What Is Avalanche Breakdown?

Avalanche breakdown is a phenomenon that occurs in a p-n junction when the reverse bias voltage exceeds a critical threshold. Under normal reverse bias, only a small leakage current flows. As the reverse voltage increases, the electric field across the depletion region rises. When the field reaches approximately 105 to 106 V/cm (depending on the semiconductor material and doping), it becomes strong enough to accelerate thermally generated free electrons to energies sufficient to break covalent bonds. These high-energy electrons collide with the crystal lattice, knocking valence electrons free and creating electron-hole pairs. Each newly generated electron can then accelerate and create more pairs, leading to a multiplicative chain reaction known as impact ionization. This avalanche of carriers results in a very large current that can damage the diode if not limited or controlled.

The breakdown voltage is determined by the doping concentration of the junction. Lightly doped junctions have wider depletion regions and thus lower electric field strength for a given voltage, leading to higher breakdown voltages. Heavily doped junctions have narrow depletion regions and breakdown at lower voltages. This relationship is fundamental to designing diodes for specific voltage ratings.

Avalanche Breakdown vs. Zener Breakdown

It is common to confuse avalanche breakdown with Zener breakdown, as both produce a sharp increase in reverse current and are used in voltage reference diodes. However, the underlying physics differ. Zener breakdown occurs at low reverse voltages (typically below 5-6 V) and is due to quantum mechanical tunneling through the narrow depletion region of a heavily doped junction. In contrast, avalanche breakdown dominates at higher voltages (above about 5-6 V) and involves impact ionization. While Zener breakdown is generally reversible and has a negative temperature coefficient (breakdown voltage decreases with temperature), avalanche breakdown has a positive temperature coefficient (breakdown voltage increases with temperature). This distinction is important when selecting voltage reference diodes or designing protection circuits. Most practical power diodes intended for high-voltage applications rely on avalanche breakdown as their primary reverse breakdown mechanism.

Physics of Avalanche Breakdown in Detail

Impact Ionization and Multiplication Factor

The avalanche process is characterized by the multiplication factor M, which describes how much the reverse current is multiplied compared to the leakage current in the absence of impact ionization. M is given empirically by the relation M = 1 / (1 - (VR / BV)n), where VR is the applied reverse voltage, BV is the breakdown voltage, and n is a material-dependent exponent (typically 3-6 for silicon). As VR approaches BV, M increases rapidly, leading to the sudden current rise. The ionization coefficients for electrons and holes, denoted αn and αp, determine the rate of pair generation. These coefficients are strong functions of electric field and temperature.

Temperature Dependence

Unlike Zener breakdown, avalanche breakdown voltage increases with temperature. Higher temperatures cause atoms in the crystal lattice to vibrate more vigorously, which increases scattering and reduces the mean free path of electrons. To achieve the same ionization energy, a higher electric field (and thus a higher voltage) is required. This positive temperature coefficient is a critical factor in thermal stability. When a power diode operates near breakdown, localized hot spots can form. Because the breakdown voltage is higher in hotter regions, current tends to redistribute to cooler areas, preventing current crowding and thermal runaway. This self-stabilizing property makes avalanche breakdown more forgiving than Zener breakdown in high-power applications.

Material Considerations

The physics of avalanche breakdown varies with semiconductor material. Silicon is the most common and well-characterized. Silicon carbide (SiC) devices, increasingly used in power electronics, have a much higher critical electric field (roughly 10 times that of silicon) and wider bandgap, enabling breakdown voltages of thousands of volts with thinner drift layers. However, the temperature coefficient of SiC avalanche breakdown is positive and stronger than that of silicon. Gallium nitride (GaN) devices also exhibit avalanche capability, though its widespread adoption in vertical power devices is still evolving. Understanding material-specific ionization rates is essential for accurate simulation and design.

Practical Implications of Avalanche Breakdown

Avalanche Energy Ratings

Power diodes designed for high-voltage operation often include a specified avalanche energy rating (typically in joules or millijoules) or a non-repetitive peak avalanche current rating (IAR). This rating indicates how much energy the diode can safely dissipate during an avalanche event without failing. The energy is dissipated as heat within the device, so the rating is tied to the diode’s thermal capacitance and ability to spread heat. Exceeding the avalanche energy rating can cause localized melting, junction spiking, or short circuits. Designers must consider worst-case transient overvoltages and ensure the chosen diode can handle the energy. For applications like motor drives, welding, or power inverters where unclamped inductive switching (UIS) is possible, avalanche ratings are especially critical.

Unclamped Inductive Switching (UIS)

UIS is a common test that stresses a diode (or transistor) with a sudden energy dump from an inductor when a switch opens. If no clamp circuit is present, the inductor current forces the diode into avalanche breakdown to dissipate the stored magnetic energy. The diode must survive this avalanche until the current decays to zero. Diode UIS capability is characterized by the avalanche energy it can absorb. Many datasheets provide curves showing the maximum allowable avalanche current versus time or temperature. Using a diode with insufficient UIS robustness can lead to immediate failure under of the first surge.

Design Considerations for Reliable Operation

  • Choose diodes with appropriate avalanche ratings. For high reliability, select devices explicitly rated for avalanche energy (e.g., "avalanche rated" series from Vishay, ON Semiconductor, STMicroelectronics, etc.). These diodes are processed to have uniform junction edges that prevent premature breakdown at corners.
  • Implement snubber circuits. An RC snubber placed across the diode can limit the rate of voltage rise (dV/dt) and reduce the peak voltage spike, lowering the stress on the junction. For very high energy pulses, a clamp circuit (e.g., a Zener or MOV) can absorb the energy instead of forcing the diode into avalanche.
  • Ensure proper heat dissipation. Avalanche events generate significant thermal energy in a short time. Use heat sinks, thermal compound, and consider the diode's transient thermal impedance. Multiple repetitive avalanche events can cause cumulative heating and eventual failure if the average junction temperature exceeds rated limits.
  • Use derating guidelines. Never operate at the absolute maximum avalanche rating under normal conditions. A typical derating factor of 80% or more is prudent, especially at elevated ambient temperatures.
  • Consider the diode’s reverse recovery characteristics. Fast recovery diodes (FREDs) are often used in switching applications. Their reverse recovery behavior can interact with avalanche to increase losses. Soft recovery diodes produce less overvoltage and are more forgiving.

Avalanche Breakdown in Different Diode Types

Not all power diodes are created equal in avalanche performance. Standard rectifier diodes (e.g., 1N400x series) are generally not rated for avalanche and may be damaged by even modest reverse voltage spikes. They are intended for low-frequency, stable reverse voltage conditions. Fast recovery diodes (FREDs) designed for switching power supplies often have avalanche ratings, as they must survive reverse recovery overvoltages and inductive ringing. Schottky diodes typically do not exhibit avalanche breakdown in the same sense because they are majority-carrier devices; their reverse leakage current increases gradually with voltage until catastrophic thermal runaway. Schottky diodes are rarely avalanche-rated and should never be intentionally operated in breakdown. Power Zener diodes, while designed for controlled avalanche breakdown, are optimized for voltage regulation rather than energy handling, though some high-power Zeners can absorb significant transient energy. Silicon carbide (SiC) Schottky diodes may have limited avalanche capability, while SiC bipolar diodes (e.g., PiN diodes) show robust avalanche characteristics similar to high-voltage silicon diodes.

Testing and Characterization of Avalanche Capability

To ensure a diode can withstand avalanche conditions, manufacturers perform tests such as the unclamped inductive switching (UIS) test. The circuit uses an inductor, a switch (often an IGBT or MOSFET), and the diode under test in series with a DC supply. The switch is turned on to charge the inductor, then turned off, forcing the current through the diode. The diode goes into avalanche, and the energy is dissipated until the current decays to zero. The test is repeated at increasing energy levels until failure, establishing the avalanche energy rating. Another common metric is the I²t rating (ampere-squared seconds), which reflects the diode's ability to handle a pulse of current during an avalanche event. This is frequently used for fuse coordination in power circuits. Engineers can also measure the avalanche breakdown voltage using a curve tracer; a sharp increase in reverse current at a specific voltage indicates the start of avalanche. Temperature-controlled measurements are necessary because the breakdown voltage drifts with temperature.

Conclusion

Avalanche breakdown is a fundamental physical process that limits the maximum reverse voltage of power diodes but also provides a mechanism for safely absorbing transient overvoltages when properly designed. Understanding the differences between avalanche and Zener breakdown, the influence of temperature and material properties, and the practical implications for circuit design enables engineers to select diodes with the right avalanche ratings and implement protection strategies that prevent failure. By considering avalanche energy, UIS capability, snubber circuits, and thermal management, designers can build robust power converters, motor drives, and other high-voltage systems that withstand real-world stresses. For further reading, see the Wikipedia article on avalanche breakdown, the Vishay application note on avalanche ratings, and the ON Semiconductor guide to UIS testing.