The influence of silicon nitride passivation on electron mobility of InGaAs/InP composite channel high electron mobility transistor structure has been studied. Different from the structures with single InGaAs channel, an increase in effective mobility u(c) with a negligible change of sheet carrier density n(s) after SiN deposition is clearly observed in the composite channel structures. The enhancement of u(c) could be explained under the framework of electrons transferring from the InP sub-channel into InGaAs channel region due to the energy band bending at the surface region caused by SiN passivation, which is further confirmed by low temperature photoluminescence measurements. (c) 2006 Elsevier B.V. All rights reserved.