406 Chapter 6. Scintillation Detectors and Photodetectors
Just like conventional photodiode detectors, the avalanche photodetectors can
also be designed and built in different configurations. Fig.6.5.23(a) shows the con-
ceptual design of a simple avalanche photodetector. The detector is made of an
intrinsic (or lightly dopedptype material) sandwiched between a heavy dopedp
side and a heavy dopednside. Anotherptype region is also established between
the intrinsic material and the heavily dopednside. The contacts are made by metal
deposition on two sides of the detector. A high reverse bias applied at the two ends
creates an electric field profile similar to the one sketched in Fig.6.5.23(b). Based on
this electric field profile two regions can be identified: an absorption regionRabsand
a multiplication regionRmult. The incident radiation passing through the intrinsic
or absorption region creates electron hole pairs along its track. The charges move
in opposite directions under the influence of moderate electric field insideRabs.The
electrons move towards thepregion, which is characterized by very high electric
field intensity. This is where avalanche multiplication takes place. The large num-
ber of electron hole pairs generated here, as a result of charge multiplication, move
in opposite directions and induce a voltage pulse that is measured by the readout
electrode. Alternatively the current generated by the motion of charges can also be
measured by the associated electronics.
Rabs Rmult
p+ n+
V
(b)
π p
(a) Figure 6.5.23: (a) Conceptual design of
a simple avalanche photodetector. (b)
The electric field profile of the struc-
ture shown in (a).πrepresents either a
lightly doped p-material or intrinsic ma-
terial while the superscript (+) refers to
heavy doping.The kind of structure shown in Fig.6.5.23 is generally known asreach-through
structure. Fig.6.5.24 shows the practical design of a reach-through APD. The guard
ring around the multiplication region is established to minimize the possibility of
electrical breakdown.
C.1 BasicDesirableCharacteristics
The desirable characteristics of an avalanche photodetector are not much different
from the conventional photodiode detectors as can be deduced from the list below.
Small Leakage Current:An idea APD does not have any leakage current.
Real APDs have extremely small dark currents. The reason to strive for no
leakage current is to avoid unwanted avalanches, something that may induce
non-linearity in the detector response.