I was looking at this old question

Hopf algebra and group structure correspondence for algebraic varieties

which says that there exists an equivalence between algebraic group structures on an algebraic variety $V$ (over a field) and Hopf algebraic structures on its algebra of regular functions $O(V)$. I would guess that this extends to an equivalence between algebraic semi-groups structures on $V$ and coalgebra structures on $O(V)$. Am I correct here?