28086Re: [midatlanticretro] Re: vintage SRAMs self healing
- Nov 13, 2012This is a very interesting anomaly. I personally can't recall this
happening before as I haven' tried this method. I would throw out the IC
in question if it failed a memory test.
However, this part is one of the first times they used depletion-mode N
channel Mosfets on the die, versus enhancement mode type. The geometry
of the mosfet incorporates a depletion mode region which isolates the
source, drain and gate regions - this is besides the gate insulation.
This is the main distinction over the original mosfet type making it's
performance comparable to the bipolar Rams of that day.
This is a highly capacitive layer within the mosfet. I wouldn't doubt a
40yr old component with this type of process would suffer from some kind
of leakage. I would garner a strong a suspicion that you are
experiencing the mosfet becoming rejuvenated, to a certain degree, by
running the test repeatedly.
The real question is if these components remain "healed" if you leave
this sitting in your parts bin for a few weeks and testing them again. I
suspect the depletion region will start to deteriorate over time again.
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