We study charge transfer in a molecular system composed of a donor and an acceptor coupled to each other via a carbon conjugated-bond bridge (D-bridge-A). The effects of the bridge length in the transport are analyzed in the presence of an external electric field by density functional theory methodology. We find a charge accumulation in acceptor group that is strongly dependent on the bridge length and composition. In particular, for bridges with more than four carbon conjugated atoms we observe resonance tunneling type conduction in the curve of charge accumulation versus voltage. For positive bias the resonance takes place for high voltages when compared to the negative biases. This behavior suggests that the system can operate as a molecular transistor (MBFET) at some bias range. In addition we perform a current calculation in a simple model in order to get some more insight about the transport.