Empirical I-V Curve Fits of SIPMs

SiPMs are pixelated arrays of reverse-biased diodes operated as efficient photon counters. At a specific (temperature dependent) bias, the ionization created by a photon in one of the pixels undergoes avalanche amplification to give a detectable signal. Operating a SiPM above such bias voltage, known as breakdown voltage (V_bd), is referred to as Geiger-mode operation. We measure the breakdown voltage through I-V curves, where it appears as a characteristic sharp kink in the current. This feature can be extracted from I-V curves by different methods. A popular approach searches for a peak in d(lnI)/dV. This is effective when the SiPM is well illuminated, but shows limits with low levels of illumination or high levels of noise. We present an alternative method which relies on empirical fits of I-V curves to a piecewise power function: I(V) = c(V - V_bd)^a + b, for V >= V_bd. This method reliably finds the breakdown voltage in both low and high illumination conditions. Notably, fits are fast and efficient, allowing the user to adjust fitting bounds easily. We will present the fitting method and results on measured SiPM V_bd versus temperature.