Insulated Gate Bipolar Transistor: The BJT-MOSFET In-Between

Key Takeaways

• Unlike the MOSFET and BJT, the IGBT has three PN junctions.

• The IGBT uses the three PN junctions to build a MOSFET and BJT back-to-back on the semiconductor die.

• With a large safe operating area, the IGBT excels in many high-power applications, provided designers heed thermal conditions and loss vectors.

The insulated gate bipolar transistor is a similar 3-pin package to the MOSFET and BJT. Yet, its semiconductor structure is more complex than either.

Engineering is awash in tradeoffs. As the saying goes, “There’s no such thing as a free lunch”; engineering disciplines exemplify this idea with parameters that constantly offset one another. Sometimes the best solution is nearly universal: the insulated gate bipolar transistor (IGBT) combines some of the advantages of both the MOSFET and BJT to offer superior performance to both for various applications. While there are certain drawbacks with this power device relative to its forebearers that designers must keep in mind for optimization and system longevity, it can function in topologies that greatly exceed the capabilities of either individual semiconductor type.

Power Device Comparison

The Insulated Gate Bipolar Transistor’s Hybrid Nature

The insulated gate bipolar transistor is a blend of MOSFET inputs and BJT outputs, offering the distinct advantages of both power devices in a monolithic package. As a hybrid power device, some aspects of its performance suffer relative to its predecessors, but the overall operation remains incredibly robust. Like BJTs and MOSFETs, the IGBT is a three-terminal device comprising a gate, emitter, and collector (note the inherited gates from both types of power devices) but is a four-layer semiconductor structure (i.e., three NP junctions). The IGBT consists of n-channel MOSFET and a PNP transistor in doped region terms. This specific construction creates a parasitic thyristor coupling between the two transistors; to prevent its activity, a resistor shorts the base and emitter of the NPN transistor in the P-doped base.

 The physical layout of an IGBT is describable by the gating and vertical structure of the device:

• Gating - A planar gate sits atop the n+-emitter/buffer and p-base regions, whereas a trench gate extends vertically through the same regions and the n- drift region. In an equivalent circuit model, the planar gate introduces a JFET-like effect that produces a voltage drop; this effect is not present in the trench gate format, reducing its on-stage voltage requirements.
• Verticality - IGBT performance is modulated depending on the thickness of the doped regions. Punch-through (PT) IGBTs utilize a thick and heavily doped p+ collector/anode region to reduce the on-stage voltage; this method has drawbacks regarding switching loss that must resolve in the drift region. Non-punch-through (NPT) IGBTs rely on a thicker drift layer to bind the depletion layer, while a thinner collector/anode region effectively controls the carrier injection rate. The NPT IGBT possesses a faster switching speed than the PT structure and suffers fewer losses in low-to-moderate current ratings, but the on-state voltage increases on the high end. Finally, a sub-variant of the PT IGBT is the thin-wafer version, which optimizes the performance of PT and NPT by combining the thin layers of the drift and collector/anode regions, respectively.

In operation, the IGBT most closely resembles the BJT, despite its physical resemblance to the MOSFET. When an applied voltage exists between the collector and emitter, the PNP transistor turns on the MOSFET and enables the flow of current; this, in turn, modifies the conduction of the collector/anode region, affecting the rate of hole injection and decreases the resistance in the drift region. After the device switches off, the current flows until charge carriers exit the drift region or recombine. Since voltage application controls the IGBT, the gate control circuitry can reduce to the minimum size and complexity to charge/discharge the gate capacitance. 

Insulated Gate Bipolar Transistor: Operations and Logistics

The standout ability of the insulated gate bipolar transistor is its large safe operating area (SOA). The SOA defines the acceptable voltage and current operating conditions that prevent performance degradation; designers can therefore use the SOA to constrain the circuit design and ensure maximum service life. The advantage of the SOA is that it graphically presents a set of curves where design teams can match the application needs to the data; for example, a switching IGBT could not run at maximum ratings without incurring damage.

The SOA has two sub-categories:

• The forward bias SOA can run the device to its maximum settings.
• The reverse bias SOA covers the turn-off state of the IGBT. When a device turns off, a surge voltage develops; circuits using IGBTs must safely suppress tail current and surge voltage with slower turn-off times and inductance reductions.

Additionally, all power designs must consider thermal performance and loss vectors for optimization. The IGBT borrows some descriptions from the BJT for thermal runaway. Secondary breakdown occurs when a negative resistance evolves from high voltage or current operations. First, the current heats and concentrates a localized area due to a negative temperature dependence of the gate-emitter voltage. As the gate-emitter voltage drops, the current builds further in a positive feedback loop until an external circuit disrupts the process or the device inevitably destroys itself.

Conduction and switching losses are the primary factors that impact IGBT efficiency. The former is the product of the saturated collector-emitter junction voltage and the collector current; component engineers can select devices with lower saturation voltage requirements. The collector current, gate resistance, and temperature are all responsible for the switching loss; these factors incorporate into the integral of the collector current and collector-emitter voltage product.

Cadence Solutions Encompass All Aspects of Power Design

Insulated gate bipolar transistors offer circuit designers a middle ground between MOSFETs and BJTs, offering a power device with fantastic suitability for high-power demands. The IGBT may lack some characteristics of either parent device, but it makes up for that with general applicability to many circuit topologies and operating conditions. Getting the best performance out of any power circuit requires parameter and parasitic modeling that accurately reflects real-world constraints. Cadence’s PCB Design and Analysis Software suite gives product development teams a comprehensive ECAD system that seamlessly translates from one design stage to another. Import simulation data into the layout with OrCAD PCB Designer for a fast and powerful PCB DFM.

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