The influence of emitter undercut on device parameters of SiGe HBT

SiGe HBT is a key device for BiCMOS process used for high-speed communication, automotive radar and defense military. For normal SiGe process, the control of emitter window undercut is often not easy. In this paper, we studied the influence of emitter undercut on HBT parameters, through both TCAD simulation and wafer results. While transistors with larger area-to-perimeter ratio are supposed to have greater current gain, it is found in our research that with the increase of this ratio along with increasing undercut, the gain decreased. The process mechanism of this gain decrease is discussed and the influences on other parameters such as cut-off frequency have also been studied in this work. It is found in our study that when the undercut increased over 100%, the device parameters changed no more than 10%. Therefore, although the emitter undercut is hard to be avoided in a given process, it can be controlled, and the influence of fluctuation is acceptable.