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We have investigated the hydrogen sensitivity of InP HEMTs with a WSiN/Ti/Pt/Au gate stack. We have found that the impact of hydrogen on the threshold voltage of these devices is one order of magnitude smaller than conventional Ti/Pt/Au-gate HEMTs. This markedly improved reliability has been studied through a set of quasi-2D mechanical and electrostatic simulations. These showed that there are two main causes for the improvement of the H-sensitivity. First, the separation of the Ti-layer from the semiconductor by a thick WSiN layer significantly reduces the stress in the active layer. Additionally, the thinner heterostructure and the presence of an InP etch-stop layer with a small piezoelectric constant underneath the gate reduces the amount of threshold voltage shift that is caused by the mechanical stress.