Metal–insulator–metal (MIM) structures have been proposed for infrared photodetectors, optical rectennas, and alternative choices to conventional solar cells. Generally, if two dissimilar electrodes are separated by a sufficiently thin insulator, electrons can travel through the insulating gap region due to the quantum tunneling effect. This tunneling through the insulating gap can be driven by an electromagnetic field or an external bias voltage. In our work, we incorporate the contributions of the Poole–Frenkel (PF) effect on the energy barrier of the MIM junction and the effect of Coulomb interaction between the electron charge and its virtual image charges. Using this modified PF-Coulombic barrier, we perform quantum calculations for electron tunneling using the transfer matrix method and Wentzel–Kramers–Brillouin approximations. This, along with the density of states, the Fermi occupation probability, and the quantum confinement effect is used to derive the dark current density for the MIM diode. To test the theory, MIM Au-TiO₂-Ti diodes are designed and fabricated in-house using photolithography, along with electron beam evaporation and sputter depositions. The dark current–voltage (I  −  V) characteristics of the fabricated MIM photodiodes are measured and show good agreement with theoretical predictions.