  In this paper, we study the sensitivity-tunable Terahertz (THz) liquid/gas
biosensor in a coupling prism-three-dimensional Dirac semimetal (3D DSM)
multilayer structure. The high sensitivity of the biosensor originates from the
sharp reflected peak caused by surface plasmon resonance (SPR) mode. This
structure achieves the tunability of sensitivity due to that the reflectance
could be modulated by the Fermi energy of 3D DSM. Besides, it is found that the
sensitivity curve depends heavily on the structural parameters of 3D DSM. After
parameter optimization, we obtained sensitivity over 100{\deg}/RIU for liquid
biosensor. We believe this simple structure provides a reference idea for
realizing high sensitivity and tunable biosensor device.
