# Progress in THz Rectifier Technology: Research and Perspectives
### THz Diode Technology: General Information on Metal Multi-Insulator Metal (MI\({}^{n}\)M) Diode  
It is very difficult to combine requirements such as FOMs and low resistance in a single MIM diode. High value of nonlinearity and high asymmetry cannot be reached simultaneously. Figure 6: Fabrication of nanoantennas and the rectenna device. (**a**) SEM image of the nanoantenna array fabricated using EBL. (**b**) Overlap fabrication process: (i) first antenna arm, (ii) second arm exposure using EBL, (iii) removing the exposed resist using a mixture of MIBK and IPA developer with ratio of 153, (iv) deposition of 0.7 nm of oxide using atomic layer deposition (ALD), (v) second arm sputtering, (vi) complete device after the liftoff process using acetone. (**c**) SEM image of the fabricated overlap. (**d**) SEM image of the antenna-integrated diode. Reprinted with permission from ref. [8]. Copyright 2014 Scientific Reports. Asymmetry is achieved with metals presenting large differences in their band gaps (energy levels increase), whereas high nonlinearity is obtained with thicker barriers and larger barrier heights. This combination leads to high diode resistance. In general, the addition of another insulator layer allows precise control of asymmetry and nonlinearity. Therefore, to obtain low resistance, a possible solution is multiple insulator layers of a few nanometers thickness sandwiched between two similar or dissimilar metal electrodes. This device is referred to as MI\({}^{\text{n}}\)M, where n represents the number of ultrathin insulator layers. The use of multi-insulator layers with dissimilar affinity and band gap leads to different band offsets obtained at metal-interface (M-I), interface-interface (I-I), and interface-metal (I-M). The tunneling current can be easily engineered by modifying the band offsets through biasing. The introduction of two insulator layers (\(n=2\)) with very thin thickness, referred to as metal-insulator-insulator-metal (MIIM), introduces three different barrier height values, localized among metal-insulator (M-I), insulator-insulator (I-I), and insulator-metal (I-M) interfaces. The barrier height value at each interface governs parameters such as tunneling efficiency and FOMs. This approach allows one to create an additional asymmetry in the tunnel barrier and to achieve precise control over nonlinearity. The asymmetry can further be increased with different metal electrode combinations [7; 23; 54; 55; 56; 57; 58]. However, metals with similar work functions can also be used as long as the electron affinity of both insulators is different, so as to keep the resistance low. Although there have been continuous improvements, this type of diode is not yet optimized for THz frequencies. In fact, there are several open issues. By inserting three or more insulator layers between two metal contacts, this leads to an increase in the tunneling resistance, reducing the current. In addition, an external bias voltage is needed to move electrons out of the quantum well (QW) created by the two insulator layers. Consequently, this rectifying diode is less attractive for EH applications. In light of what has been said, the mechanisms that govern this device are based on resonant tunneling and step tunneling. Both these mechanisms are explained in detail as follows. Figure 7 shows the schematic energy band diagram of the resonant tunneling MIIM diode in forward and reverse conditions. Resonant tunneling occurs when a triangular quantum well (QW) (an area where electrons may exist) is created between the two insulators with different band gaps (one with a large band gap and another one with a smaller band gap). An external bias voltage controls the flow of electrons. Figure 7: Resonant tunneling: (**a**) forward bias, (**b**) reverse bias. In the forward bias condition in Figure 7a, a negative bias voltage is applied to the electrode with higher work function (to the left metal contact M1), whereas the electrode (M2) with lower work function is grounded.