Thermoelectric contributions to electronic transport and THz responsivity of TeraFETs – simulations and experiments

• Applicant: Professor Dr. Hartmut G. Roskos
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering ; Experimental Condensed Matter Physics
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 399177913

Antenna-coupled field-effect transistors (FETs) establish themselves successfully as sensitive detectors of terahertz (THz) radiation. The detection mechanism is based on plasmon-enhanced distributed resistive mixing in the FET's channel (Dyakonov-Shur mechanism) in combination with thermoelectric hot-carrier effects arising from diffusive contributions to energy and charge transport. Extending the hydrodynamic Dyakonov-Shur model, which does not take thermal effects into account, we derived in the first phase of this project a hydrodynamic model for charge and energy transport from the Boltzmann equation using the method of moments. Based on the current/voltage characteristics of the FETs, we also developed a charge-voltage model for graphene FETs (for other material systems, we had derived them earlier). The complete physical model with its four differential equations describing charge and energy transport was implemented in the circuit simulation tool Keysight ADS for the material systems Si, GaN, GaAs and Graphene. As input parameters for the simulations, only data extracted from DC measurements of the transistors and antenna simulations are needed. The model was found to reproduce the measured THz responsivity and noise-equivalent power of detectors in various material systems in a near-quantitative manner from 0.1 THz up to at least 4 THz. The contribution of the thermoelectric effect in transistors made in the III/V material systems and in Graphene could be determined; for III/Vs, it is detrimental at the operation points of the gate voltage.In the second phase of the project, for which we apply for funding now, we aim at various model refinements. One of them relates to the details of the potential distribution at the channel’s boundaries both at DC and THz frequencies. The AC potential distribution is expected to strongly influence the diffusive transport and thus the thermoelectric effect. Another subject of the studies will be saturation effects at high radiation intensities. Further refinements are mainly for graphene devices and involve a close cooperation with a team at Aalto University, Finland. With specifically designed device structures, we aim for the extraction of more robust data for the energy relaxation rates of the charge carriers. Using the advanced model, we will then embark on the design of optimized graphene-based detector structures, to be implemented by the Aalto team and to be measured at Aalto and by us. A significant part of the project will also be devoted to the preparation of publications (i) on the simulation tool, its underlying physical model assumptions and its algorithmic implementation in ADS, (ii) on the application of the simulation tool for the analysis of device performance, and (iii) on the predictive use of it for the development of improved detectors.

• DFG Programme: Research Grants
• International Connection: Finland
• Cooperation Partner: Andrey Generalov, Ph.D.