A modeling and experimental study is carried out to understand why low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE) of AlGaAs in nitrogen atmosphere differs from that in hydrogen in a horizontal tube type of reactor. To this end flow, heat transfer as well as the key chemical species’ mass transport are considered. The increased uniformity in N 2 atmosphere is related to the higher molecular weight and, therefore to the higher gas density of the carrier resulting in a flow structure that is more favorable for improved growth rate uniformity of AlGaAs on the substrate. Due to the so called “cold finger” [L. Stock, W. Richter, J. Crystal Growth 77 (1986) 144; D.F. Fotiadis, M. Boekholt, K.F. Jensen, W. Richter, J. Crystal Growth 100 (1990) 577.] effect as well as the enhanced inertia of the carrier gas and lower diffusion coefficients of the growth rate limiting chemical species in N 2, lower total flow rates are found to be optimal for material quality and layer thickness uniformity when using N 2 as carrier gas. The dependence of growth rate uniformity on the carrier gas and total flow rate can only be understood by the detailed numerical modeling of three-dimensional flow, heat and species’ mass transfer with resulting layer deposition on the susceptor. The results of experiments are in good agreement with the modeling computations.