Influence of the emitter-base junction depth on the low frequency noise of Si/SiGeC heterojunction bipolar transistors

Abstract

This work presents low frequency noise results in high-speed Si/SiGeC heterojunction bipolar transistors (HBTs). In this new generation of HBTs carbon doping is processed during of the deposit of the epitaxial SiGe base layer in order to suppress boron out-diffusion. Low frequency noise study is performed on three type of transistors that differ by the thickness of the Si cap layer. The Si Cap layer is a non intentional doped Si layer deposit after the SiGeC base layer and prior the contact emitter structure. Thus, the results on the three different Si Cap HBTs allow us to study the influence of the Emitter-Base junction depth on the low frequency noise of these HBTs. Measurements of the equivalent input noise spectral density (Si_B) showed that spectra are composed of a 1/f component and the white noise is always reached at low bias. For the smallest transistors we observed the presence of Lorentzian(s) component(s). The excess noise sources are mainly located at the intrinsic emitter-base junction. Concerning the 1/f noise level, a quadratic dependence on base current bias and an inverse dependence on the emitter area are found. The normalized figure of merit, K_b = K_fxA_E, is ranging between 1.7 and 2.1 10^-9 μm² and is among the best results published concerning SiGe HBTs, this shows that the incorporation of carbon do not have any consequence for the 1/f noise level and more generally for the LF noise characteristics. In the Si Cap thickness range used in this work, no noise degradation is observed when the electrical emitter-base junction is getting closer to the poly/mono emitter interface. Hence DC and AC characteristics could be optimized without changing the LF noise performances. Finally, from measurements at the input and at the output, the emitter series resistance is extracted and is found to be proportional to the Si Cap thickness.