[time-nuts] TAPR TICC boxed (input protection)

Charles Steinmetz csteinmetz at yandex.com
Fri Apr 7 04:09:38 EDT 2017


David wrote:

>> what doping is used for PNP RF transistors and saturated switches
>> if it is not gold?  Does it also increase leakage?

I replied:

> Gold doping doesn't affect the speed of BJTs in the active region very
> much -- its purpose is to reduce minority carrier lifetime and, thereby,
> to reduce storage time when a transistor recovers from saturation.  I'm
> not sure how manufacturers deal with this in the case of PNPs.

After I posted, I recalled learning in a long-ago device physics course 
that both Gold and Platinum doping were used to reduce minority carrier 
lifetime in PNP saturated switches.  According to Motorola, the 
MPS3639/3640, 2N4209, and 2N5771 were gold-doped PNP saturated switches 
(all are now obsolete, although SMD versions of the 3640 and 5771 appear 
to still be available).

And yes, doping PNPs with either Gold or Platinum does increase reverse 
leakage current (Platinum less so than Gold).

Best regards,

Charles




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