Insulated Gate Bipolar Transistor (IGBT) – Structure, Operation & Applications

IGBT is a voltage controlled device. It has high input impedance like a MOSFET and low on-state conduction losses like a BJT. In other words, this power device combines the voltage control of the MOSFET gate with the superior conduction characteristics of the bipolar device. Nonetheless, the IGBT utilizes injected-charge modulation in the base region and due to the need for this charge to be extracted or extinguished at turn-off, it has higher switching losses compared to the MOSFET. But as it can be realized at much higher power ratings it has become the power device of choice for a wide range of medium to high power electronic applications. The key advantages of the IGBT are the simplicity with which it can be driven (i.e. comparable to a power MOSFET), its lower on-state conduction losses and the capability of switching high voltages. These features in addition to the ability of IGBT to survive a wide reverse bias safe operating area make it superior to the power MOSFET in high-voltage applications. Usually, IGBTs are used for switching circuits requiring high voltage (up to 3300 V) and high current (up to 3000 A) with a switching frequency of the order 1-40 kHz.

The Structure of IGBT

The figure below shows a basic silicon cross-section of an IGBT. Its construction is similar to MOSFET except that the n⁺ layer at the drain in a power MOSFET is replaced by P⁺ substrate called collector.

The IGBT has three terminals: gate (G), collector (C) and emitter (E).

IGBT Symbol

The Operation of IGBT

With collector and gate voltage positive with respect to emitter the device is in forward blocking mode. When the gate to emitter voltage becomes greater than the threshold voltage of IGBT, an n-channel is formed in the p-region. Now the device is in forward conducting state. In this state p⁺ substrate injects holes into the epitaxial n^– layer. The increase in collector to emitter voltage will result in an increase of injected hole concentration and finally a forward current is established.

Characteristics of IGBT

The figure below shows the circuit diagram that can be used to obtain the characteristics of an IGBT.

Output Characteristics of IGBT

An output characteristic is a plot of collector current I_c versus collector to emitter voltage V_CE for given values of gate to emitter voltage V_GE.

Transfer Characteristics of IGBT

A plot of collector I_C versus gate-emitter voltage V_GE for a given value of V_CE gives the transfer characteristic. This is illustrated in the figure below:

Controlling parameter is the gate-emitter voltage V_GE in IGBT. If V_GE is less than the threshold voltage V_T then IGBT is in OFF state. If V_GE is greater than the threshold voltage V_T then the IGBT is in ON state.

Switching Characteristics of IGBT

The figure below illustrates the switching characteristics of an IGBT. Turn-on time consists of delay time t_d(on) and rise time t_r.

The turn on delay time is the time required by the leakage current I_CE to rise to 0.1 I_C where I_C is the final value of the collector current. Rise time is the time needed for the collector current to rise from 0.1 I_C to its final value of I_C. After turn-on collector-emitter voltage V_CE will be very small during the steady state conduction of the device.

The turn-off time comprises of delay off time t_d(off) and fall time t_f. Off time delay is the time during which the collector current falls from I_C to 0.9 I_C and V_GE falls to threshold voltage V_GET. During the fall time t_f the collector current falls from 0.90 I_C to 0.1 I_C. During the turn-off time interval collector-emitter voltage rises to its final value V_CE.

IGBT Applications

IGBTs are used in medium power applications such as DC and AC motor drives, medium power supplies, servo controls, solid state relays and contactors, general purpose inverters, robotics, cutting tools, induction heating,  welding equipment, and so forth.

Typical Ratings of IGBT

• Voltage rating = 1400 V.
• Current rating = 600 A.
• Maximum operating frequency = 20 kHz.
• Switching time ≈ 2.3 μs (t_ON ≈ t_OFF).
• ON state resistance = 600 m𝝮 = 60 x 10^-3𝝮.

Conclusion

To sum up, insulated gate bipolar transistors (IGBTs) are voltage controlled power transistors. They are faster than BJTs but not somewhat as fast as MOSFETs. The IGBTs provide much superior drive and output characteristics when compared to BJTs. Generally, IGBTs are appropriate choice for high voltage, high current and frequencies up to 20 kHz. Some of the IGBTs commercially available are up to 1400 V, 600 A and 1200 V, 1000 A.

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