An n-channel or ambipolar field-effect transistor including an organic semiconductive layer having an electron affinity EA.sub.semicond; and an organic gate dielectric layer forming an interface with the semiconductive layer; characterized in that the bulk concentration of trapping groups in the gate dielectric layer is less than 10.sup.18 cm.sup.-3, where a trapping group is a group having (i) an electron affinity EA.sub.X greater than or equal to EA.sub.semicond and/or (ii) a reactive electron affinity EA.sub.rxn greater than or equal to (EA.sub.semicond.-2 eV).