We examined the critical layer thickness (CLT) of mismatched epitaxial layers and strained heterostructures. Samples consisting of InxGa1-xAs/InP and In0.52Al0.48As/InxGa1-xAs/In0.52Al0.48As/InP were grown on InP substrates by metalorganic vapour phase epitaxy (MOVPE). The atomic force microscopy (AFM) was used to observe misfit dislocation generation. When the layer is buried in the heterostructure, its critical layer thickness increases. Our investigations have shown how many times this value may be exceeded in chosen technological conditions. A model was proposed, which explains difference between CLT of InGaAs with free surface and CLT of buried InGaAs. Heterostructures mentioned above were employed for producing InAlAs/InGaAs/InP HEMT transistors.