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Working principle
of the Silicon Drift Detector (SDD)
The silicon drift detector (SDD) is fabricated on high purity n-type silicon by providing at the entrance side of the radiation a large area homogeneous abrupt pn-junction and on the opposite side a central spot-like n-doped anode, which is surrounded by a number of concentric p-type drift electrodes.
During operation the pn-junctions at both sides of the device are biased in reverse generating a potential minimum for electrons within the bulk.
By superposition of a voltage gradient at the drift electrodes a transversal electric field is generated which bents the potential valley and forces signal electrons to drift to the anode. The small capacitance of the anode together with the low leakage current of the device enable low noise and fast read out of the electron signal by use of a charge sensitive preamplifier.
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The animation shows the working principle of an SDD. When bias is applied at the front and rear side of the SDD, the whole device becomes totally depleted from charge carriers and can now be used for x-ray spectroscopy. The small line within the depletion layer represents the borderline between both depletion layers, which is identical with the course of the potential minimum for electrons.
Next the animation demonstrates the absorption and measurement of three x-ray quanta, which hit the detector at the same time at different positions. Three charge clouds of e/h-pairs are generated, which are separated under the electric field.
The holes (red) are running to the electrodes, whereas the electrons (blue) are rolling down to the bottom of the potential valley, where they are drifting to the anode.
X-ray no. 1 is registered immediately at the input of the preamplifier as the collection time is very short. The electrons from x-ray no. 2 have to drift a longer distance to the anode, where they influence a charge, as soon as they pass the inner drift ring. The electrons of x-ray no. 3 have to drift the longest path, therefore, they arrive at the anode much later.
The rise time of the first signal is short, as the electrons are collected without drift. The rise time of the second signal is longer due to diffusion of the charge cloud on the way to the anode. The situation is even worse for the third signal, which results from the electrons with the longest drift distance and largest diffusion of the charge cloud.
The characteristic features of an SDD are:
- During drift the charge cloud is shielded and only at the moment, when the electrons pass the inner drift ring, it generates the signal.
- The rise time of the signal is depending upon the drift distance due to the diffusion of the charge cloud during the drift to the anode.
- In an SDD it is possible, to detect several x-ray quanta which are absorbed simultaneously, if they hit the detector at different positions and if the drift times are longer than the rise times of the previous signals.
- For large area detectors the charge diffusion during drift may result in ballistic deficit of the signal, if the shaping time of the electronics is too short.
