Gold was used as a lifetime killer. Usually put in just a tiny
region of the base, because you can't afford to kill the lifetime
in the active base, just want to recombine the stored charge.
I haven't seen this since the 80s in ICs, and even then it was
not real popular with the fab guys (how to keep it off the
tube walls and not have it end up on the next non-gold-doped
boat was a problem; I think they kept special tubes for the
gold doped stuff, which wasn't much of a volume runner).
Other metallic contaminants can have the same effect.