Strained-Silicon Heterojunction Bipolar Transistor

Experimental and modeling results are reported for high-performance strained-silicon heterojunction bipolar transistors (HBTs), comprising a tensile strained-Si emitter and a compressively strained Si_0.7Ge_0.3 base on top of a relaxed Si_0.85Ge_0.15 collector. By using a Si_0.85Ge_0.15 virtual substrate strain platform, it is possible to utilize a greater difference in energy band gaps between the base and the emitter without strain relaxation of the base layer. This leads to much higher gain, which can be traded off against lower base resistance. There is an improvement in the current gain β of 27 × over a conventional silicon bipolar transistor and 11× over a conventional SiGe HBT, which were processed as reference devices. The gain improvement is largely attributed to the difference in energy band gap between the emitter and the base, but the conduction band offset between the base and the collector is also important for the collector current level.