A combination of structural and electrical characterization techniques was applied to polymer Ta capacitors with in situ and prepolymerized poly(3,4-ethylenedioxythiophene) (PEDOT) cathodes with a broad range of Ta2O5 dielectric thicknesses. The in situ PEDOT was produced by polymerizing the EDOT monomer using an oxidizer/dopant. The prepolymerized material was an aqueous suspension of doped polymer which was dried to produce the PEDOT film. Experimental data show that polymer Ta capacitors with prepolymerized PEDOT have lower dc leakage and higher breakdown voltage (BDV) compared to polymer Ta capacitors with in situ PEDOT. The difference in dc leakage and BDV between these two types of capacitors becomes greater with increasing thickness and improved structure of the Ta2O5 dielectric film. These experimental results are inconsistent with current theories presented in the literature. An alternative model is presented based on classical metal/insulator/semiconductor (MIS) theory, where in this case M corresponds to the Ta metal, I corresponds to the Ta2O5 insulator, and S corresponds to the semiconducting PEDOT. According to this model, a potential barrier for the current carriers at the insulator/semiconductor interface controls the current flow through the Ta2O5 dielectric under normal operating conditions (positive polarity on the Ta anode and a temperature range of -55 degrees C <= T <= 105 degrees C). In situ polymerization of the PEDOT adversely affects this barrier, while the prepolymerized PEDOT suspension leaves it essentially intact. The different migratory ability of the dopants in in situ and prepolymerized PEDOT also contributes to the differences in electrical performance of polymer Ta capacitors with in situ and prepolymerized PEDOT cathodes.