Physics and Engineering of Radiation Detection

6.5. Photodetectors 415

whereBis the bandwidth of the system,〈Ge〉is the mean electron multiplication
factor,Ieis the mean electron current, andFeis the excess noise factor for the elec-
tron current. To compute the signal-to-noise ratio we also need the signal current,
which for this case is simply given byIe〈Ge〉sinceIeis the signal current before
multiplication. The expression for the signal-to-noise ratio can now be written as

S/N =

Ie〈Ge〉

σout

=

Ie〈Ge〉

√

σ^2 c+σl^2 +σt^2

=

Ie〈Ge〉

√

2 eIeB〈Ge〉^2 Fe+2eIlB〈Gl〉^2 Fl+4FtkBTBR−eqv^1

. (6.5.81)

The signal-to-noise ration depends on the mean gain which in turn has a depen-
dence on the ratio of hole and electron ionization ratesu.InfactS/N is highly
sensitive to the value ofuas can be deduced from Fig.6.5.26. This implicit de-
pendence onuputs a tough constraint on the value of gain that would yield high
signal-to-noise ratio. This is specially true for higher values ofu. For smallu,the
curve is more or less flat after a certain gain and therefore gives some flexibility in
terms of choosing the gain according to the particular application.

u = 10 −3

u = 10 −2

u = 10 −1

S/N

(re

lat

ive

)

0

0

0.5

1.0

100 200

Mean Gain

Figure 6.5.26: Dependence

of signal-to-noise ratio of an

APD on the mean gain at

different values of the ratio

of the hole ionization rate to

the electrion ionization rate.

C.6 RadiationDamage

The radiation damage to semiconductor detectors has already been discussed in
the previous Chapter. The damage mechanisms in the APDs are the same as in
conventional semiconductor detectors. The overall effect of the damage is, however,
more dramatic due to the higher sensitivity of these detectors. In an APD being
operated in a hostile radiation environment, the following two distinct types of
radiation induced damages can occur.