308 Chapter 5. Solid State Detectors
we can write
τef f≈τext. (5.2.2)
5.2.C DriftofChargePairs.......................
The high mobility of free charges in diamond is a positive factor for radiation de-
tection since it implies fast charge collection and consequently fast overall response
of the detector. The electric field profile of drift velocity of charge pairs in diamond
is qualitatively similar to semiconductors. The velocity increases with increase in
electric field intensity and ultimately reaches a saturated value. Diamond detectors
are operated at such high potentials that the charge carriers quickly attain their sat-
urated velocities after their production by the incident radiation. Typical operating
electric field is 1kV/mm.
Figure 5.2.1: Variation of electron mobility in CVD diamond
with respect to the absolute temperature at three impurity
(phosphorus) levels: 100ppm(), 500ppm(•), and 1000ppm
(◦)(32).Besides having a value higher than semiconductors, the mobilities of electrons and
holes in diamond do not show strong temperature dependence at and around room
temperature. Fig.5.2.1 shows the variation of electron mobility in diamond with
temperature and concentration of impurity (phosphorus). The interesting thing to
note here is that at temperatures lower than the room temperature, the mobility
has a strong dependence on the impurity level. If the material is highly doped, the
electron mobility decreases very rapidly, certainly not a very desirable effect as far
as radiation detection is concerned. On the other hand, for temperatures higher
than 300Kthe electron mobility shows small variations even if the the dopant level
changes by a factor of 5. The best fit to the data shows that the electron mobility