When engineers design circuits for power conversion, motor drives, or load switches, one of the most common sudden faults they may encounter is the abrupt failure of MOSFETs. There are no obvious early warning signs, yet it can shut down the entire circuit and even affect the mass production yield of products. Recently, our FAE (Field Application Engineering) team assisted a customer in resolving a case involving the breakdown failure of an AO3400 MOSFET in an SOT-23 package. During this process, we identified the most common causes of MOSFET failure and outlined how to avoid these issues from a fundamental principle perspective.

I. Typical Case: Why Did the AO3400 Burn Out?

The customer used the AO3400 in the switching circuit of a portable power supply and noticed abnormal electrical performance after use. After the samples were sent back, our FAE team conducted a comprehensive analysis and confirmed that the provided samples were used and disassembled units. No obvious abnormalities were found in the appearance or internal structure during inspection; however, electrical testing revealed breakdown failure after use. Further analysis of the chip (via decapsulation and ball corrosion inspection) showed abnormal burning on the internal chip—this was the root cause of the MOSFET failure.

The failure was determined to result from abnormal overcurrent during the customer’s application testing, which exceeded the component’s rated current tolerance. This overcurrent caused burning damage to the MOSFET’s internal chip, leading to its performance failure. Finally, the FAE team recommended that the customer check for overcurrent or leakage anomalies in their application.

Through failure analysis, the failure process was reconstructed: the drain-source voltage (VDS) of the failed device exceeded its avalanche breakdown threshold, causing an instantaneous surge in internal carriers. The drain current (ID) spiked to more than 10 times the rated value; the chip junction temperature rose to 200°C (the critical temperature of silicon material) within a few microseconds, and the device was ultimately burned out due to thermal runaway.

II. Four Underlying Causes of MOSFET Failure

MOSFET failure is never a random event—it occurs when the device’s operating conditions exceed its characteristic limits. Combining the case study and past experience, we have summarized four most common causes of failure, each directly related to the electrical principles of MOSFETs.

1. Avalanche Failure: A Chain Reaction of Exceeding Voltage Tolerance

When the drain-source voltage (VDS) exceeds the avalanche breakdown threshold of the internal PN junction, the PN junction is "broken down." At this point, a large number of carriers are generated instantaneously, leading to a sharp increase in drain current. Without overcurrent protection, power loss surges in an extremely short time, eventually causing burnout due to thermal runaway. The core of this failure is that the voltage exceeds the MOSFET’s voltage withstand limit, and it commonly occurs in scenarios with transient overvoltage, such as motor startup and power switching.

2. SOA Failure: Voltage + Current Exceeding the Safe Boundary

The Safe Operating Area (SOA) of a MOSFET is jointly defined by voltage, current, and power. For example, if high voltage and large current occur simultaneously during motor startup, the device’s power consumption will exceed its heat dissipation capacity. As the temperature rises continuously, thermal breakdown occurs when the junction temperature exceeds 150°C. The essence of this failure is that power consumption exceeds the MOSFET’s heat dissipation limit, and it is common in high-power scenarios such as high-power motor drives and high-power load switches.

3. Gate Voltage Failure: Dual Risks of the Gate Oxide Layer

The gate of a MOSFET consists of an extremely thin oxide layer. Excessively high gate voltage will cause breakdown, while excessively low voltage will prevent the MOSFET from turning on normally. The key to this failure is that the gate voltage deviates from the safe range, and it often happens in scenarios where the drive circuit is improperly designed or voltage fluctuations occur.

4. ESD Failure: Massive Damage from Latent Risks

The extremely high gate input impedance of MOSFETs makes them unusually sensitive to electrostatic discharge (ESD). Static electricity from the human body or equipment can instantly break down the gate oxide layer like a lightning strike. This type of failure leaves no obvious traces on the device’s appearance but can cause batch MOSFET failures. It is common in scenarios where ESD protection is inadequate during production, transportation, or installation.

III. How to Avoid Failure from a Principle Perspective?

The reliability of a MOSFET essentially lies in keeping it operating within its characteristic limits. Engineers can improve reliability through the following approaches:

• Prevent Overvoltage: Use simulation software to simulate the VDS waveform in the circuit and ensure it does not exceed BVDSS (Breakdown Voltage Drain-Source); add overvoltage protection components such as TVS diodes and RC snubber networks to the circuit.
• Control Power Consumption: Ensure the MOSFET’s operating point stays within the Safe Operating Area (SOA) to avoid simultaneous peaks in voltage and current; enhance heat dissipation by adding heat sinks or reducing PWM duty cycles.
• Stabilize Gate Voltage: Add clamping diodes to the drive circuit to suppress voltage spikes; ensure the gate voltage stays between the threshold voltage (VGS(th)) and the maximum rated gate voltage.
• ESD Protection: Wear anti-static wristbands and use conductive packaging during production and installation; avoid direct contact with MOSFET pins by hand.

As MOSFET designers, we optimize these risk points at the device level. For example, we improve BVDSS through doping processes, enhance ESD resistance with thick gate oxide layers, and clearly mark parameters such as avalanche energy and SOA in datasheets to help engineers accurately evaluate application scenarios. However, the more critical factor is that engineers need to understand the characteristics of MOSFETs and avoid failures from the source of design.

MOSFET failure is never uncontrollable—it is an inevitable result of electrical principles. By understanding these underlying logics, engineers can transform MOSFETs from failure-prone components into stable core parts of circuits. We share this experience in the hope of working with engineers to make circuit design more reliable through professional knowledge.

Company Introduction

Heketai, founded in 1992, is a high-tech and "Specialized, Refined, Characteristic, and Innovative" enterprise integrating R&D, design, production, and sales of professional electronic components. We focus on providing cost-effective component supply and customized services to meet the R&D needs of enterprises.

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We offer a full range of discrete devices and passive components such as chip resistors, mainly including: MOSFETs, TVS diodes, Schottky diodes, Zener diodes, fast recovery diodes, bridge rectifiers, diodes, transistors, resistors, and capacitors.

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