The most important demand required for emitting semiconductor element is the condition of their being possessed of high external quantum yield ηext, which provided obtaining maximum possible power of the luminous flux. The aim of this work is to establish a functional connection with the quantum yield cathodoluminescence (CL) of the technological mode of cultivation in the quasi-closed volume (QCV) specially undopped epitaxial layers n-CdSe/mica and their level of electron excitation. The layers of CdSe were prepared at a temperature of evaporation of the starting material ts = 650, 680°С and substrate temperatures of mica tev = (480 ’ 600)° С. Excitation layers are pulsed with electron beam, the electron energy is maintained at Ee = 40 keV. Their penetration depth in the layers n-CdSe was d ≃ 5 mkm. Duration of excitation impulses was 0.2 mks, and their repetition rate - 200 Hz, this consistent duty cycle q = 2.5·104. The diameter of the electron beam on the irradiated surface layers was ≃ 1 mm. Excitation current density layers ranged je = (0.22 ’ 25.5) А/cm2, that with Ee = 40 keV corresponds to the rate of generation of electron-hole pairs in the range of (2.0·1025 ’ 2.3·1 027) cm-3·c-1. The external quantum yield is the ratio of the number of layers n-CdSe of photons [1], emanating from the layer in half the sample, the number of electron-hole pairs generated by the electron beam inside the layer per unit time. If Wli and We - power flow of light and electron beam respectively. Eg – band gap CdSe. Power flow of light Wli emitted by the sample was calculated from measurements of the amplitude of the voltage impuls. At the experiment were taken into account that when the production of one CL nonequilibrium electron-hole pairs in the average energy is expended equal 3Eg [2]. Established that in the layers n-CdSe, the light output CL increases linearly with increasing current density excitation. Dependence of ηext (j) practically the entire range of current densities used excitation portion characterized saturation. The maximum value of the external quantum efficiency CL at equal levels of excitation observed in layers n-CdSe grown under high temperature conditions epitaxial close to thermodynamic equilibrium. It was of the (78 K) = 2.2%. The maximum value of the internal quantum efficiency of these samples was equal (78 K) ≃ 75%. Epitaxial layers n-CdSe grown in QCV crystals of mica and having relatively large value of the external quantum efficiency, a high degree of purity and crystalline perfection. With 78 K in the spectra of the boundary layers of CL line appears only free A-exciton.