Schottky-diode action in thiophene oligomer is investigated by current-density--voltage (J-V) and capacitance-voltage (C-V) measurements. An energy-band diagram is deduced that explains the diode characteristics for both unintentionally and highly doped thiophene oligomers. We conclude that the diode consists of a thin layer of low ionizable acceptor density (${\mathit{p}}^{\mathrm{\ensuremath{-}}}$) at the metal-oligomer interface and a semiconductor bulk layer that has a higher dopant concentration (${\mathit{p}}^{+}$). The presence of the lower doped ${\mathit{p}}^{\mathrm{\ensuremath{-}}}$ layer leads to a built-in voltage of 0.5 V, which is both experimentally observed and predicted using standard Schottky theory. Differences in J-V characteristics upon doping the thiophene semiconductor are explained by the Schottky-barrier lowering effect for the reverse current density, and by a higher conductivity of the bulk for the forward current density.