Katy Perry -- Hits On College Football Star ... 'Trevor, Call Me'

Katy Perry unleashed her inner Demi Moore on ESPN this morning -- obsessing over a college football star who's 10 YEARS YOUNGER ... and telling the guy to "Call me!"

30-year-old Perry appeared on ESPN's "College Gameday" this morning as the celebrity guest picker -- but before she weighed in on the big game, she picked Oklahoma quarterback Trevor Knight as the guy she's currently CRUSHING ON.

Perry held up heart-shaped sign featuring Knight's pic -- and urged the QB to hit her up, ASAP.