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

Question

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?

Answer 1

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.