Comparison of experimental and theoretical results on polysilicon emitter bipolar transistors

Two types of polysilicon emitter transistors have been fabricated using identical processing except for the surface treatment prior to polysilicon deposition. The first type was given a dip etch in buffered hydrofluoric acid, which was intended to remove any interfacial oxide, while the second type was given an RCA clean, which was intended to grow an interfacial oxide of known thickness. Detailed electrical measurements have been made on these devices including the temperature dependence of the gain over a wide temperature range. The transistors given an RCA clean have gains approximately five times higher than those given an HF etch. In addition, the temperature dependence of the gain is different for the two types, with the HF devices exhibiting a much stronger dependence at high temperatures than the RCA devices. A detailed comparison is made with the theory and it is shown that the characteristics of the HF devices can largely be explained using a transport theory, while those of the RCA devices can be fully explained using a modified tunneling theory.