298 Chapter 5. Solid State Detectors
Charge yield:The number of charge pairs created should be enough to yield
a good signal to noise ratio. For very low fields, an internal amplification of
charge pairs is sometimes done to enhance the signal.Resolution:Energy resolution is of prime importance in low radiation fields.5.1.K Specific Semiconductor Detectors
Semiconductor detectors can be build in different geometries with different structures
and can be tuned according to applications. Their main disadvantage is that it
is not possible to build large area detectors since the process of crystal growth
has its size limitations. Nevertheless, large area position sensitive detectors have
been built by integrating many smaller detectors together. Furthermore it has been
shown that it is possible to use semiconductors to build detectors for not only very
high radiation fields but also for extremely low radiation environments at the single
photon level. Hence semiconductors have provided the long sought for versatility.
Intense research has gone into and is still going on to build detectors with better
operational characteristics. As a result state-of-the-art detectors have been built
with qualities that far exceed their gaseous counterparts. In this section we will
look at some specific types of semiconductor detectors that have been built and are
in extensive use.
K.1 PINDiode
Although a simple pn junction diode can be used as a radiation detector but it
suffers from several disadvantages, some of which are described below.
Small Depletion Region: Although the charge pairs are generated in the
whole material but the ones generated inside the depletion region constitute
the measurable current. Hence it is desired that the depletion region is as wide
as possible to allow the radiation to deposit most of its energy. Increasing
the depletion region in a simple pn diode requires increase in the bias voltage
and therefore large depletion widths require very high external electric fields.
Increasing the voltage to very high values may not always be possible due to the
possibility of breakdown of the crystal structure. Hence for a simple pn-junction
the depletion region can not be widened more than the value determined by
the crystal structure’s breakdown voltage. The work-around to this problem is
to use such a thin material, which could be fully depleted at moderate voltages.
This, in fact, is generally done in some detectors made with 100-300μmthick
wafers of semiconductor materials.Large Capacitance: Since the capacitance is inversely proportional to the
width of the depletion region therefore simple pn diodes operating at moderate
voltages have high capacitances. This is certainly not a desirable effect for
using the junction as a detector.Slow Response:In general, the pn detectors are operated in fully depleted
mode. But this is only possible for very thin detectors, since, as we saw earlier,
increasing the depletion width requires introduction of high potential across the
junction. For thicker detectors, which can not be fully depleted, the creation