due vs unpaid

due is a noun but unpaid is not a noun.

Word	Noun	Adjective	Verb	Adverb
due	Yes	Yes	No	Yes
unpaid	No	Yes	No	No

due (noun)	unpaid (noun)
that which is deserved or owed
a payment that is due (e.g., as the price of membership)

due (adjective)	unpaid (adjective)
owed and payable immediately or on demand	not paid
suitable to or expected in the circumstances	engaged in as a pastime
capable of being assigned or credited to	without payment
scheduled to arrive

due (adverb)	unpaid (adverb)
directly or exactly; straight