5 Solid State Detectors
189
5.11.2 Low Intensity Light Detection
An optical photon in its primary interaction will create a single electron-hole
pair, a charge too small to be detected by standard electronics. However, intrinsic
amplification in an avalanche process makes single photon detection possible. The
avalanche diode of Fig. 5.51 is such a device. Operation in proportional mode will
result in an output signal proportional to the number of (optical) photons, with some
statistical fluctuations of the avalanche process added and additional contributions
from the non-uniformity of the electric field in the avalanche region. Operation in
limited Geiger mode will result in a signal independent of the number of incident
photons. The charge signal will be approximately given by the product of the diode
capacitance times the difference of the applied voltage and the voltage at which the
avalanche process stops.
As the charge multiplication probability is a strong function of the electric field
strength, high uniformity over the active area is required and high field regions at the
edge of the device have to be avoided by proper design. Edge breakdown is avoided
in Fig. 5.51 by the less strongly doped n region at the rim. This leads to a spacecharge region extending deeper into the bulk and to a reduction of the maximum
field.
If the structure of Fig. 5.51 is to be operated in proportional mode (with only
electrons multiplying), primary charge produced by radiation entering from the
top has to be generated below the high field multiplication region in order to be
properly amplified. Therefore for blue light, with its submicron penetration depth,
the efficiency is low for this design.
In choosing the width of the depleted region, one has to consider several partially
conflicting requirements. Based on noise considerations, this region should be large
in order to reduce the capacitive load to the amplifier. The same is required for
the detection of deeply penetrating radiation such as X-rays or energetic charged
particles. One may even extend the depleted region all the way to the bottom surface.
Then the backside p-doped surface can also be used as a radiation entrance window.
This can be an advantage for low penetrating radiation such as optical photons,
since such an entrance window can be made thin. The disadvantage of a large
depleted region is the large volume for thermal generation of electron-hole pairs,
the electrons being capable of initializing the avalanche process and, depending on
the application, a not wanted sensitivity to deeply penetrating radiation.
The electric field configuration in the avalanche region is shown in an idealized
way in Fig. 5.52, assuming abrupt doping changes. Such a distribution is not only
unrealistic but also far from optimal for proportional operation: Breakdown should
be avoided as much as possible which can be achieved by an extended amplification
region and lower hole-to-electron multiplication ratios, as is the case for lower fields.
Such a design can be realised by suitably doping the avalanche region.
189
5.11.2 Low Intensity Light Detection
An optical photon in its primary interaction will create a single electron-hole
pair, a charge too small to be detected by standard electronics. However, intrinsic
amplification in an avalanche process makes single photon detection possible. The
avalanche diode of Fig. 5.51 is such a device. Operation in proportional mode will
result in an output signal proportional to the number of (optical) photons, with some
statistical fluctuations of the avalanche process added and additional contributions
from the non-uniformity of the electric field in the avalanche region. Operation in
limited Geiger mode will result in a signal independent of the number of incident
photons. The charge signal will be approximately given by the product of the diode
capacitance times the difference of the applied voltage and the voltage at which the
avalanche process stops.
As the charge multiplication probability is a strong function of the electric field
strength, high uniformity over the active area is required and high field regions at the
edge of the device have to be avoided by proper design. Edge breakdown is avoided
in Fig. 5.51 by the less strongly doped n region at the rim. This leads to a spacecharge region extending deeper into the bulk and to a reduction of the maximum
field.
If the structure of Fig. 5.51 is to be operated in proportional mode (with only
electrons multiplying), primary charge produced by radiation entering from the
top has to be generated below the high field multiplication region in order to be
properly amplified. Therefore for blue light, with its submicron penetration depth,
the efficiency is low for this design.
In choosing the width of the depleted region, one has to consider several partially
conflicting requirements. Based on noise considerations, this region should be large
in order to reduce the capacitive load to the amplifier. The same is required for
the detection of deeply penetrating radiation such as X-rays or energetic charged
particles. One may even extend the depleted region all the way to the bottom surface.
Then the backside p-doped surface can also be used as a radiation entrance window.
This can be an advantage for low penetrating radiation such as optical photons,
since such an entrance window can be made thin. The disadvantage of a large
depleted region is the large volume for thermal generation of electron-hole pairs,
the electrons being capable of initializing the avalanche process and, depending on
the application, a not wanted sensitivity to deeply penetrating radiation.
The electric field configuration in the avalanche region is shown in an idealized
way in Fig. 5.52, assuming abrupt doping changes. Such a distribution is not only
unrealistic but also far from optimal for proportional operation: Breakdown should
be avoided as much as possible which can be achieved by an extended amplification
region and lower hole-to-electron multiplication ratios, as is the case for lower fields.
Such a design can be realised by suitably doping the avalanche region.