Lesser known reasons: as bipolar transistors went to polysilicon gates (which have a tiny, ~1-2 Angstrom thick layer of oxide in them) and MOS transistors started to become leakier through their ever-shrinking gate oxides, the distinction between MOS and bipolar transistors became fuzzier. Modern MOS transistors leak less current through their gate oxides than the bipolar transistors did through the polysilicon emitter, but the physics of the two is not as different as it was twenty years ago.
https://web.archive.org/web/19970712065424/http://www.byte.c...
300 MHz / 115 W. Ten years later, you'd have Pentium 4 at 3000 MHz and 115 W.
https://en.m.wikipedia.org/wiki/VAX_9000
It was an enormously expensive failure.
Quoting you below...
"The most unusual circuit is the BiCMOS driver. By adding a few extra processing steps to the regular CMOS manufacturing process, bipolar (NPN and PNP) transistors can be created. The Pentium extensively used BiCMOS circuits since they reduced signal delays by up to 35%. Intel also used BiCMOS for the Pentium Pro, Pentium II, Pentium III, and Xeon processors. However, as chip voltages dropped, the benefit from bipolar transistors dropped too and BiCMOS was eventually abandoned."
I didn't realize that BiCMOS lasted so long. I thought it was only used on the original Pentium, but I really didn't look hard.
Edit: BiCMOS has a wiki.
Bipolar has for example lower noise than CMOS when it comes to opamps.
We always enjoy reading your articles, Ken!