As demanding applications such as x-ray spectroscopy push transition-edge sensors (TESs) to even better energy resolution, it is critical to understand all their potential noise sources. Since the early days of TESs, many groups have observed a broadband voltage noise that could not be explained by known noise mechanisms. In 2004, Ullom et al. showed this unexplained noise could be suppressed using both device geometry and an applied magnetic field. However, despite more than a decade of effort, the magnitude of the unexplained noise in TESs is still not understood. Some progress was made in 2006 , when Irwin predicted an enhancement of the Johnson noise based on an analysis of non-linear bolometers near equilibrium. This analysis predicted that the noise level in a TES is equal to the Johnson noise due to the TES resistance at its operating point multiplied by a factor of $(1+2\beta_I)$. In some scenarios the (1+2$\beta_I$) term has reasonably predicted the amount of Johnson noise. In many other scenarios, especially low in the transition, and for devices with high $\alpha_I$ and high $\beta_I$, the $(1+2\beta_I)$ expression dramatically underpredicts the observed noise .
To resolve this mystery, we present a new noise model for TESs that takes into account their junction nature and the shape of the $R(I,T)$ surface. We present analytical expressions for the unexplained noise within the context of the RSJ and two-fluid models. We also present an expression for the unexplained noise for an arbitrary $R(I,T)$ surface. We then compare these expressions with the measured noise in different TESs and at different operating resistances. Initial testing shows good agreement between measurement and theory.
 Ullom, Joel N., et al., Appl. Phys. Lett., 84 (2004) 4206.
 Irwin, Kent D., Nucl. Instrum. Methods Phys. Res. A, 559 (2006) 718.
 Jethava, Nikhil, et al., AIP Conference Proceedings., 1185 (2009) 31.
|Student (Ph.D., M.Sc. or B.Sc.)||N|
|Less than 5 years of experience since completion of Ph.D||N|