Negative differential resistance (NDR) with a room temperature peak-to-valley ratio of 8 was observed in a graphene-based ballistic field-effect transistor (FET) with an oblique top gate. Graphene FETs with a top gate inclination angle of 45° and a drain-source distance of 400 nm were fabricated on a chip cut from a 4-inch graphene wafer grown by chemical vapor deposition (CVD). Of the 60 measured devices, NDR was observed only in the regions where the CVD graphene displayed a Raman signature of defectless monolayers. In other specific positions on the wafer, where graphene quality was not high enough, and the Raman signature indicated the presence of defects, the ballistic character of transport was lost and the graphene FETs displayed nonlinear drain-voltage dependences tuned by the top-gate and back-gate voltage.