Vertex detector for BELLE II

The DEPFET Pixel detector

Fig. 1: FET principle. The lightly n-doped substrate(colorless) and the p-doped regions drain and source.In addition, there is a p-doped back and amore heavily doped internal gate (green). — Fig. 1: FET principle. The lightly n-doped substrate (colorless) and the p-doped regions drain and source. In addition, there is a p-doped back and a more heavily doped internal gate (green).

Fig. 1: FET principle. The lightly n-doped substrate
(colorless) and the p-doped regions drain and source.
In addition, there is a p-doped back and a
more heavily doped internal gate (green).

The DEPFET (Depleted P-channel Field Effect Transistor) is a active pixel sensor, based on a FET on a completely depleted substrate. A field-effect transistor consists of the following areas Drain, source and gate. For example, be with us (see Fig. 1) the drain and the source are made of p-doped silicon, the rest of the silicon is only slightly n-doped (colorless). You put it a voltage between drain and source, so no Electricity flows as there is always an area, depending on the polarity of the Tension, locks. If a negative voltage is now applied to the gate, then collect "holes" underneath, these are also the ones Charge carriers that are present in the drain and source. It this creates a conductive channel between them Areas where an electric current flows.

If a charged particle passes the detector, it creates in the silicon electrical charge pairs (electrons and holes). These are caused by an electric field inside the silicon Separate pairs, the electrons become one Potential minimum directed. This minimum, also called the internal gate, is below the conductive channel of the transistor. The more electrons in the internal the transistor becomes more conductive. You can tell whether a particle has now penetrated the detector at the increase in transistor current. The difference of the current and a quiescent current. The measurement allowed therefore not only conclusions as to whether a particle was present, but the size of the stream can also depend on the amount electrons in the internal gate are closed (~ 0.5 nA / e -). This gives an indication of the nature of the passing particle. So that the DEPFET is sensitive again after the readout, the charge can be removed from the internal gate. This happens due to a sufficiently high positive voltage at the clear contact. This causes the electrons to drift from the internal gate to the clearer DEPFET is ready for the next measurement.

Fig. 2: DEPFET Pixel

Fig. 2: DEPFET Pixel

The advantages of the DEPFET are:

 Large signal due to the depleted substrate
 Low noise due to the small detector capacity
 Low power consumption
 Fast selection