Analysis of square-law detector for high-sensitive detection of terahertz waves

Theoretical analysis of a square-law detector composed of a field effect transistor has been conducted to develop a circuit model for the terahertz (THz) wave detection. Mathematical formulae that indicate the detection characteristics of the detector are derived by applying the unified charge control model of FET channel carriers and by considering drift and diffusion current. The circuit model with an external circuit similar to the actual system is considered. The analysis of the circuit of the detectors reveals the effects of the subthreshold slope and the gate length of FETs on the sensitivity. In addition, square-law detectors have been fabricated using a high-electron-mobility transistor (HEMT) with an InGaAs/InAs/InGaAs double heterostructured channel on a glass substrate. The device has been fabricated using the layer transfer technology and showed electron mobility as high as 3200 cm2/Vs. Detection performance is characterized by directly inputting 1.0 THz waves through a THz probe to detectors. Detection results agree well with the characteristics predicted from the circuit model. Furthermore, our analysis expresses the contribution of drift and diffusion to the total detection characteristics. Experiments carried out using HEMT detectors also prove that the sensitivity, such as maximum voltage responsivity and minimum noise equivalent power of the detectors, is related to the subthreshold slope and the gate length. In other words, a small subthreshold slope and a short gate length of an FET lead to a high-sensitive detection.