Silicon Drift Detectors (SDDs) are well suited for high resolution high count rate X-ray spectroscopy. A low leakage current level allows to operate them close to room temperature with moderate cooling.
SDDs are used in large quantities on industrial scale in applications like electron microscopy (SEM-EDX) and X-ray fluorescence analysis (XRF). In addition PNSensor develops SDDs in dedicated shapes and operational modes for scientific experiments within the frame of international collaborations.
The basic form of the Silicon Drift Detector (SDD) has been proposed in 1983 by Gatti & Rehak . It consists of a volume of fully depleted high-resistivity silicon, in which an electric field with a strong component parallel to the surface drives electrons generated by the absorption of ionising radiation towards a small sized collecting anode. The electric field is generated by a number of increasingly reverse biased field strips covering one surface of the device . The radiation entrance window on the opposite side is made up by a non-structured shallow implanted junction giving a homogeneous sensitivity over the whole detector area.
The unique property of this type of detector is the extremely small value of the anode capacitance, which is independent of the active area. This feature allows to gain higher energy resolution at shorter shaping times compared to conventional photo diodes and Si(Li) detectors, recommending the SDD for high count rate applications .
To take the full advantage of the small output capacitance the front-end transistor of the amplifying electronics is integrated on the detector chip and connected to the collecting anode by a short metal strip. This way the stray capacitance of the interconnection detector - amplifier is minimised and moreover noise by electric pickup and microphony effects is insignificant. The anode is discharged from signal electrons in a continuous mode. Thus the SDD can be operated with dc-voltages only and there is no detector dead time caused by a clocked reset mechanism .
Due to the elaborated process technology used in the SDD fabrication the leakage current level is so low that the SDD can be operated with moderate cooling by means of a single stage Peltier element.
The SDD's energy resolution (FWHM < 145 eV @ MnKα at -20°C) can be compared to that of a Si(Li) detector requiring no expensive and inconvenient liquid nitrogen cooling. It surpasses the quality of pin-diodes.
To improve performance of the standard Silicon Drift Detector with regard to energy resolution and peak-to-background a new SDD-layout was developed. In Figure 2 the design of the Silicon Drift Detector Droplet named SD³ is shown. The integrated FET is no longer centered but moved to the outside margin of the structure. If a proper collimator is used, the FET is not irradiated by incoming photons.
The new layout of the SD³ structure allows the design of a readout anode which has a smaller area. As the capacitance of the readout node is mainly given by the area of the anode, reducing the area helps greatly to minimize the electronic noise contributions of serial white and 1/f noise. The detector capacitance of a SD³ determined from ENC² vs. shaping time measurements is about 120 fF compared to 200 fF for standard SDDs.
In next figure a mounted and bonded SD³ chip is shown. The chip is mounted on a TO8 housing with 16 pins for the supply voltages. A thermoelectric element is included in the housing. The droplet like shape of the detector can easily be seen. The homogenous radiation entrance window which is shown on the picture consists of a flat p-implant covered by a thin aluminum layer to keep the entrance window radiation hard.
When irradiating silicon drift detectors with high doses of X-ray photons, radiation damages in the oxides and at the interface oxide/silicon occur. The most important effect after irradiation is the increased surface leakage current which leads to a worse energy resolution.
Radiation hardness measurements were performed for standard SDDs as well as for the new SD³ chips. The devices have been irradiated by X-ray photons of about 18 keV which impose maximum radiation damage. Even for this energy SDDs show no loss in spectroscopic performance until an absorbed dose of about 1013 photons. For a supposed count rate of 10,000 cps this means that the detector could be continuously operated for 30 years without degradation in energy resolution because of radiation damage.
Classic SDDs (hexagonal shaped) and High Resolution SD³-structures are assembled in various multielement structures:
Linear, lumped or ring shaped multielement structure with hexagonal shaped SDDs:
 E. Gatti, P. Rehak, Semiconductor Drift Chamber - An Application of a Novel Charge Transport Scheme, Nucl. Instr. and Meth. A 225, 1984, pp. 608-614.
 P. Lechner et al., Silicon drift detectors for high resolution room temperature X-ray spectroscopy, Nucl. Instr. and Meth. 1996; A 377, pp. 346-351.
 L. Strüder, H. Soltau, High Resolution Silicon Detectors for Photons and Particles, Radiation Protection Dosimetry 61, 1995, pp. 39-46
 C. Fiorini, P. Lechner, Continuous Charge Restoration in Semiconductor Detectors by Means of the Gate-to-Drain Current of the Integrated Front-End JFET, IEEE Trans. on Nucl. Sci. 46 (3), 1999, pp. 761 764
 C. Fiorini, P. Lechner, Charge Sensitive Preamplifier with Continuous Reset by Means of the Gate-to-Drain Current of the JFET Integrated on the Detector, IEEE Trans. on Nucl. Sci. 49 (3), 1999, pp. 1147 1151