Breakdown: Understanding Secondary Breakdown in Transistors

Secondary Breakdown is a destructive phenomenon primarily associated with certain types of power semiconductor devices, particularly bipolar junction transistors (BJTs). It is characterized by the formation of localized hot spots within the device, leading to thermal runaway and irreversible damage.

BJT: Why Secondary Breakdown Occurs in BJTs

Bipolar Junction Transistors (BJTs) are susceptible to secondary breakdown due to their specific operating characteristics and internal structure. Here's why it occurs in BJTs:

• Negative Temperature Coefficient of Resistance: In BJTs, as the junction temperature increases, the collector current (\(I_C\)) tends to increase for a given base current, especially at high current densities. This creates a positive feedback loop: increased temperature leads to increased current, which in turn generates more heat.
• Current Crowding: At high current levels, especially during saturation or breakdown conditions, the current flow within the BJT emitter is not uniform. The current tends to concentrate at the edges of the emitter region due to resistive drops in the base spreading resistance. This phenomenon is known as current crowding.
• Localized Hot Spots: Current crowding leads to uneven power dissipation, causing highly localized regions within the transistor to become significantly hotter than others. These are called hot spots.
• Thermal Runaway: Once a hot spot forms, the increased temperature in that region further lowers its effective breakdown voltage and increases current flow, leading to even more localized heating. This self-accelerating process, where heat generation outpaces heat dissipation, is called thermal runaway. Eventually, this can cause a drastic and irreversible drop in the device's voltage-handling capability, leading to device failure.

MOSFET: Absence of Secondary Breakdown in MOSFETs

In contrast to BJTs, Power MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) are generally immune to secondary breakdown. This fundamental difference stems from their inherent electrical characteristics:

• Positive Temperature Coefficient of ON-Resistance: For MOSFETs, the ON-resistance (\(R_{DS(on)}\)) typically increases with increasing temperature.
• Current Spreading: If a hot spot starts to form in a MOSFET, the resistance in that hotter region increases. This increased resistance reduces the current flow through that specific region, effectively diverting the current to cooler, less resistive parts of the device.
• Self-Limiting Behavior: This current redistribution mechanism prevents the formation of localized thermal runaway. The device exhibits a self-limiting or self-correcting behavior, ensuring more uniform power dissipation across the entire die and protecting it from secondary breakdown.

Identifying: Where Secondary Breakdown Predominantly Occurs

Based on the distinct operating principles and temperature dependencies, Secondary Breakdown is a critical failure mode primarily observed in Bipolar Junction Transistors (BJTs). MOSFETs, due to their positive temperature coefficient of ON-resistance, inherently protect themselves from this destructive phenomenon.