The first kind of defect in a semiconductor relates to the materials used. Because of the extreme dependence of the device on the exact makeup of the chip, any variance or impurities in the material will cause it to operate outside the specified range. Detecting these variances isn't trivial and many sensitive methods are used to detect it such as optical emission spectroscopy.
Another problem might not be with the semiconductor materials, but with mechanical integrity. Cracks caused due to stresses and problems with the attachment to the die can also cause malfunctions. For these issues, dye penetrant testing and acoustic microscopy are used in various ways depending on the exact type of suspected failure.
The third type of error can relate to defective electronics within the chip. These are very difficult to track down due to a large number of components and techniques tend to use indirect methods to analyze the package. For example, curve tracing is used to measure the deviation of the chip's output from the expected output of a known working chip and hotspot detection is used via microthermography techniques in order to identify heat centers which can assist in isolating the problem. We can use many techniques within microthermography to get the job done. Liquid crystal imaging and fluorescent microthermal imaging are two such processes.
Before actually applying these techniques, electronic failure analysis engineers examine the circumstances surrounding the errors in order to try and isolate the defect. For example, those caused due to environmental issues such as excess humidity or dust need not indicate a problem in the chip itself.
This is a very brief overview of the common semiconductor defects we see in our day-to-day electronic failure analysis activities. Semiconductor defects come in a wide variety of types, and it often takes significant effort to get to the root of the problem of a malfunctioning chip. And like many aspects of engineering, analysis of semiconductor defects is as much an art as it is a science.