Secondary breakdown occurs in

Question

Examoogle · Accepted Answer

BJT but not in MOSFET

Explanation: Secondary breakdown is a failure mechanism inherent in Bipolar Junction Transistors (BJTs) caused by localized current constriction, leading to hot spots. It is not observed in MOSFETs because MOSFETs possess a positive temperature coefficient of resistance, which naturally promotes uniform current distribution.

Working Principle: In a BJT, when the device is operated in the reverse-biased collector-base junction region, current flows non-uniformly. As temperature increases in high-current density regions (hot spots), the local resistivity decreases, drawing even more current and causing localized thermal runaway. Conversely, MOSFETs have a negative temperature coefficient (resistance increases with temperature), which causes the current to redistribute away from hot spots, preventing localized secondary breakdown.

Key Formulas: $P_{loss} = V_{CE} 	imes I_C$; $\frac{dR}{dT}; $\frac{dR}{dT} > 0$ (MOSFET - Self-protecting)

Key Points: Secondary breakdown is a non-destructive failure if limited quickly, but usually leads to permanent damage.; It typically occurs in the reverse-biased safe operating area (RBSOA).; BJTs are minority carrier devices, making them susceptible to carrier injection non-uniformities.; MOSFETs are majority carrier devices with inherently stable thermal properties.

Advantages: Understanding this helps in designing robust snubber circuits.; Crucial for selecting appropriate gate drive protection for BJTs.

Disadvantages: Limits the high-voltage high-current switching capability of BJTs.; Reduces the effective Safe Operating Area (SOA) of BJTs.

Applications: Power Inverter Design; High-speed switching circuit protection; Power Supply thermal management

Additional Info: Secondary breakdown is strictly a phenomenon involving thermal instability and current focusing.. Option A is incorrect because MOSFETs are known for their immunity to secondary breakdown.. Option B is incorrect as MOSFETs exhibit excellent parallelability due to their positive temperature coefficient.

Answer: C is correct — Secondary breakdown occurs in BJTs due to thermal runaway in localized current filaments, a phenomenon avoided in MOSFETs due to their positive temperature coefficient of resistance.

Exam Tip: When analyzing switching devices, always prioritize the temperature coefficient; it dictates the thermal stability and parallel operation capabilities of the semiconductor.

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