Heterojunction structure, also referred to as a heterostructure, of semiconductor material, in particular for a high electron mobility transistor (HEMT), includes a substrate, a stack of at least three buffer layers of a same semiconductor material with a wide bandgap EG1 based on a column-III nitride, namely an unintentionally doped first buffer layer, a second buffer layer, an unintentionally doped third buffer layer, an unintentionally doped intermediate layer, and a barrier layer arranged on the intermediate layer, said barrier layer being of a semiconductor material with a wide bandgap EG2 based on a column-III nitride; the second buffer layer has constant P+ doping throughout some or all of its thickness; and the third buffer layer includes a first region which is unintentionally doped throughout its entire thickness and at least one second region adjacent to said first region with N+ doping surrounding the first region.