Study of frequency-dependent strip admittance in silicon microstrip detectors

We report on detailed interstrip admittance measurements performed on double-sided, AC-coupled and punch-through biased silicon microstrip detectors. The sensors chosen have been fabricated on very high resistivity substrates, which translates in very low depletion voltages, in the range 10–20 V. This, together with the absence of bias resistors, allows a careful study of the strip admittance components over a wide range of bias voltages and frequencies. In some instances, beyond total depletion the measured interstrip capacitance and dissipation factor exhibit a marked voltage and frequency dependence, linked to the presence of some resistive component. A simple lumped electrical model has been developed to explain the observed features and 3-D numerical simulations have been performed, supporting the interpretation of the phenomena. These features of the admittance have been found to be closely correlated with a non-standard noise term, exhibiting a peculiar frequency/time dependence, which adds in quadrature to the well known parallel and series noise sources.