Diodes are one of the few semiconductor components that can have higher temperature ratings than other semiconductors, so they are sometimes overlooked when reliability is concerned. But just like any other component, diodes can fail and they have distinct failure modes that can be determined from inspection. When you know the principle failure modes for diodes in circuits, you can determine whether a circuit needs to be redesigned with a larger diode or alternative operation mode to ensure reliability.
Root Causes of Diode Failure
All diode failures that are not due to mechanical fracture or environmental contamination are related to electrical overstress. The electrical conditions that lead to different types of failures are:
Large pulsed voltages or currents - leads to open or closed circuit failure
Large voltages in reverse bias - leads to pn junction failure (closed circuit)
Long-term operation at extreme conditions - leads to degradation failure
Now let’s look at each of these failure modes individually.
Closed Circuit Failure
Typically a diode fails as a closed circuit (i.e., short circuit) due to overvoltage, such as what might occur in high voltage pulses. For example, this is how ESD protection diodes (TVS diodes) fail when driven excessively into reverse bias by strong voltage pulses. When a conventional diode is overdriven into reverse bias beyond the breakdown voltage, the diode will conduct a high negative current. Unless this current is limited by external circuits, the diode may be permanently damaged due to overheating. During overheating, the pn junction could fail and leave a short circuit.
Image showing pn junction failure resulting in a short circuit. (Image source)
The mechanism by which the pn junction fails is thermal runaway, similar to what happens in MOSFETs. Essentially, the diode’s reverse current raises the temperature of the pn junction, which then increases the reverse current. This continues until the pn junction fails as described above. However, this mechanism could also produce an open circuit failure as described below.
Open Circuit Failure
If a diode fails as an open circuit, it is due to a different failure point in the internal structure of the diode. During overcurrent conditions in forward or reverse bias, the metallization on the pn junction can overheat and burn up. This leaves behind burnt up metal leads that leave a high resistance contact or an open circuit internally in the diode packaging. This is more typical in LEDs due to their lower current carrying capability compared to rectifier diodes.
Degraded Device Failure
A device that has become degraded will be more difficult to diagnose. Over time, a diode that has degraded tends to exhibit an increase in the reverse leakage current while the device is operated within its rated voltage range. Related changes in the diode’s specifications could include reduction in the device’s breakdown voltage and forward voltage drop. The two common factors contributing to a degradation failure are high operating temperature and repeated overstress.
Verify With Simple Measurements
Verifying a diode is still minimally functional does not require special equipment, it can be measured with most handheld multimeters. While a degraded component requires a measurement of the reverse leakage current to detect failure, it is possible to measure the forward voltage of a diode with most multimeters. Remove the diode from its test circuit or PCB, and take the measurement of the diode with the component isolated.
A forward voltage measurement is sufficient to determine an open or closed failure. As was mentioned above, most diode failures result in a closed failure (short circuit), in which case the diode would measure a zero forward voltage because there will be no voltage being dropped across the diode. In an open failure (open circuit), the diode forward voltage measurement will probably result in an overload reading on the meter.
Another point to consider early in the design phase is to perform an electrical stress analysis in a circuit simulator. SPICE results showing voltage and current values on diodes (and on any other components) can be used to identify potential component failures in a design before the design is prototyped. If high-reliability is a top concern, make sure you add this capability into your ECAD software suite.
Anytime you need to identify potential overstress in your components, use the Smoke Analysis feature in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.