Disclosed is a semiconductor device (1) for determining NO concentrations in fluids such as exhaled breath. The device (1) typically comprises a pair of electrodes (18) separated from each other to define a channel region (16) in an organic semiconductor (14), a gate structure (10) for controlling said channel region, and a receptor layer (22) at least partially overlapping said channel region, said receptor layer comprising a porphine or phtalocyanine coordination complex including a group III-XII transition metal ion or a lead (Pb) ion for complexing NO. Such a semiconductor device is capable of sensing NO concentrations in the ppb range.