Low-frequency noise and random telegraph signal noise in SiGe:C heterojunction bipolar transistors: impact of carbon concentration

We have investigated the influence of carbon concentration on the low frequency noise (LFN) of Si/SiGe:C Heterojunction Bipolar Transistors (HBTs). The HBTs are supplied by ST-Microelectronics Crolles and are based on a 0.13 &mgr;m BiCMOS technology. Three types of transistors were studied; they only differ by the amount of carbon incorporated. When carbon is incorporated, representative noise spectra of the input current spectral density, SiB, show important generation-recombination (G-R) components, while no such components are observed in carbon free transistors. When the 1/f noise component is unambiguously observed, the associated figure of merit KB has a very good value close to 4.10-10 &mgr;m2. In this paper we focus on the analysis of the G-R components associated with the presence of the carbon. Most of the observed Lorentzians are associated with Random Telegraph Signal (RTS) noise. No RTS noise is found in carbon free devices. The RTS noise appears to be due to electrically active defects formed by the addition of carbon, typically observed for concentrations above the bulk solid solubility limit in silicon. The RTS noise, amplitude &Dgr;IB and the mean pulse widths (tH, tL), are analyzed as a function of bias voltage and temperature. The RTS amplitude is found to scale with the base current and to decrease exponentially with temperature, independently of the carbon concentration. The mean pulse widths are found to decrease rapidly with bias voltage, as 1/exp(qVBE/kT) or stronger. Our results confirm that electrically active C-related defects are localized in the base-emitter junction, and the RTS amplitude is explained by a model based on voltage barrier height fluctuations across the base-emitter junction induced by trapped carriers in the space charge region. The observed bias dependence of mean pulse widths seems to indicate that two capture processes are involved, electron and hole capture. These C-related defects behave like recombination centers with deep energy levels rather than electron or hole traps involving trapping-detrapping process.