NO2 (nitrogen dioxide) gas is known as a toxic gas, and its concentration in ambient air needs to be measured to determine whether the concentration is still below the quality standard or not. NO2 gas can absorb visible light and ultraviolet light. To measure the concentration of this gas, someone can use a semiconductor as a light sensor. To absorb light at a specific wavelength, it is necessary to know the optical properties of the semiconductor and the appropriate quantum well width. In this study, the author used an AlN/GaAs/AlN (aluminum nitride / gallium arsenide / aluminum nitride) semiconductor, where GaAs serves as a quantum well and AlN serves as a barrier layer. The purpose of this study is to design an AlN/GaAs/AlN semiconductor as a sensor for measuring NO2 gas. From the calculation results, the author could make AlN/GaAs/AlN as a sensor for measuring NO2 gas. To absorb wavelengths of 341–489 nm absorbed by NO2, the width of the GaAs quantum well is 17–24 angstroms. Meanwhile, for GaAs electrons to penetrate the AlN barrier layer, the width of the AlN layer must be less than 92 and 14 angstroms for electron energies of 3.12E-12 and 1.56E-12 erg, respectively.