The influence of silicon nitride (SiN) passivation on the electron mobility of InGaAs/InP composite channel high electron mobility transistor (HEMT) structures has been studied. Hall measurements were used to characterize the influence of SiN passivation on electrical properties. An increase in effective mobility mu(e) with a negligible change of sheet carrier density n(s) after SiN deposition is clearly observed. This behavior is different from the previous report on the single InGaAs channel HEMT structure, in which the increase in sheet carrier density with a negligible change of electron mobility was found. Photoluminescence (PL) measurements were carried out to evaluate the effects of SiN passivation on electrical and optical properties. The variations of the PL peak position, the transition intensity, and the linewidth of different passivated samples were analyzed to clarify the mechanism for the change in electrical properties. Our results suggest that the enhancement of mu(e) could be explained under the framework of electron transfer from the InP subchannel into the InGaAs channel region due to energy band bending at the surface region caused by the SiN passivation. (c) 2006 American Vacuum Society.