The forward and reverse bias capacitance–voltage (C–V) and conductance–voltage () characteristics of Al/polymer/PEDOT-PSS/ITO heterojunction diodes have been investigated in the frequency range of 10 kHz to 5 MHz at room temperature. The obtained C–V and characteristics of these diodes at various gate biases show fairly large frequency dispersion especially at low frequencies and forward bias due to surface states N_ss in equilibrium with the polymer semiconductor and series resistance R_s. These observations indicate that at low frequencies, the charges at interface states can easily follow an ac signal and yield an excess capacitance and conductance. In addition, the forward and reverse bias current–voltage (I–V) characteristics of Al/polymer/PEDOT-PSS/ITO heterojunction diode were measured at room temperature. The main diode parameters such as barrier height Φ_B0, ideality factor n and average series resistance R_s obtained from the forward bias ln I–V characteristics were found as 0.53 eV, 4.79 and 195 Ω, respectively. Experimental results show that both N_ss and R_s values should be taken into account in determining frequency and voltage-dependent I–V, C–V and characteristics.