Q:

Why Intel 8080 CPU chip required three voltages (+5, -5, +12 V)?

Why Intel 8080 CPU chip required three voltages (+5, -5, +12 V) ?
In wikipedia article I found:

The 8080 integrated circuit used non-saturated enhancement load nMOS gates, demanding extra voltages (for the load-gate bias). It was manufactured in a silicon gate process using a minimum feature size of 6 µm.

... but where can I read more about this?

A:

Most chips from that timeframe used these three supplies (check for instance old DRAMs or EPROMs). I think you will need books or articles about details of the (IIRC NMOS) technology of that time.
(edit)
As far as I have been able to find out:
The 8080 used N-MOS FET logic, the pull-ups that are required (because there are no P-FETs to do the pulling up) were N-MOS FETs that had their gate at -5V, putting them in their linear (resistor-like) region.
The internal logic worked at 12V. This is visible in the requirements for the clock inputs: they are 0/12V, not 0/5V.
The +5V was used used for the I/O, to be compatible with the common TTL chips of that day.

