A question came up about common failure mode of a transistor. Searching past posts I actually found varying answers, either open or short. Maybe it depends on the application, and I'm missing something?
"The output of a TTL NAND gate is being used to drive a relay ON by pulling the source low. The diode for the relay was installed backwards, effectively shorting the gate's output to the power supply. Which component has most likely failed?
- The gate's output transistor has failed open
- The gate's output transistor has failed shorted"
simulate this circuit – Schematic created using CircuitLab
The book provides that 'open' is the correct answer, not 'short', but doesn't explain why.
Best Answer
In my experience, your specific example will have two distinct failures.
I'm assuming that the power supply can supply several Amps of current when I describe the following:
First: the transistor fails SHORT
Second: the bonding wire that runs from the die to the package pin burns open.
Depending upon the energy levels involved, the second part of that failure could be silent, smelly, noisy, or both smelly and noisy.