I know a diode can short out and fail closed. Is it possible for a diode to fail in the open position, and what would cause this to happen?
A diode fails closed due to overvoltage. This is called punch-through. This is is the principle used in ESD diodes. If they can't handle the voltage the PN juntion fails and short to ground, protecting any circuitry after them. On the image you see a small black dot where the voltage went through the junction. eg:
*Second picture added that shows this better on a BJT.
A diode typically fails to open happens due to over current. This is called metallization burnout and can occur from things like EOS (Electrical Over Stress). Image shown bellow. Over current causes excessive heating and literally burns the metal away. As mentioned above this is easy to demonstrate on LEDs as their current carrying capability is much lower than rectifier didoes.
Diodes fail open all the time. Sure, they are of the light emitting variety, but they are still diodes.
Why do they fail open? Thermal Runaway, cause by increased voltage increasing the current through the led junction, causes Heat to literally fry the diode junction. It then goes poof, no more junction. It physically can't conduct any more electrons.
This is separate from the led bond wires melting or breaking.
A diode will typically fail open if one of the bond wires from the leads to the actual semiconductor opens like a fuse.
To verify the contention made in a recent answer to another question, I applied a high voltage pulse without current limiting, using a capacitor bank, first to a couple of no-name LEDs, once forward and once in reverse bias, and later did the same to a couple of 1n4148 diodes.
All the diodes failed open. The LEDs were more spectacular, with the cases cracking open, but the 1n4148 diodes failed with the same end result.
So yes, a diode can fail open.
By "fail open" I assume you mean fail in such a way that it doesn't conduct in either direction. It is possible for a diode to fail open. I've applied too high a voltage to a diode and had it fail open.
As far as I know, the only way for a diode to fail open is when it fails destructively, i.e.:
- Leads or bond wires get destroyed (this very often happens with LEDs, which have comparatively long and thin bond wires from the lead to the silicon/sapphire)
- For Schottky diodes: the metallization-silicon interface gets heated beyond the range of temperatures where they thermally match, and heat stress peels them loose
For low and medium energy situations, the dominant failure mode of diodes is to fail closed. Open failure is usually a subsequent event, not a seperate failure mode.
Sure. Just a little while ago I pulled an IN749A Zener out of a Tektronix T922 oscilloscope. It had been said to be a failure-prone part in this scope and it affected triggering, which is what's jacked up so that's why I went after it first thing. I pulled the diode and went at it with a DMM, and sure enough it was reading many megohms in both directions. In the circuit it sits in series with a 620-ohm resistor between the -8VDC supply and ground, and the node in between the two is apparently used as a 4.3V reference so the diode is reverse-biased into its Zener voltage drop all the time. I guess this poor IN749A could only live for so long like that (the scope was built in 1977)
I repair and design vintage electronics and every failure mode ends up being weird. Just pulled out 2 1n4002 diodes out from a bipolar power supply of a 1970's Maestro ps1a phaser that looked to be slowly cooking for a while. The positive 12v supply would only show .7v under load of the op amp and transistor circuit (which probably has a failure yet to be found) but when isolated worked properly +12v and tested properly except a varying forward drop off voltage. .5 to .6v on my DMM. After pulling them out and testing new ones sussessfully, I tested the forward resistance at 100k. Hope this helps.