Extracted delay times provide information useful for device scaling. In this work a novel parameter extraction technique that permits delay times associated with the physical operation of the transistor, along with element values for an equivalent circuit, to be determined from terminal S-parameter measurements. The technique is employed to investigate the operation of mm-wave AlInAs/GalnAs/InP heterojunction bipolar transistors. High current phenomena, such as the onset of the Kirk Effect, are clearly evident. The results indicate that the base region delay is dominant in determining the high frequency operation of these devices.