What is IGBT?

IGBT stands for insulated gate bipolar transistor. It is a three-terminal device that is primarily used as an electronic switch which was developed to combine high efficiency and fast switching. IGBT is a conductivity-modulated device and consists of four layers, P-N-P-N which is controlled by a metal oxide semiconductor gate (MOS gate) structure. In the entire device operation range, the thyristor is completely suppressed and only the transistor action is permitted although the structure of IGBT is similar to the thyristor with a "MOS" gate. In higher power module, it has many applications. It is used in switching power supplies such as variable frequency drives, trains, electric cars, lamp ballasts, variable speed refrigerators, air conditioners, and arc welding machines. It is also used in switching amplifiers in industrial control systems and sound systems since it is designed to turn off and on rapidly because the IGBT can synthesize complex waveforms with low pass filters and pulse width modulation.

Device structure

Similar to an n-channel vertical construction power MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), an IGBT cell is constructed but only the exception is that the n+ drain is replaced by a p+ collection layer which forms a vertical PNP bipolar junction transistor. A cascade connection is created by this p+ layer of a PNP bipolar junction with the surface N-channel power MOSFET.

History

The MOSFET was invented by Mohammad M. Atalla and Dawon Kahng in 1959 at Bell Labs. The IGBT mode of operation which was basically where a MOSFET drives the PNP transistor was first proposed by K. Yamagami and Y. Akagiri of Mitsubishi Electric in 1968. In 1977 B. Jayanta Baliga submitted a patent in the general electric (GE) describing a power semiconductor device with the IGBT mode of operation which includes the MOS gating of thyristors, a VMOS(V-groove) structure with four-layer and the use of MOS gated structure to control a four-layer discrete semiconductor device. In 1978 with the assistance of Margaret Lazzeri he began fabricating the IGBT device and successfully completed the project in 1979. In 1979 the results of the experiments were reported. The development of IGBT led to the complete suppression of the thyristor device operation or the latch-up in the four-layer device because fatal device failure was caused by the latch-up. When the latch-up of the parasitic thyristor had been achieved the technology of the IGBT had thus been established. After the invention of non-latch-up IGBTs, it became a de facto standard and then the patent of the non-latch-up IGBTs became the basic IGBT patent. In order to suppress the latch-up of the parasitic thyristor, all the developers tried to increase the latch-up current in the early development stage of IGBT. Because the IGBT could not conduct high current, the efforts failed enormously. By limiting the maximal collector-emitter current the successful suppression of the latch-up was made possible. The IGBT could now conduct below the latch-up current by reducing or controlling the saturation current of the inherent power MOSFET.

Applications

The second most widely used transistor is the IGBT as of 2010 after the power MOSFET. In the power transistor market, the IGBT accounts for 27% which is second to the power MOSFET which accounts for 53%, and ahead of RF amplifier which accounts for 11%, and bipolar junction transistor which accounts only for 9%. The IGBT device is mostly and widely used in industrial technology, consumer electronics, aerospace electronic devices, the energy sector, and transportation. IGBTs are used in inverters instead of SCR. It is used in solar inverter applications.

Advantages

The IGBT is the combination of the low saturation voltage capability of bipolar transistors and simple gate-drive characteristics of power MOSFETs. For the control input, the IGBT combines an isolated gate FET and as switching in a single device with the help of a bipolar transistor. In a medium to high-power applications, the IGBTS's are used such as switching mode power supplies, induction heating, and traction motor control. Many devices in parallel are present in the insulated gate bipolar transistors and can have very high-current handling capacities in the order of hundreds of amperes and blocking voltages up to 6500 V. Loads of hundreds of kilowatts can be controlled by these IGBTs.

Comparison with power MOSFETs

A significantly lower forward voltage drop is present in IGBT as compared to a MOSFET in higher blocking voltage rated devices although MOSFET exhibits much lower forward voltage drops at lower current densities due to the absence of the body diode. The body diode is absent in the IGBT's output BJT. The depth of the n- region must increase and the doping must decrease as the blocking voltage rating of both the devices that is MOSFET and IGBT increases resulting in roughly square relationships decrease in forwarding conduction mode Vs voltage blocking capacity of the device. For IGBTs high-voltage, high-current and low switching frequencies are favorable and for MOSFETs low-voltage, medium-current and high switching frequencies are favorable.

IGBT models

By using various circuit simulating computer programs such as Saber, SPICE, and other programs the circuits with IGBTs can be developed and modeled. In order to simulate the insulated gate bipolar transistor circuit, the device consists of a model which predicts or simulates the device responses to various currents and voltages on their electrical terminals. The effect of temperature on various parts of the IGBT can be included with the simulation for more precise simulations. The two common methods of modeling are- device physics-based models and equivalent circuits or macro models. SPICE simulates IGBTs in a Darlington configuration where the components like BJTs and Field-effect Transistor are ensembled with the help of a macro model.

Context and Applications

This topic is significant in the following professional exams for graduate and postgraduate courses:

• Bachelors in Electrical Engineering
• Bachelors in Electronics and Telecommunication
• Masters in Electrical Engineering
• Masters in Electronics and Telecommunication

Practice Problems

1. What is the full form of IGBT?

1. Insulated gate bipolar transistor
2. Induced gate bipolar transistor
3. Induction gate bipolar transistor
4. None

Correct option- a

Explanation: IGBT stands for an insulated-gate bipolar transistor.

2. IGBT consists of how many layers?

1. 2 layers
2. 3 layers
3. 4 layers
4. 5 layers

Correct option- c

Explanation: IGBT consists of four layers. They are P-N-P-N which is controlled by a metal oxide semiconductor gate (MOS gate) structure.

3. In the power transistor market, the IGBT accounts for how much share?

1. 9%
2. 11%
3. 27%
4. 50%

Correct option- c

Explanation: In the power transistor market, the IGBT accounts for 27% which is second to the power MOSFET which accounts for 53%, and ahead of RF amplifier which accounts for 11%, and bipolar junction transistor which accounts only for 9%.

4. What is the switching frequency speed in IGBT?

1. Low
2. High
3. Very high
4. None

Correct option- a

Explanation: For IGBTs high-voltage, high-current and low switching frequencies are favorable, and for MOSFETs, low-voltage, medium-current, and high switching frequencies are favorable.

5. Which of the following can be used to model the IGBT circuits?

1. SPICE
2. GNU
3. Naver
4. None

Correct option- a

Explanation: By using various circuit simulating computer programs such as Saber, Spice, and other programs the circuits with IGBTs can be developed and modeled.

Related Concepts

• Bipolar junction transistor
• Bootstrapping
• MOSFET
• Power semiconductor devices.