A thin Al2O3 layer was atomic layer deposited on a 50 - or 70 nm-thick, polycrystalline Mn-doped BiFeO3 (BFMO) thin film to fabricate a dielectric/ferroelectric bi-layer capacitor. The thin Al2O3 layer worked as a tunnel switch to improve the ferroelectric performance of the BFMO films. The BFMO thin film and BiFeO3 film contains a high density of charged defects, such as oxygen vacancies, Bi vacancies, and Fe2+ ions reduced from Fe3+, which induce a large leakage current and cause instability of ferroelectric polarization in one direction. An asymmetric hysteresis loop and severe depolarization was observed in the BFMO capacitor due to the local field induced by the defect complexes that are mainly formed near the Pt/BFMO interface from interactions between charged defects. Depositing a thin Al2O3 tunnel switch layer reduced these detrimental phenomena, which could be attributed to a decrease in the amount of oxygen vacancies as well as the suppression of polarization back-switching after the polarization switching and during non-switching. The defects in the BFMO films also caused domain wall pinning during the relaxation time so the switching speed decreased with increasing relaxation time. Adopting an Al2O3 tunnel switch layer also improved this problem.