CMOS
CMOS was introduced to ICs in the late 1960s. Before then, NMOS was used for digital logic. NMOS isn’t very energy efficient because the pullup resistor is dissipating energy when the N type transitor is on.
To get better energy efficiency, an extra P type transistor replaces the pullup resistor. Only one transistor is on at a time.
https://en.wikipedia.org/wiki/CMOS:
Complementary Metal Oxide Semiconductor (CMOS) is a type of MOSFET fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFETs to build logic functions.
Here is a CMOS inverter showing the complementary N and P type MOSFETs.
And here’s how it could be laid out using a tool like Magic
This video shows a sped up screen capture of me laying out a DRC clean inverter with Magic.
Course feedback
The Zero To ASIC course took me on a fantastic journey from drawing and simulating a MOSFET, formal verification leading up to implementing a custom design with an open PDK and completely open source tools. The course is crammed full of interesting material with great pacing and support from Matt, and it's been a fantastic opportunity to meet other folks with shared interests and different backgrounds. The course has left me excited with opportunities for new projects and optimism for some working silicon!