Hydrogen sensitivity of InP HEMTs with a thick Ti-layer in the Ti/Pt/Au gate stack

We have investigated the hydrogen sensitivity of InP HEMTs with a gate stack containing a thick Ti-layer (order of 1000 Å). We have found that the hydrogen-induced piezoelectric effect in these devices is an order of magnitude smaller than conventional Ti/Pt/Au-gate HEMTs with a thin Ti layer (order of 250 Å). This markedly improved reliability can be explained through the diffusion mechanism of H in Ti which limits hydrogenation of the Ti layer to a thin sheet at the top. Using Auger electron spectroscopy, we have confirmed that under the studied conditions, TiH_x is only formed in the top 250 Å of the Ti-layer. In some devices located in the periphery of the wafer, we have observed a second hydrogen degradation mechanism that induces a large positive ΔV_T. This appears to be related to an improperly fabricated gate recess. The use of a thick Ti layer in the gate stack allows for a simple and effective mitigation of the H-induced piezoelectric effect in InP and other III-V HEMTs.