Negative Differential Resistance (NDR) is obtained at room temperature for Metal-Insulator-Insulator-Insulator-Semiconductor (MIIIS) diodes after band offset engineering. After post-metallization annealing (PMA), this characteristic is lost due to an inner diffusion of elements. The MIIIS diodes (as-prepared and PMA) were fabricated using a gate stack of atomic-layer deposited ultra-thin (2 nm/1 nm/2 nm) high-k oxides (Al2O3/HfO2/Al2O3) and their I-V, C-V and I-V-T characteristics were studied. A quantum well of 1.3 eV is obtained in the intermediate oxide for the as-prepared sample, promoting quantization of energy levels and resulting in three NDR zones at 0.1, 0.28 and 0.5 V due to the effects of resonant tunneling (RT). These zones are non-existent in the PMA diode. RT was found to be dominant at low voltages due to the discrete energy levels at the conduction band of the HfO2 oxide. After RT, the main conduction mechanism for both diodes is Poole-Frenkel.