The observation of astrophysical objects in the mid-infrared requires Blocked Impurity Band (BIB) detectors
based on n-doped Silicon. It is desirable to observe faint astronomical objects with such a detector, which can
be achieved with a high Signal to Noise ratio. One possibility is an implemented Depleted P-channel Field Effect
Transistor (DEPFET) Active Pixel Sensor (APS)<sup>1</sup> on the BIB detector in order to be free of interconnection
stray capacitance. A noise of 2 <i>e</i><sup>-</sup> ENC and a current amplification of 300 pA per <i>e</i><sup>-</sup> can be obtained at room temperature.
These detectors operate at a temperature range from 6 K to 12 K. The DEPFET is operated under these
conditions to investigate the functional principle of the detector. We show results of characteristic and dynamic
measurements of the single pixel DEPFET at low temperature. We irradiate the DEPFET single pixel with
x-rays originating from the nuclear decay of Fe55 and take a spectrum of the K<sub>α- </sub>and Kβ-line. Uncomplete clear
is identified with freeze-out of the signal charge into ionized shallow donor states in the heavily doped internal
Gate of the DEPFET due to low thermal energy. We found a solution to emit the localized signal charges into
the conduction band in order to ensure the transport from the internal Gate to the Clear contact.