Analysis of Substrate Effect in Field Effect Transistor Terahertz Detectors

The development of terahertz (THz) detectors based on field effect transistors (FETs) with planar antennas requires improving the detector responsivity, thus it needs higher coupling efficiency between the antenna and THz radiation. Substrate effect and its impact on the detector responsivity have been investigated for the case of semiinfinite substrate, and four different types of planar dipole antennas are compared. The ratio ( R ) of the power radiated into the air to the total power radiated by planar antenna shows several peaks when the substrate thickness increases. Independent of antenna structure and source impedance, the ratio R would only depend on the substrate thickness, the dielectric constant of substrate, and the operation frequency. Furthermore, the simulation result shows that the radiation power ratio R is correlated with the detector responsivity. This correlation is independent of FET input impedance and antenna structure. It indicates that the ratio R can be used as a reference to choose proper substrate parameters. All the results above can be extended to common materials and broad range of frequency. The comparison of four antennas shows that besides substrate effect, the antenna design would also have a significant impact on the detector responsivity.