Self-aligned gate
Beginner
Using a switch’s gate, its on/off control, as the stencil for its two ends, so they always line up perfectly.
Novice
The gate is patterned first and then blocks the source/drain implant beneath it, so the source and drain end exactly at the gate edges without a separate alignment.
Expert
Removes the gate-overlap margin that misalignment once forced, cutting parasitic overlap capacitance. Sidewall spacers then offset the heavy implant from the channel; extension implants done before the spacers set the overlap that remains.