Physics and Engineering of Radiation Detection

310 Chapter 5. Solid State Detectors

the effective lifetime of a carrier in diamond is dominated by its extrinsic lifetime (see
equation 5.2.2). Therefore the lifetimes appearing in the above equation are actually
the extrinsic lifetimes of electrons and holes. This equation is also sometimes written
as
dD=μτ E, (5.2.6)

whereμ=μe+μhand
1
τ

=

1

τe

+

1

τh

The reader should note that definingμas a sum of electron and hole mobilities hints
towards the correct definition ofdDas the distance by which the electrons and hole
move apart under the influence of applied electric field.
Now that we know the actual distance traveled by the charges, that isdD,we
can determine the charge induced at the readout electrodes through the relation

Qind=Q 0

dD

d

, (5.2.7)

wheredis the physical length of the material andQ 0 represents the total charge
produced by the radiation. Since the charge collection efficiency of a detector is
defined as the ratio of the total charge observed to the actual charge, therefore using
the above equation we can write the efficiency as

η=

Qind

Q 0

=

dD

d

. (5.2.8)

The good thing aboutηis that it can be used to determine the optimum thickness
of a material such that the efficiency is nearly perfect ( 100%). An important point
to note here is that the CVD crystals grown in laboratories do not show spatially
uniform properties. Small localized variations in trap densities have been observed
in practical systems.

5.2.D LeakageCurrent

At room temperature the resistivity of diamond is about 6 orders of magnitude higher
than that of silicon (see Table 5.2.1). Due to such a high resistivity (and large band
gap) the leakage current in a well grown CVD diamond crystal is extremely small.
The variation of resistivity with temperature in diamond is shown in Fig.5.2.3. It can
be seen that even at very high temperatures the resistivity of diamond still remains
higher than most semiconductor materials kept at lower than room temperature.

5.2.E DetectorDesign

As we noted earlier, because of the large band gap in diamond the leakage current is
extremely small. The direct implication of this advantage with regard to radiation
detection is that one does not need to establish a pn junction. The detector design
is thus simpler as compared to semiconductor detectors where creating a junction
and stretching it is a major task in itself.
A simple diamond detector can be built by simply establishing metal contacts
on two sides of the material. These contacts act can then be used to apply bias