In this work we studied the possibility of improving InGaAs/InAlAs-on-InP high electron mobility transistor (HEMT) devices by using temperature-graded InAlAs buffer layers with growing temperatures in the range of T-g = 250-560 degrees C. Our specimens were grown by molecular beam epitaxy and we analyzed them using plane view and cross-sectional transmission electron microscopy, atomic force microscopy, scanning electron microscopy and Hall effect measurements. We found that growth at the optimum temperature (T-g = 530 degrees C) of a thin InAlAs buffer sublayer between the InP substrate and the thick InAlAs buffer layer (grown at low or high T-g) can dramatically improve the crystalline quality of our HEMT devices. Therefore the growth temperature of the buffer could be used as engineering parameter to tailor the electronic properties of InAlAs layers without interfering with the layers＇ crystalline quality, already assured by the first optimized nanometers.