This adder is fully discrete and uses 6 transistors per full adder (not including the output driving transistors). This circuit is a bit of a mess with three full adders but a single full adder circuit can be seen in one of my other posts.
The first two numbers are the inputs and the third number is the output sum.
I have built this adder circuit on a custom PCB to add two 8-bit numbers and it works well (despite being comparatively slow).
6 transistors per full adder is the lowest I can get for now, I'd be interested to see a full adder with a lower count.
