Geometric effect in a vertical stack-up metal-insulator-metal tunnel diode

The geometric effect was investigated in a vertically designed metal-insulator-metal (MIM) tunnel diode for which a narrow tunneling distance can be controlled easily and reliably, with the goal of enhancing rectifying efficiency, based on the angle of a pointed shape electrode and various thicknesses of insulator material. Although MIM tunneling diodes can provide ultra- high working speeds (>THz), the very low contrast ratio between forward and reverse currents results in poor rectifying efficiency. An asymmetric geometry design with two metal electrodes can be an effective approach for enhancing the contrast ratio between the tunneling currents. Using a sharp electrode with a pointed shape, it was determined that the rectifying efficiency and tunneling probability could be increased impressively depending on the angle of the pointed shape of the electrode. Moreover, the selection of insulation material was also important for improving efficiency. Although the band gap of Al2O3 is larger than that of HfO2, the rectifying efficiency was significantly improved by blocking reverse current well. In general, large band gap insulator materials are inappropriate for a tunneling device, due to the low tunneling current. However, in our approach, since the issue of low tunneling probability is compensated by the sharp tip structure, the larger band gap insulator produced better rectifying efficiency with the appropriate current density. The results of this study demonstrated that geometric design could be a possible solution to increase rectifying efficiency. If the geometric effect in the tunneling diode structures will be optimized more, it can improve the applicability of vertical stack-up MIM tunnel diode to THz switching devices, tunneling transistors and ultra-high speed electronics.