In this paper, we analyze the problem of quasiballistic electron transport in ultra small of mercury -cadmiumtelluride (Hg0.8Cd0.2Te -MCT) n-nn devices from hydrodynamic point view. From our study, we note that, when the size of the active layer is low than 0.1μm and for low bias application ( ) mV 9 ≥ , the quasi-ballistic transport has an important effect. Keywords—Hg0.8Cd0.2Te semiconductor, Hydrodynamic mode, Quasi-ballistic transport, Submicron diode