Low-frequency noise in self-aligned bipolar transistors

We report the first study of the low-frequency noise in self-aligned npn bipolar transistors, which use the polysilicon emitter. Some of these devices showed excess noise spectra different from the 1/f law generally observed in diffused junction transistors, and the spectral shape S(f) was found to vary from sample to sample. For instance, we have observed two different characteristics, S(f)∼1/f and S(f)∼1/[1+(f/f0)2], in two adjacent transistors on the same chip. We attribute the latter to carrier trapping in the oxide barrier at the poly-/monosilicon boundary, whose inhomogeneity could explain the wide variation of the noise spectra.