David wrote:
I mentioned this in connection with some manufacturers using gold
doping in transistors which would not normally be expected to have
gold doping. So you end up with a bunch of lessor named 2N3904s which
meet the 2N3904 specifications but are useless if you were looking for
low leakage diodes.
I believe all 2N3904s and 2N3906s are gold doped. National's certainly
were (Processes 23 and 66), and TI's and Fairchild's are. Not heavily
doped, like 2N2369s (with storage times of ~20nS), but just enough to
bring the storage time down to ~100nS. 2N2219s, 2N2222s, and 2N4401s
are also lightly gold doped.
If [4117 leakage is] not being tested, then where is the maximum specified
leakage number coming from? For a small signal bipolar transistor it
will typically be 25nA, 50nA, or 100nA, but the InterFET datasheet (1)
shows 10pA maximum and 1pA maximum for the A versions.
* * *
When this discussion of low leakage input protection started, I did a
quick search for inexpensive alternatives to the 4117/4118/4119 JFETs
and came up with nothing; all of the inexpensive JFETs are much worse
Same as any "guaranteed by design" spec -- by the device design. The
4117 series is unlike any other JFET -- the geometry is TINY, and the
4117 Idss is only 30-90uA (hundreds of times lower than other low-Idss
JFETs). [BTW, lowest Idss is why I recommend the 4117 over the 4118 and
4119 for use as a low-leakage diode. The 4118 and 4119 have higher Idss
-- up to 240uA for the 4118 and 600uA for the 4119 -- and tend to have
higher gate leakage, as well.]
Best regards,
Charles
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