In this paper, the growth and material properties of Al(2)O(3) layers grown by thermal atomic layer deposition (ALD) with water vapor, and plasma ALD with oxygen plasma are examined by spectroscopic ellipsometry and X-ray reflectivity on Si substrates and InGaAs/InP epilayers. The thermal-ALD and plasma-ALD deposited Al(2)O(3) layers have subsequently been used to passivate the surface of InGaAs/InP heterojunction bipolar transistors (HBTs). The impact and efficiency of the ALD-Al(2)O(3) passivation layers have been evaluated using the dc current gain and breakdown voltage of the InGaAs/InP HBTs. For comparison, the results from these experiments are contrasted with results from similar samples passivated with SiO(2) using conventional plasma enhanced chemical vapor deposition (PECVD). The thermal-ALD-Al(2)O(3) passivated InGaAs/InP HBTs show higher current gains as compared to structures passivated using the plasma-ALD or PECVD processes, suggesting differences in the dielectric-semiconductor interface properties. More importantly, as compared to PECVD-SiO(2), the common emitter characteristics of both (thermal and plasma) ALD-Al(2)O(3) passivated HBTs show fairly stable device breakdown voltage, which is of particular importance, especially in analog applications. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.060112jes] All rights reserved.