The goal of this thesis is to contribute to the understanding of charge transport in organic field-effect transistors (OFETs) made of pentacene. Organic thin-film transistors (OTFTs) with active layers thicknesses of 5, 10, 20, and 100 nm were fabricated in order to examine their structure and electrical characteristics and identify the key parameters that affect the charge transport in these devices. The conductivity of the pentacene films determined via four probe measurements suggests the presence of extrinsic charge carriers, or residual carriers, which have a large influence on device performance and electrical characteristics. The origin of these carriers is discussed in terms of a charge-transfer process occurring between the organic semiconductor and the electron acceptor states of the gate oxide surface. The gate interface is studied by varying the density of residual carriers via modification of the oxide surface by different plasma treatments with or without using subsequent deposition of various molecular monolayers. The OFETs yielded residual carrier densities ranging from 5 × 109 et 1 × 1013 cm-2 depending on the gate interface modification. The electrical characteristics such as the film conductivity and field-effect mobility are shown to be dependent on the density of residual carriers. Based on the measured density of residual carriers the OFETs are classified into two groups : devices based on doped and on un-doped pentacene. Temperature-dependent measurements of the field-effect mobility and the film conductivity performed on these devices reveal a thermally-activated character. This suggests that charge transport in the examined devices occurs via hopping between localized states. Based on the activation energies of the film conductivity and the field-effect mobility, the difference in energy between the Fermi and the transport level Δε is estimated to be between 10 and 160 meV depending on the density of residual carriers. This range of values is in agreement with thermoelectric power measurements performed on the same devices. The thermoelectric power is discussed in terms of two contributions : one that is dependent on Δε and another constant term that is independent of temperature and gate interface modification. The latter contribution was measured to be 265 ± 40 μV/K and originates from the creation of the phonon cloud associated with local changes in intermolecular interaction. This suggests that the charge carrier in the channel is dressed with a cloud of polarization, an electronic polaron.