A New Model for the Low-Frequency Noise and the Noise Level Variation in Polysilicon Emitter BJTs

This work presents a new, physically-based model for the low-frequency noise in high-speed polysilicon emitter bipolar junction transistors (BJTs). Evidence of the low-frequency noise originating mainly from a superposition of generation-recombination (g-r) centers is presented. Measurements of the equivalent input noise spectral density ($S _{{I}_B}$) showed that for BJTs with large emitter areas $(A _{E}) S _{{I}_B}$ is proportional to 1/$f$, as expected. In contrast, the noise spectrum for BJTs with submicron $A _{E}$ showed a strong variation from a 1/$f$-dependence, due to the presence of several g-r centers. However, the average spectrum $\langle S _{{I}_B}\rangle $ has a frequency dependence proportional to 1/$f$ for BJTs with large as well as small $A _{E}$. The proposed model, based only on superposition of g-r centers, can predict the frequency-, current-, area-, and variation-dependency of $\langle S _{{I}_B}\rangle $ with excellent agreement to the measurement results. The SPICE parameter $K _{F}$, extracted from $\langle S _{{I}_B}\rangle $ is found to be proportional to 1/$A _{E}$ with the product $K _{F} \times A _{E} =4.3\times 10 ^{-17}~\hbox{cm}^{2}$. The relative variation in the noise level is found to be proportional to $A _{E} ^{-0.5}$, resulting in an absolute variation proportional to $A _{E} ^{-1.5}$. The g-r centers are most likely located next to the thin $\hbox{SiO}_{2}$ interfacial layer between the polysilicon and monosilicon emitter. The areal trap density, responsible for the low-frequency noise within 1−104Hz, is estimated to be $n _{T} =3\times 10 ^{9}~\hbox{cm}^{-2}$. From temperature measurement of one clearly observed g-r center, the extracted trap energy level and capture cross-section are 0.31 eV and $2\times 10 ^{-19}~\hbox{cm}^{2}$, respectively.