The frequency-dependent electrical characteristics of Au/Poly (3-Substituted thiophene) (P3DMTFT)/n-GaAs Schottky barrier diodes have been investigated by using capacitance-voltage (C-V) and conductance-voltage (G/omega-V) measurements at room temperature. Negative capacitance behavior has been observed in the C-V characteristic for each frequency. The magnitude of absolute value of C was found to increase with decreasing frequency in the forward bias region. The value of G/omega increases with decreasing frequency in the positive region. This can be attributed to the increase in the polarization at low frequencies and to the fact that more carriers are introduced into the structures. Negative capacitance phenomenon can be explained by the loss of interface charges from the occupied states below the Fermi level, caused by impact ionization process. According to obtained result, the values of C and G/omega are strong functions of frequency and applied bias voltage, particularly in the accumulation an inversion region. Doping concentration (N-d), diffusion potential (V-d), Fermi energy level (E-f), and barrier height (Phi(b)(C-V))values have been calculated from reverse bias C-2-V plots for 3 MHz. Finally, the obtained value of R-s in the accumulation region increases with decreasing frequency.