Failure analysis of a semiconductor LED can actually be quite complex, as there’s really only one failure mode – insufficient or completely absent light output – that can be seen. For obvious reasons, photoemission microscopy is tailor-made for finding defects on LEDs. The photoemission system will often reveal that what the naked eye perceives as the lower-than-specified light output is, in fact, uneven illumination (in other words, only part of the LED is lighting up, not the whole device). This uneven lighting can often be attributed to a damaged junction; often, a cross-section of the failing device will show a junction that has been spiked with metal causing a localized short, an ESD flash point creating a leaky junction or even a processing defect.
Similarly, failure analysis of semiconductor photodetectors often depends on leveraging the unique characteristics of these types of devices. For example, failure analysis of a photovoltaic die (usually an array of diodes that generates a voltage when exposed to light) may involve shining a pulsed light source onto the surface of the die and monitoring the output of the device with an oscilloscope. By characterizing the photovoltaic’s output, an analyst may be able to narrow down the type of failure afflicting the array. An output that is stuck low, for example, may imply something completely different than an output that reaches its nominal voltage but decays back to nothing extremely quickly. At this point, an analyst must combine experimental data with experience and knowledge of semiconductor physics in order to successfully proceed with the failure analysis.
Though tearing apart cutting-edge technology is certainly the most (relatively) glamorous part of semiconductor failure analysis, analysis of more commonplace devices like LEDs is just as important. Indeed, finding the failure on an LED or photocell can be at least as complex, if not more so, than hunting down a failure on a microprocessor.