Inverts and Negative Voltage

[stampedeboss]

But aren't the HCT versions TTL?

[SeanB]

No, the HCT are TTL level compatible, done by shifting the threshold voltages slightly in manufacture. They are pure CMOS inside. They do not have any bipolar transistors other than the parasitic ones that are part of every CMOS process. The voltage levels are shifted so that they operate properly with loaded TTL outputs, and so they can sink more current in the low output so they have a standard TTL sink current ability at 5V. This though does limit the voltage range to TTL supplies, so they run from 3V to 7V only instead of going to 15V.