An n-channel or ambipolar field-effect transistor including an organic semiconductive layer having an electron affinity EAsemicond; and an organic gate dielectric layer forming an interface with the semiconductive layer; characterised in that the bulk concentration of trapping groups in the gate dielectric layer is less than 1018cm−3, where a trapping group is a group having (i) an electron affinity EAX greater than or equal to EAsemicondand/or (ii) a reactive electron affinity EArxngreater than or equal to (EAsemicond. −2eV).