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.