From someone who discussed with the author of "Instruments of Amplification":
"Friedrichs also had tinkered with cuprous oxide transistors.
What he didn't tell in the book: only one of 10 cuprous oxide transistors had worked as intended,
and they tend to die at 5V+ or so. He wasn't aware of this when testing them with a curve tracer."
But still, 5V is not a crazy working voltage and many circuits use 3.3V.