Copyright ©-International Centre for Diffraction Data 2010 ISSN 1097-0002
BRIDGING THE PRICE/PERFORMANCE GAP BETWEEN SILICON DRIFT
AND SILICON PIN DIODE DETECTORS
Derek Hullinger, Keith Decker, Jerry Smith, Chris Carter
Moxtek, Inc.
ABSTRACT
Use of silicon drift detectors continues to increase as new XRF applications drive performance
improvements beyond existing silicon PIN detector capabilities. Increasing the thickness of PIN
diodes (and, correspondingly, the thickness of the depletion region within the diode) significantly
reduces the diode capacitance and results in improved performance. For example, the new
Moxtek XPIN-6 detector (which uses a diode with a 6mm2 area and 625-µm thickness) has a
typical improvement in resolution of 20 eV over the Moxtek XE600 detector (with a 400-µm
diode), at a peaking time of 5 µs and an energy of 5.9 keV. It also has a 50% higher absorption
efficiency at energies above 20 keV. Further improvements, including thicker diodes, are
expected to further bridge the gap between existing PIN detectors and SDDs. These
improvements result in detectors that, in some applications, offer an attractive alternative to
silicon drift detectors at a reduced cost.
SILICON PIN VS. SDD DETECTORS
Figure 1 shows the resolution of a typical Moxtek XE600 detector and a typical high-end silicon
drift detector (SDD) at an energy of 5.9 keV at different peaking times. Both sets of
measurements are made at -25°C. The resolution performance of an SDD is much better, but the
cost is also much higher—typically by about 3 times.
Figure 1: Typical Resolutions of an XE600 PIN Diode Detector and an SDD
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Copyright ©-International Centre for Diffraction Data 2010 ISSN 1097-0002
PEAKING TIME AND COUNT RATE
Whenever the resolution of a detector is cited, a particular peaking time is assumed. Therefore,
it is helpful to discuss the meaning of “peaking time” and how it relates to the attainable count
rate.
When the voltage signal produced by an x-ray photon detection passes through a shaping
amplifier, the result is a voltage pulse with a particular shape (commonly, a triangle). A set of
consecutive photon events result in a series of pulses—each pulse representing a detected
photon, with the pulse height proportional to the photon energy (or, more properly, to the energy
absorbed in the detector). The peaking time of the shaping function is the time between the
beginning of the voltage pulse and its peak (see Figure 2).
Pulse
Height
Peaking
Time
Time
Figure 2: Voltage Pulses Resulting from Consecutive Photon Detections
If one uses a shaping function with shorter peaking time, one can collect more photons in a given
amount of time without the pulses substantially overlapping one another. Thus, there’s a one-toone relationship between the count rate a detector can handle and the peaking time used. If one
were to decrease the peaking time by a factor of ten, one could accommodate a factor of 10
higher count rate. For example, if we assume a 50% dead time and no losses to pile-up, a 10 µs
peaking time would result in approximately 25,000 counts per second, whereas a 1 µs peaking
time would result in ten times that count rate (see Figure 3).
Figure 3: Peaking Time vs. Count Rate
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Copyright ©-International Centre for Diffraction Data 2010 ISSN 1097-0002
CONTRIBUTIONS TO DETECTOR RESOLUTION
The various noise sources which contribute to the resolution of a detector result in resolution
components that have different dependences on the peaking time of the shaping amplifier (Gatti
and Manfredi 1986) (see Figure 4). The series thermal noise component (due substantially to the
JFET) is inversely proportional to peaking time, so that as peaking time increases, thermal noise
decreases. On the other hand, parallel shot noise (which largely depends on the leakage current
of the diode) is directly proportional to peaking time, so that as peaking time increases, shot
noise increases. There is also a significant series “flicker” (or “1/f”) noise component that is
independent of peaking time. The statistical noise, which represents the fluctuation in the actual
number of electron-hole pairs resulting from a photon, is also independent of peaking time.
Figure 4: Resolution contributions typical of an XE600 detector at -25°C
To illustrate how leakage current affects the overall resolution at different peaking times, Figure
5 shows the shot noise contribution (green) and total resolution (blue) that would result from an
XE600 detector with three different leakage currents. The solid lines indicate the resolution due
to shot noise and total resolution typical of 10 pA of leakage current, the dashed lines show
resolutions typical of 1 pA, and the dotted lines show resolutions typical of 0.1 pA. With 10 pA
of leakage current, the contribution of shot noise becomes significant above a peaking time of
about 5 µs. With a leakage current of 1 pA, shot noise is significant above about 20 µs. With a
leakage current of 0.1 pA, shot noise isn’t significant except at very long peaking times of 100 µs
or more.
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Copyright ©-International Centre for Diffraction Data 2010 ISSN 1097-0002
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10 pA
1 pA
0.1 pA
Figure 5: Resolution for Different Leakage Currents
Referring again to Figure 4, we note that at short peaking times (where shot noise has little
impact on the resolution), the two dominant noise sources (thermal noise and 1/f noise) depend
strongly on the total capacitance between the anode and any grounded or fixed-voltage
conductors. Thus, reducing the anode capacitance has an enormous impact on resolution. This
is the reason that silicon drift detectors have excellent resolution performance—the anode
capacitance of the chip itself is so small as to be negligible compared to other capacitance
sources, such as the JFET.
REDUCING DIODE CAPACITANCE
In considering ways to reduce the total capacitance of a PIN diode detector, one possibility that
presents itself is to increase the thickness of the diode. Making use of the parallel-plate
approximation, (which is a good approximation for the anode-to-backside capacitance), a 6 mm2
diode that is 400 µm thick (like the XE600) has an anode-to-backside capacitance of 1.6 pF,
whereas a fully-depleted 625-µm diode of comparable area has a capacitance of only 1.0 pF.
One obstacle to this approach is that the anode-to-backside capacitance depends upon the
thickness of the depletion region within the diode, rather than on the thickness of the diode itself.
Thus, the maximum possible reduction in capacitance is realized only if the diode is fully
depleted. Full depletion of a thicker diode requires either a higher bias (V in Equation 1) or a
Copyright ©-International Centre for Diffraction Data 2010 ISSN 1097-0002
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substrate with higher resistivity (corresponding to a lower doping concentration Nd in Equation
1) (Sze 1981).
(1)
å: Permittivity of silicon (1.04×10-10 F/m)
V: Reverse bias applied to the diode
q: Charge of an electron (1.602×10-19 C)
Nd: Doping concentration of the diode’s intrinsic region
A second obstacle involves leakage current. The leakage current scales roughly with the volume
of the depletion region (Sze 1981). Therefore, either a very low leakage current process or else a
very low operating temperature is necessary.
PERFORMANCE OF NEW XPIN-6 DETECTOR
Moxtek has just introduced a new PIN diode detector called the XPIN-6 which uses a 625-µm
diode. Figure 6 shows typical 5.9 keV resolutions of this detector alongside those of a typical
XE600 (with a 400-µm diode) and a high-end SDD. All measurements are made at -25°C, and
the XPIN-6 and XE600 are both biased to 130 V (at which both are fully depleted). As can be
seen in the figure, the lower capacitance of the XPIN-6 results in significant improvement at
short peaking times. For example, at a peaking time of 5 µs, the XPIN-6 has a resolution that is
20 eV lower than that of the XE600.
Copyright ©-International Centre for Diffraction Data 2010 ISSN 1097-0002
Figure 6: Resolution of XPIN-6
Another advantage of a thicker diode is that it is able to absorb higher energy photons more
efficiently. This is equivalent, in terms of count rate, to a larger active area, but without the
disadvantage of increased capacitance and correspondingly poorer resolution. Figure 7 shows
the absorption curve of the XPIN-6 compared to the XE600. Above 20 keV, where the average
depth of interaction in silicon is much larger than the thickness of the diode, the XPIN-6 has
about a 50% greater absorption efficiency than the XE600.
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Copyright ©-International Centre for Diffraction Data 2010 ISSN 1097-0002
Figure 7: Absorption Efficiency of XE600 and XPIN-6
NEXT GENERATION PIN DIODES
Modeling suggests that additional improvements (including a further increase in thickness of the
diode) should result in resolutions that further bridge the gap between the XE600 and high-end
SDDs. Figure 8, for example, shows the expected resolution from a 6-mm2 875-µm diode that is
in development. An 875-µm diode would also offer improved absorption, as shown in Figure 9.
Such PIN diode detectors may offer resolutions that are adequate for some applications and at a
much lower cost than typical SDDs.
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Copyright ©-International Centre for Diffraction Data 2010 ISSN 1097-0002
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Figure 8: Resolution from an 875-µm, 6mm2 diode in development
Next Gen PIN (875 µm)
Figure 9: Absorption Efficiency of 875-µm-thick silicon
Copyright ©-International Centre for Diffraction Data 2010 ISSN 1097-0002
Larger area diodes are desirable for some applications—for example, when a much higher count
rate is needed. Modeling suggests that the same types of improvements we’ve described could
result in significant gains in resolution performance over Moxtek’s current 13mm2 diode—the
XE1300 (see Figure 10). Even 25mm2 diodes should be possible with resolutions that aren’t too
far from the resolutions of typical 6mm2 diodes of just a few years ago.
Figure 10: Theoretical resolutions of larger-area pin diodes
SUMMARY
In conclusion, an increase in diode thickness permits a larger depletion thickness, which in turn
leads to better resolution performance due to its lower anode capacitance, as long as acceptable
leakage current is maintained. This performance begins to approach the performance of silicon
drift detectors and at a much lower cost than SDDs. As an added benefit, thicker diodes also
result in improved x-ray absorption.
Bibliography
Gatti, E., and Manfredi P.F. (1986). “Processing the Signals from Solid-State Detectors in ElementaryParticle Physics,” Rivista Del Nuovo Cimento. 9, 1-146.
Sze, S. M. (1981). Physics of Semiconductor Devices (Wiley, New York).
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