Ambipolar operations in organic field effect transistors (OFETs) with heterojunction structures have been demonstrated. We have selected a biphenyl capped thiophene oligomer (BP2T) as p-type and fullerene (C-60) as n-type materials in the active layer of the OFETs. To investigate their intrinsic behaviors we measured the OFET characteristics in vacuum without breaking vacuum after device fabrication. Their electric characteristics depended on the heterostructure configurations. The OFET prepared with a co-deposited thin film of BP2T/C-60 showed high carrier transport performance and both carriers were efficiently injected into the channel of the active layer. In the bi-layered device, ambipolar characteristics were only observed when the n-type C60 molecules penetrated deeply into the BP2T layer. (c) 2007 Published by Elsevier B.V.