Anatomy of a Transistor Failure

We recently purchased a new video microscope and were looking for something interesting to inspect. Digging through our box of bad electronic parts produced a bad power transistor. Utilizing a set of sharp cutters, we performed surgery on this poor TO3 package device.

Before showing the images from the inside of the actual device, here are the analog signatures that led us to decide this device was bad (green signatures are good; red signatures are bad).

 

bad power transistor signatures

Signatures from bad power transistor; (left to right) Emitter to Collector, Base to Emitter, Base to Collector

Note that the base to emitter and base to collector signatures are showing a resistive short circuit indicated by the angled horizontal leg. That angled portion should flat to the horizontal axis of the display. Putting a resistor across the component legs would have the same effect. Checking between the emitter and collector shows a hard short circuit. There is no doubt that this device would raise havoc if inserted into a circuit board and powered on.

Here are images of the transistor itself. The image below shows our surgical prowess.

bad-transistor

Bad TO3 power transistor cut open

This image shows the damage resulting in the internal faults.
The resistive short is likely caused by the carbon arc (circled in yellow) shown between the emitter (lead on right) and base (lead on left) connections to the transistor chip. The collector to emitter hard short is likely caused by the burned portion of the collector plate indicated by the red arrow.

bad-transistor-closeup

Inside of bad TO3 power transistor showing damage

In conclusion…
Being curious in this case led to discovering some interesting details on how an electronic component can fail.

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