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answers:
Mr. Un-couth say: Variations in doping among other things.
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Duane say: As a Armature Radio Operator, HAM, even I am having a hard time figuring out you problem. Like several have said, there are many reasons. I would need to know the circuit and why you think it has variations.
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mustanger say: Temperature shifts (especially high temps) cause instability and eventually failure in transistors.
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Jack say: Context? Without a context this cannot be answered. The answer could be temperature variation, extreme conditions, pressure, light, being used at the limit for so long that it "wears out", or manufacturing variances/defects.
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Second say: temperature
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