# Progress in THz Rectifier Technology: Research and Perspectives
### THz Diode Technology: General Information on Metal Insulator Metal (MIM) Diode  
The metal-insulator-metal (MIM) diode is an ultra-fast switching device able to operate at THz frequency range, thanks to the quantum tunneling effect. The MIM diode consists of a thin dielectric layer within two similar or dissimilar metal electrodes. The purpose of an MIM is to receive a very high frequency AC signal and to rectify it into DC signal. The time it takes for the electron to cross the forbidden band gap of the insulator layer must be less than the inversion time of the AC. This is possible if an insulator barrier is less than 5 nm, so that the tunneling effect can be maintained at the switching frequency THz. Currently, this type of diode can rectify signals up to a frequency of 343 THz [21], useful for reaching the visible regime. Figure 2 shows an energy-band diagram of an MIM tunnel barrier. The barrier height \(\varphi\) at the metal-insulator (M-I) interface is determined by the difference between the work function \(\Psi\) and the electron affinity \(\chi\); the width of the barrier is defined by the insulator thickness; and \(\Delta\varphi\) represents the difference of heights of the barrier between the upper and the lower metal contacts [7; 22]. The conduction mechanism of an MIM diode is observed under three different bias conditions (zero bias, reverse bias, and forward bias). In Figure 3, the band structures under zero bias condition for symmetric and asymmetric MIM diodes are shown. In zero bias condition, when two metals are in contact, a net flow of electrons is observed from both sides, until the equilibrium condition is reached with an alignment of their Fermi levels (\(E_{F}\)) and a potential barrier is formed between the metal contacts. A diode is symmetric if similar electrodes create equal energy barrier heights at each M-I interface due to the same work function, as shown in Figure 3a [7; 23; 24; 25]. In this condition, the shape of the energy band diagram of the junction of the metal-insulator-metal with no bias voltage is rectangular. Consequently, a larger potential barrier at an M-I interface reduces the tunneling probability. This type of diode at zero bias condition is not able to rectify an electromagnetic wave and an external bias is required to have a nonzero current density. Figure 2: Energy-band profile of an MIM tunnel diode. In Figure 3b, dissimilar electrodes presenting different work functions create different energy barrier heights at each M-I interface. As a result, an asymmetric tunneling current is generated and a trapezoidal barrier replaces the rectangular barrier so that the tunneling occurs more easily and a net flow of current through the device will be possible under zero bias conditions. If the electron affinity of the insulator is close to one of the metal work function values, the band bending results in a triangular shape and this increases the tunneling current density through the barrier. Figure 4 shows the band structure of an asymmetric MIM diode for (a) forward bias, (b) reverse bias, and (c) more forward bias. In Figure 4a, the negatively biased metal contact M1 has a higher work function than the second grounded metal contact M2. Consequently, an increase in \(E_{F}\) is observed in the M1 side with a decrease in the barrier height at M1-I. A net flow of electron tunnels through the entire dielectric thickness and direct tunneling (DT) represents the dominant conduction mechanism. This condition is known as the forward bias condition. In Figure 4c, under more forward bias conditions, the bias voltage \(V\) further increases, so the trapezoidal barrier becomes triangular and a net flow of electrons will cross a reduced tunneling distance. In this case, the Fowler-Nordheim tunneling (FNT) represents the dominant conduction mechanism. Finally, in Figure 4b, under a reverse bias condition, the negatively biased metal contact M2 has a higher work function than the first grounded metal contact M1.