INSTRUMENTATION FOR TXRF AND GI-XRF - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 168

Figure3.16. Cross-section of a silicon drift detector or SDD. It consists of a small disc of a high-
purity Si wafer with a thin P + -doped cathode at the front area and several concentric drift electrodes
on the opposite side (on top). X-rays coming from the front area (from below) produce several
electron-hole pairs in the Si crystal. The electrons are drifted down the field gradient kept by the
drift rings and finally are collected at the small central anode ring. The charge pulse is converted
into a voltage pulse either by a closely attached FET or even by an integrated“on-chip”JFET.
From Ref. [53], reproduced with permission from PNDetector GmbH, Munich, Germany.
The rear shows a series of several concentric p + -type rings with a small anode
ring in the center (area < 0.5 mm 2 ). The homogeneous front side may be
covered by an ultrathin layer of Al (30 nm) instead of a gold layer, which is
normally used for Si(Li) detectors. Consequently, the absorption of the front
layer is strongly reduced for soft X-rays. The front layer is switched as cathode
with negative voltage while the rings on the rear are provided with an
increasingly negative voltage from the inside to the outside [52-55].
Each X-ray photon hitting the crystal produces electrons within the pene-
tration depth of silicon, which is < 0.5 mm for E < 17 keV. These electrons drift
sideward to the central anode via the horizontal voltage field created by the
concentric“drift”rings and produce a chain of electron-hole pairs on its
sideward path to the anode, which is lengthened up to some 5 mm. The small
anode in the center guarantees for an input capacitance that is extremely low in
comparison to that of conventional Si(Li)s leading to a small electronic noise
and good spectral resolution. A capacitance of only 35 fF instead of 100 pF for
gas-filled detectors means an improvement of 3000-fold.
The charge pulses at the anode have to be converted into voltage pulses and
linearly amplified. For that purpose, a junction field effect transistor (JFET) is
monolithically integrated into the center of the sensor chip by implantation [53].