Effect of interfacial oxide thickness on 1/f noise in polysilicon emitter BJTs

The role of the interfacial oxide (IFO) between the polysilicon and monosilicon emitter regions on the noise behavior of n-p-n poly-emitter bipolar transistors was investigated through 1/f noise measurements. Bipolar junction transistors with different IFO thickness, and emitter geometry were utilized. Measurements with variable external base bias resistance (R_S) were used to investigate the relative contribution of each individual noise source from the base current (S_IB), the collector current (S_IC) and, the internal emitter and base series resistances (S_Vr). When the voltage noise power spectral densities S_VC and S_VB were measured across resistances in series with the collector and base, respectively, using a relatively large R_S (∼1 MΩ), S_IB was found to have the dominant noise contribution at lower bias currents. On the other hand, when the voltage noise power spectral densities S_VC and S_VE were measured across resistances in series with the collector and emitter, respectively, in a different experimental setup with a low R_S value, S_Vr was found to have the dominant noise contribution at higher bias currents. IFO was found to increase S_IB, S_IC, and S_Vr. S_IB was modeled as a combination of tunneling and diffusion fluctuations of the minority carriers in the emitter; whereas S_IC was modeled as a combination of number and diffusion fluctuations of the minority carriers in the base. S_Vr was attributed to the internal emitter resistance noise originating from the fluctuation in the majority carrier flow through the IFO.