The current-voltage characteristics of InP-based HBTs with InAlAs-InP composite emitters have been measured as a function of the thickness of the InP layer in the emitter. As the thickness varies, characteristics such as the gain and the ideality factor vary qualitatively as expected from the changes in position of the InAlAs barrier in the emitter. Quantitatively, however, the variations indicate that the interfaces vary systematically with InP thickness, becoming more abrupt for emitters with thicker InP layers.