with bipolar transistors, it has excellent current conduction capabilities. It also has excellent forward and reverse blocking functions.

basic structure

Figure 1 shows the basic schematic diagram of a typical N-channel IGBT based on DMOS technology. This is one of several possible structures for this device. Obviously, except for the P + injection layer, the silicon cross-section of the IGBT is almost the same as the silicon cross-section of the vertical power MOSFET. It shares a similar MOS gate structure and P well with the N + source region. The top N + layer is the source or emitter, and the bottom P + layer is the drain or collector. It is also feasible to make a P-channel IGBT, for this reason, the doping profile in each layer will be reversed. The IGBT has a parasitic thyristor including a four-layer NPNP structure. The conduction of this thyristor is undesirable.

Schematic diagram of general-purpose N-channel IGBT[2]

Some IGBTs without an N+ buffer layer are called non-punch-through (NPT) IGBTs, and IGBTs with this layer are called punch-through (PT) IGBTs. If the doping level and thickness of the layer are appropriately selected, the presence of the buffer layer can significantly improve the performance of the device. Despite the physical similarities, the operation of the IGBT is closer to the operation of the power BJT than the operation of the power MOSFET. This is because the P + drain layer (injection layer) is responsible for injecting minority carriers into the N- drift region and causing conductivity modulation.

Equivalent circuit model of IGBT[2]

According to this structure, a simple IGBT equivalent circuit model can be drawn, as shown in Figure 2. It contains MOSFET, JFET, NPN and PNP transistors. The collector of the PNP is connected to the base of the NPN, and the collector of the NPN is connected to the base of the PNP through a JFET. NPN and PNP transistors represent parasitic thyristors that form a regenerative feedback loop. The resistor RB represents the base-emitter short circuit of the NPN transistor to ensure that the thyristor will not lock up, which will cause the IGBT to lock up. JFET represents the current contraction between any two adjacent IGBT cells. It supports most voltages and allows the MOSFET to be a low-voltage type, so the RDS(on) value is low. The circuit symbol of IGBT is shown as in Fig. 3.

IGBT circuit symbol

Operation mode

When a positive voltage is applied between the collector to the emitter terminal and the gate is short-circuited to the emitter, as shown in Figure 1, the device enters the forward blocking mode, and the junctions J1 and J3 are forward biased, and the junction J2 is reverse biased. The depletion layer partially extends into the P base region and the N drift region on both sides of the junction J2.

When a negative voltage is applied across the collector-emitter shown in Figure 1, the junction J1 becomes reverse biased, and its depletion layer extends to the N-drift region. The breakdown voltage during the reverse blocking period is determined by the open base BJT formed by the P + collector / N-drift region / P base region. If the doping of the N-drift region is very light, the device can easily break down. The required reverse voltage capability can be obtained by optimizing the resistivity and thickness of the N-drift region.