An ideal semiconductor switching device is the Insulated Gate Bipolar Transistor, also known as an IGBT for short. It is a kind of hybrid between a conventional Bipolar Junction Transistor (BJT) and a Field Effect Transistor (MOSFET).

The MOSFET and bipolar transistor’s high input impedance and fast switching rates and low saturation voltage are combined in the IGBT transistor to create a new type of transistor switching device that can handle large collector-emitter currents with almost no gate current drive.

Typical IGBT (Insulated Gate Bipolar Transistor)

The Insulated Gate Bipolar Transistor (IGBT), as its name implies, combines the insulated gate (hence the first part of its name) technology of the MOSFET with the output performance properties of a conventional bipolar transistor.

The “IGBT Transistor” is the end product of this hybrid arrangement,. And it has the output switching and conduction characteristics of a bipolar transistor while being voltage-controlled like a MOSFET. IGBTs are primarily used in power electronics applications, such as inverters, converters, and power supplies,. Where power bipolars and power MOSFETs are unable to fully satisfy the demands of the solid state switching device. Power MOSFETs may have faster switching speeds than high-voltage and high-current bipolars, but the latter are more expensive and challenging to produce. High-current and high-voltage bipolars are available, but their switching speeds are slow.

The insulated gate bipolar transistor device has an advantage over a BJT or MOSFET because it provides more power gain than a typical bipolar type transistor while also operating at higher voltages and with lower input losses. In actuality, it is a Darlington type configuration where a FET is integrated with a bipolar transistor.

Insulated Gate Bipolar Transistor

As we can see, the insulated gate bipolar transistor is a three terminal, transconductance component that joins an insulated gate N-channel MOSFET input with a PNP bipolar transistor output connected in a manner akin to the Darlington configuration. The terminals are therefore identified as Collector, Emitter, and Gate. The conductance path through which current travels is connected to two of its terminals (C-E), while its third terminal (G) controls the device. The ratio of the output signal to the input signal determines how much amplification the insulated gate bipolar transistor achieves. The amount of gain for a typical bipolar junction transistor (BJT) is roughly equal to the Beta (output current to input current ratio).

The gate of a metal oxide semiconductor field effect transistor, or MOSFET, is isolated from the primary current-carrying channel, so there is no input current. As a result, a FET and an IGBT are both transconductance devices because their gain is equal to the ratio of output current change to input voltage change. After that, we can treat the IGBT as a power BJT whose MOSFET supplies the base current. Similar to BJT or MOSFET type transistors, the Insulated Gate Bipolar Transistor can be used in small signal amplifier circuits. But as the IGBT combines the low conduction loss of a BJT with the high switching speed of a power MOSFET an optimal solid state switch exists which is ideal for use in power electronics applications.

Also, the IGBT has a much lower “on-state” resistance, R_ON than an equivalent MOSFET. This means that the I²R drop across the bipolar output structure for a given switching current is much lower. The forward blocking operation of the IGBT transistor is identical to a power MOSFET.

The insulated gate bipolar transistor has voltage and current ratings. That are comparable to those of the bipolar transistor when used as a static controlled switch. However, an IGBT is much easier to drive than a BJT because it has an isolated gate, requiring much less drive power. A simple way to “ON” or “OFF” an insulated gate bipolar transistor is to activate and deactivate its Gate terminal. The device remains “ON” when a positive input voltage signal is applied across the Gate and the Emitter,. But turns “OFF” when the input gate signal is zero or slightly negative, much like an eMOSFET or a bipolar transistor. Another advantage of the IGBT is that it has a much lower on-state channel resistance than a standard MOSFET.

IGBT Characteristics

Unlike BJTs, which need the base current to be continuously supplied in a sufficient amount to maintain saturation,. IGBTs are voltage-controlled devices and only need a small voltage on the gate to maintain conduction through the device. In addition, unlike MOSFETs, which have the ability to switch current in both directions (controlled in the forward direction and uncontrolled in the reverse direction),. IGBTs can only switch current in the “forward direction,” or from Collector to Emitter.

The N-channel power MOSFET and the insulated gate bi-polar transistor have very similar operating principles and gate drive circuits. The primary distinction is that an IGBT has a much smaller main conducting channel resistance. When current is flowing through the device in the “ON” state. Due to this, when compared to an equivalent power MOSFET, the current ratings are significantly higher.

Insulated Gate Bi-polar Transistor applications include moderate speed, high voltage pulse-width modulated (PWM), variable speed control, switch-mode power supplies, and solar powered DC-AC inverter and frequency converter applications. These applications benefit from the Insulated Gate Bi-polar Transistor’s high voltage capability, low ON-resistance, ease of drive, relatively fast switching speeds,. And combined with zero gate drive current.

A general comparison between BJT’s, MOSFET’s and IGBT’s is given in the following table.

IGBT Comparison Table

As we have seen, the Insulated Gate Bi-polar Transistor is a semiconductor switching device. That controls like a metal oxide field effect transistor (MOSFET). While having the output characteristics of a bi-polar junction transistor (BJT). The ease with which the IGBT transistor can be switched “OFF” by making the gate signal zero. Or slightly negative allows it to be used in a variety of switching applications. This is one of the main benefits of the IGBT transistor. For use in power amplifiers, it can also be driven in its linear active region.

The Insulated Gate Bi-polar Transistor is the best choice for driving inductive loads like coil windings, electromagnets,. And DC motors due to its lower on-state resistance, conduction losses,. And ability to switch high voltages at high frequencies without being damaged.