. For MID-IR application a low RC time constant is mandatory. Diodes based on thermal activation, on tunneling effect and ballistic effect will be discussed. Schottky barrier diodes where electron moves across the barrier by a thermal activation are theoretically limited to frequencies of a few THz (5 THz), due to relatively higher resistance of semiconductors compared to metals. Diodes based on tunneling effect are divided into two classes: low voltage tunnel diodes and ultralow-voltage diodes. An example of low voltage tunnel diode in MID IR rectennas are the metal-insulator- metal (MIM) diode and Metal Insulator Insulator Metal (MIIM). MIM/MIIM diodes have fast electron switching speeds, of the order of 10 fs, and use metal electrodes, thus providing a low resistance. Therefore, they can be used at higher frequencies than Schottky barrier diodes and they can be operational at up to 150 THz. An example of ultralow-voltage diode is geometric diode based on ballistic effect. This planar structure avoids the capacitance/resistance tradeoff because it is not a sandwich structure. Made out of a conductive material, it can have both low resistance and ultra-low capacitance. They show to be operational at up to 28 THz.
4.5 Schottky diode detector
A junction formed by metal in contact with a moderately doped n type semiconductor material is named Schottky barrier diode. It is a unilateral device where current flow in one direction from metal to semiconductor and it is prevented in the other.