Physics and Engineering of Radiation Detection

6.5. Photodetectors 407

p+

n n+

Guard Ring Intrinsic

SiO 2

Metal Contact Antireflective Coating

p

Metal Contact

Photon

Figure 6.5.24: Typical structure of a sili-

con based reach-through type avalanche

photodetector.

High Gain:Since the sensitivity of the detector depends on its gain therefore

the APDs are designed such that they have gains of up to 10^8. However de-

pending on the application, one can opt for lower gain, which can be achieved

by applying lower electric field.

Good Frequency Response: Good frequency response is one of the most

desirable characteristics of APDs, specially the ones that are to be used in high

rate environments.

It should be noted that the last two characteristics are similar to the ones desired
in photomultipliers, which of course is not a surprise since both detectors have essen-
tially the same purpose. We will shortly see that modern APDs have characteristics
near to their ideal values.

C.2 MultiplicationProcessandGainFluctuations........

The process of electron multiplication in an APD is very much different from the one
that occurs in a PMT. It resembles the avalanche process in gases that we studied
in the Chapter on the gas filled detectors.
The process through which the charges multiply in an APD is known ascarrier
impact ionization. Intuitively thinking, it might seem a very simple process since it
refers to the subsequent ionizations when high energy charge carriers make collisions
with molecules along their tracks. However, the reality is that it is an extremely
complicated process and is very difficult to evaluate analytically. The difficulty lies
mainly in modeling the stochastically spread distribution of charges after impulse
ionization. A detailed account of different existing models and the related analytical
computations is beyond and scope of this book and therefore the interested reader
is referred to other sources (see, for example (37; 24; 31) and the references therein).
The problems mentioned above lead to the difficulty in the determination of
the probability density function of the electrons that make up the output signal.
One might assume these electrons to have a Poisson distribution if the absorption
of the incident light could be described by the Poisson process. This, however, is
not really the case. In face, the distribution of electrons is fairly complicated and
extremely difficult to evaluate numerically ((33)). Due to this difficulty a number of