Optimal asymmetry of transistor-based terahertz detectors
Detectors of terahertz radiation based on field-effect transistors (FETs) are among the most promising candidates for low-noise passive signal rectification both in imaging systems and wireless communications. However, it was not realized so far that geometric asymmetry of common FETs with respect to source-drain interchange is a strong objective to photovoltage harvesting. Here, we break the traditional scheme and reveal the optimally asymmetric FET structure, providing the maximization of THz responsivity. We fabricate a series of graphene transistors with variable top gate positions with respect to a mid-channel and compare their subterahertz responsivities in a wide range of carrier densities. We show that responsivity is maximized for input gate electrode shifted toward the source contact. Theoretical simulations show that for large channel resistance, exceeding the gate impedance, such a recipe for responsivity maximization is universal and holds for both resistive self-mixing and photo-thermoelectric detection pathways. In the limiting case of the small channel resistance, the thermoelectric and self-mixing voltages react differently upon changing the asymmetry, which may serve to disentangle the origin of nonlinearities in novel materials.