Mesaelements of the focal plane array (FPA) of p – i – n diodes based on Al x Ga 1– x N for p–i–n heteroepitaxial structures (HESs) grown by the molecular-beam epitaxy and hydride epitaxy methods with the use of metalorganic compounds are formed. Elements of 320 × 256 FPAs with a pitch of 30 μm are separated by means of ion-beam etching through a photoresist mask in the argon ion stream produced by the Kaufmann ion source in a vacuum plant. To determine the required etching depth, contact profilometry and ultraviolet spectrophotometry methods allowing one to determine positions of the n -layer and sufficient etching depth of the sample are used. The thickness accuracy of the HES functional layers stated in manufacturer’s certificates does not exceed 28%. Rates of ion-beam etching of Al x Ga 1– x N for p–i–n layers with different compositions are determined.