This paper reports on in situ passivation studies of GaAlN/GaN High electron mobility transistors (HEMT) structures grown by low-pressure metalorganic vapour phase epitaxy (LP-MOVPE) on sapphire and silicon carbide substrates. GaN, GaAlN, AlN and AlN/GaN superlattice (SL) layers have been deposited at low temperature (LT) by LP-MOVPE on the surface of GaAlN/GaN HEMT structures. These high resistivity LT materials behave like dielectric films and seem very promising insulating materials for HEMT applications. Growth optimisation of GaAlN/GaN HEMT structures with the different LT cap layers mentioned has been carried out. The critical impact of the induced strain on the physical properties of the LT cap layer/GaAlN/GaN HEMT structures was identified and studied using high-resolution X-ray diffraction (HR-XRD), atomic force microscopy (AFM), C V and eddy current measurements. HEMT structures based on LT-GaN and LT-AlN/GaN SL cap layers were found to have the best structural and electrical properties in term of surface roughness or sheet resistance of the 2DEG. Devices were made on these two samples in order to validate the statement. The static characteristics of these devices such as maximum drain current I-dss and transconductance g(m) are comparable to those obtained on standard structures and the pinch-off voltage is shifted in good agreement with the thickness of the related insulating layers as observed by C-V measurements on wafers. Such observations indicate that LT cap layers deposited by MOCVD can be used as dielectric films to passivate the surface states of HEMT wafers. (c) 2006 Elsevier B.V. All rights reserved.