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We report on the temperature and count-rate dependent noise behavior of an Si-drift detector system using an on-sensor integrated JFET in a source-follower configuration. The readout chain of a 0.8-μm BiCMOS chip consists of a fully differential low-noise postamplifier, current-mode shaper with gated integrator, and analog storage cell. Six channels are processed in parallel using a 6 : 1 multiplexer buffered by a 100-Ω line driver. The readout chain's power dissipation is ∼15 mW/channel. The indexes for parallel, serial, and 1/f noise of the time-variant signal processor are calculated using weighting functions. For a detector capacitance of ∼140 fF and low count rates, the chip's and total electronics' input-referred equivalent noise charge is about 20 and 23 rms electrons, respectively. Due to an almost quadratical increase of the noise indexes with increasing count rate, the signal current deteriorates so that the spectral resolution of a Cu-Kα-emission line at 20°C decreases from ∼300 eV (full-width at half-maximum) at low count rates to ∼850 eV at 600 kilocounts per second. The investigation of the temperature-dependent leakage current for different detectors leads to current densities between 1.5 pA/mm2 and 3 pA/mm2 at 20°C. The simulated and experimental data verify the theoretical results for a wide range of count rates and sensor temperatures.