nitrogen (N)-doped polycrystalline diamond was reported to have excellent electron emission properties. One of the obstacles to practical application of this diamond was its localized and uncontrolled electron emission site. Even though we have reported the broad area electron emission from homoepitaxial diamond in our previous work, the difficulty remained to fabricate the injection contact for homoepitaxial diamond because its substrate is an insulator, which is not suitable for practical applications. In this study, we have obtained heavily N-doped heteroepitaxial diamond films on Iridium (Ir), and its electron emission properties are investigated. It is found that the emission threshold voltage of heteroepitaxial diamond (100) is higher than that of other heavily N-doped diamonds and has linear relationship between the spacing as is not observed in metal-insulator-vacuum type emission, which is a model proposed for electron emission from polycrystalline and homoepitaxial diamonds. From the characterization results, obtained film is confirmed to be identical to heavily N-doped polycrystalline and homoepitaxial diamond. The difference in the emission properties, thus, is caused by high electron affinity of diamond (100) and the absence of negative charges at the Ir/diamond interface. (C) 2004 American Vacuum Society.