IGBT Fundamentals: Device Structure, Loss Calculation, Parallel Design, Reliability

Insulated Gate Bipolar Transistors (IGBTs) are semiconductor devices widely employed in power electronics, combining the advantages of Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) and Bipolar Junction Transistors (BJTs). They feature both high input impedance and low on-state voltage drop characteristics. Despite the increasing adoption of wide-bandgap semiconductors such as SiC and GaN, IGBTs have long been the preferred choice for numerous high-power applications due to their efficiency and reliability. They remain well-suited for diverse operational scenarios even as these newer technologies emerge.

This article provides an in-depth exploration of key IGBT concepts, covering device structure, loss calculations, parallel operation design, and reliability testing. It offers a comprehensive guide to mastering the essential knowledge of IGBTs.

IGBT Device Structure

The IGBT is a power semiconductor transistor comprising four alternating layers (P-N-P-N), controlled by a voltage applied to its metal-oxide-semiconductor (MOS) gate. This fundamental structure, through gradual refinement and optimisation, reduces switching losses and enables thinner device profiles. Onsemi's IGBT integrates a trench gate with a field-stop structure to suppress inherent parasitic NPN behaviour. This approach reduces the device's saturation voltage and on-resistance, thereby enhancing overall power density.

Figure 1: Trench Field-Effect Gate-Cutoff IGBT Structure

IGBT Loss Calculation

To ensure efficient operation of IGBTs within a system, precise calculation of losses is paramount! IGBT losses can be broken down into conduction losses and switching losses (turn-on and turn-off), whilst diode losses encompass both conduction and turn-off losses. Accurate measurement of these losses typically requires an oscilloscope, monitoring waveforms during device operation via voltage and current probes. Measuring energy involves mathematical functions. After determining the total energy for one switching cycle, dividing this by the switching cycle time yields the power dissipation.

IGBT parallel configuration

When confronted with ultra-high loads of tens or even hundreds of kilowatts, a single IGBT device often proves inadequate. In such instances, ‘parallel design’ emerges as the core solution for high-power systems. Parallel devices may comprise discrete packaged components or bare chips assembled within modules. This approach not only enables higher rated currents and improved thermal management but also facilitates system redundancy. It is important to note that variations in process and layout between components can affect the static and dynamic current distribution of parallel devices. System design engineers must understand these factors to create reliable systems. Consequently, they should prioritise consideration of the following key points: static variations, dynamic variations, thermal coefficients, gate resistance, and empirical data.

IGBT reliability

As the ‘core device’ of power electronic systems, the reliability of IGBTs is crucial for ensuring the operational safety of the entire system. IGBTs must undergo a series of extensive reliability tests to validate consistency. These tests are designed to accelerate failure mechanisms encountered in practical applications, thereby ensuring satisfactory reliability performance in ‘real-world’ deployments.

Conventional reliability tests for IGBTs include: High Temperature Reverse Bias (HTRB), High Temperature Gate Bias (HTGB), High Temperature Storage Life (HTSL) testing, High Humidity High Temperature Reverse Bias (H3TRB), Unbiased High Accelerated Stress Test (UHAST), Intermittent Operating Life (IOL), Temperature Cycling (TC), Low Temperature Storage Life (LTSL) testing, Steady State Operating Life (SSOL) testing, and others.