Silicon Drift Detector
Silicon Drift Detectors are a derivative of the basic principle of sideward depletion. A volume of a high resistivity semiconductor, in our case n-type silicon, is covered by rectifying p-doped junctions on both surfaces. A small sized substrate contact in reverse bias to the p‑regions depletes the Si bulk. In an SDD the p-junctions are strip-like segmented and biased in such a way that they generate an electric field with a strong component parallel to the surface. Signal electrons released within the depleted volume by the absorption of ionising radiation drift towards the substrate contact, which acts as collecting anode. Due to the anode’s small physical dimension, its capacitance has a small value almost independent of the detector area. Compared to a conventional diode of equal area, this feature translates to a larger amplitude and a shorter rise time of the output signal.
Originally, SDDs were designed and used as position sensitive detectors in particle physics where the particle’s interaction point is reconstructed from the signal electrons’ drift time.
Silicon Drift Detectors for Spectroscopy
The semiconductor laboratory of the Max-Planck-Society (MPG‑HLL) has developed an advanced design for X‑ray and particle spectroscopy. In this device the structures for the drift field generation and the collecting anode are placed on one side of the device, while a non-structured p‑junction acting as thin, homogeneous radiation entrance window covers the opposite surface (figs. 1, 2). SDDs of this type combine are large sensitive area and a small value of the readout anode's capacitance. To take the full advantage of the small capacitance the first transistor of the amplifying electronics is integrated on the detector chip. The integration of the transistor minimises the interconnection stray capacitance between sensor and amplifier. It guarantees an excellent spectroscopic resolution and operation at extremely high signal rates exceeding 105 counts per second. In addition, it makes the device largely insensitive to electronic pickup and microphonic noise. The advanced process technology at MPG-HLL results in very small leakage current levels allowing for SDD operation at room temperature or with moderate cooling. As the SDD's bulk is fully depleted and irradiated through the non-structured thin backside entrance window, it has a high quantum efficiency and a good low-energy response.
The SDD concept is very flexible in shape and size. E.g., the KDK experiment uses single cell devices with an active area of 1 cm². To provide even larger sensors, multi-cell SDDs combine a large sensitive area with the energy resolution and the count rate capability of a single SDD. A multi-cell SDD is a monolithic, gapless arrangement of a number of SDDs with individual readout, but with common voltage supply, entrance window, and guard ring structure. For instance, the electron detector of the TRISTAN experiment at KIT is based on a multi-cell SDD with 166 cells and a total sensor area of almost 12 cm².
SDDs are found in a variety of scientific experiments and commercial systems. With their count rate capability they set new boundary conditions for electron microprobe analysis. The fact that SDDs don’t require expensive and inconvenient cooling by liquid nitrogen initiated new applications in X-ray spectroscopy. In combination with a micro focus X-ray tube the SDD makes up a compact, portable spectrometer for XRF measurements in the field, i.e. independent of laboratory infrastructure, developed for the use in archeometry. The ESA comet lander mission ROSETTA as well as NASA’s 2003 Mars Exploration Rovers Spirit and Opportunity are equipped with an Alpha-Proton-X-ray-Spectrometer (APXS) including an SDD for PIXE (particle induced X-ray emission) analysis.
Schematic view of a cylindrical Silicon Drift Detector. Signal electrons drift in the electric field towards the small sized collecting anode and the integrated amplifying transistor in the center of the device.
Simulated electron potential of a cylindrical Silicon Drift Detector in operating conditions. The plot shows a cross section perpendicular to the surface through the center of the device. The arrows indicate the paths of electrons drifting to the anode