What is Insulated Gate Bipolar Transistor?

Thyristors are widely utilized components in contemporary electronics, commonly employed in switching and amplification within logic circuits. BJTs (Bipolar Junction Transistors) and MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) are the most frequently used types of transistors, each offering distinct advantages and some limitations. The IGBT (Insulated Gate Bipolar Transistor) combines the best features of both BJTs and MOSFETs into a single transistor. It integrates the high input impedance characteristics of a MOSFET (Insulated Gate) with the output characteristics of a BJT (Bipolar nature).

What is IGBT?

The IGBT, or Insulated Gate Bipolar Transistor, is a hybrid device that combines features of both BJTs and MOSFETs. Its name reflects this integration. “Insulated Gate” denotes the input aspect from the MOSFET, which has a very high input impedance and operates based on the voltage at its gate terminal, drawing no input current. “Bipolar” pertains to the BJT’s output characteristics, where current flow is driven by both types of charge carriers, enabling it to handle significant currents and voltages with small voltage signals. This hybrid nature makes the IGBT a voltage-controlled device.

Structurally, it is a four-layer PNPN device with three PN junctions and three terminals: Gate (G), Collector (C), and Emitter (E). The terminal names also reflect their origins; the Gate is from the MOSFET, acting as the input, while the Collector and Emitter are from the BJT, serving as the output.

Construction of IGBT

The IGBT is constructed using four layers of semiconductor material, forming a PNPN structure. The collector (C) electrode is connected to the P layer, while the emitter (E) is connected between the P and N layers. A P+ substrate is employed in the construction of the IGBT. An N- layer is placed on top of the P+ substrate to create the PN junction J1. Two P regions are fabricated on top of the N- layer to establish the PN junction J2. These P regions are designed to leave a central path for the gate (G) electrode. Additionally, N+ regions are diffused into the P regions, as illustrated in the diagram.

The emitter and gate are metal electrodes. The emitter is directly connected to the N+ region, while the gate is insulated by a silicon dioxide layer. The base P+ layer injects holes into the N- layer, hence it is referred to as the injector layer. The N- layer is known as the drift region, and its thickness correlates with the voltage blocking capability. The P layer above is termed the body of the IGBT.

The N- layer is designed to provide a pathway for current flow between the emitter and collector via the channel created by the voltage applied at the gate electrode.

Equivalent Structure of IGBT

Since the IGBT combines the input characteristics of a MOSFET and the output characteristics of a BJT, its equivalent structure resembles an N-channel MOSFET and a PNP transistor in a Darlington configuration. The resistance of the drift region can also be included in this equivalent structure.

If we examine the structure of the IGBT above, we can see that there are multiple paths for the current flow. The current flows from the collector to the emitter. The primary path is “collector, P+ substrate, N-, P, emitter.” This path is already represented by the PNP transistor in the equivalent structure. The second path is “collector, P+ substrate, N-, P, N+, emitter.” To account for this path, an additional NPN transistor needs to be incorporated into the structure, as illustrated in the figure below.

Working of IGBT

The two terminals of the IGBT, collector (C) and emitter (E), are used for current conduction, while the gate (G) is used to control the IGBT. Its operation is based on the biasing between the Gate-Emitter terminals and the Collector-Emitter terminals.

The collector-emitter is connected to Vcc such that the collector is at a higher positive voltage than the emitter. This causes junction J1 to become forward-biased and junction J2 to become reverse-biased. At this stage, if there is no voltage at the gate, the IGBT remains off and no current flows between the collector and emitter due to the reverse-biased J2.

When a gate voltage VG is applied that is positive relative to the emitter, negative charges accumulate just beneath the SiO2 layer due to capacitance. Increasing VG increases the number of charges, eventually forming a conductive layer in the upper P-region when VG exceeds the threshold voltage. This layer creates an N-channel that connects the N- drift region and the N+ region.

Electrons from the emitter then flow from the N+ region into the N- drift region. Simultaneously, holes from the collector are injected from the P+ region into the N- drift region. The presence of both excess electrons and holes in the drift region enhances its conductivity, allowing current to flow and thus switching the IGBT ON.

Types of IGBT

There are two types of IGBTs distinguished by the presence of an N+ buffer layer. Their classification into symmetrical and asymmetrical IGBTs depends on the inclusion of this additional layer.

Punch-through IGBT

The Punch-through IGBT, also known as an asymmetrical IGBT, incorporates an N+ buffer layer. These IGBTs possess asymmetric voltage blocking capabilities, with different forward and reverse breakdown voltages. Their reverse breakdown voltage is lower than their forward breakdown voltage. Additionally, they exhibit faster switching speeds.

Punch-through IGBTs are unidirectional and cannot withstand reverse voltages. Consequently, they find application in DC circuits such as inverters and chopper circuits.

Non Punch through IGBT

Also referred to as symmetrical IGBTs, they lack an additional N+ buffer layer. The symmetrical structure ensures equivalent breakdown voltage characteristics, meaning the forward and reverse breakdown voltages are identical. Consequently, they are employed in AC circuits.

V-I Characteristics of IGBT

Unlike a BJT, the IGBT operates as a voltage-controlled device, requiring only a small gate voltage to regulate collector current. However, the gate-emitter voltage (VGE) must exceed the threshold voltage.

The transfer characteristics of the IGBT illustrate the relationship between the input voltage (VGE) and the output collector current (IC). When VGE is 0V, IC is also 0, and the device remains off. As VGE is gradually increased, but remains below the threshold voltage (VGET), the device remains off, but a leakage current may be present. Upon surpassing the threshold voltage, IC begins to increase, and the device turns on. As a unidirectional device, current flows in only one direction.

The provided graph depicts the correlation between the collector current (IC) and the collector-emitter voltage (VCE) at various levels of gate-emitter voltage (VGE). When VGE is less than the threshold voltage (VGET), the IGBT operates in cutoff mode, and IC remains at 0 regardless of VCE. As VGE exceeds VGET, the IGBT transitions into active mode, where IC increases with rising VCE. Additionally, for each increment in VGE, such that VGE1 < VGE2 < VGE3, the corresponding IC varies.

It is crucial to ensure that the reverse voltage does not surpass the reverse breakdown limit, similarly to the forward voltage. If either voltage exceeds its respective breakdown limit, uncontrolled current will flow through the IGBT.

Advantages & Disadvantages of IGBT

Advantages

IGBT integrates the benefits of both BJT and MOSFET.

• It boasts higher voltage and current handling capabilities.

• It features very high input impedance.

• It can switch very high currents using very low voltage.

• Being voltage-controlled, it has no input current and low input losses.

• The gate drive circuitry is simple and inexpensive.

• It can be easily switched ON by applying positive voltage and OFF by applying zero or slightly negative voltage.

• It exhibits very low ON-state resistance.

• It has a high current density, allowing for a smaller chip size.

• It offers a higher power gain than both BJT and MOSFET.

• It has a higher switching speed than BJT.

Disadvantages

• It has a lower switching speed than MOSFET.

• It is unidirectional and cannot conduct in reverse.

• It cannot block higher reverse voltage.

• It is more expensive than BJT and MOSFET.

• It has latching issues due to the PNPN structure resembling a thyristor.

Applications of IGBT

IGBTs are utilized in a variety of AC and DC circuits. Some key applications include:

• Used in SMPS (Switched Mode Power Supply) to supply power to sensitive medical equipment and computers.

• Employed in UPS (Uninterruptible Power Supply) systems.

• Applied in AC and DC motor drives for speed control.

• Utilized in choppers and inverters.

• Used in solar inverters.