Application of Polycrystalline SiGe for Gain Control in SiGe Heterojunction Bipolar Transistors

This paper reports a method of controlling the gain of a bipolar transistor by incorporating Ge in the polysilicon emitter. Measured results show that varying the Ge content in the polySiGe from 0 to 33% gives a change in base current of approximately four. The competing influences of the Ge and the interfacial layer at the polySiGe/Si interface are investigated theoretically using an effective surface recombination velocity for the polySiGe emitter. Good agreement between theory and measured results is obtained. When incorporated into a SiGe HBT, the polySiGe emitter will allow the best trade-off between gain and BVCEO to be achieved for a given Ge and B profile in the base.