5.1. Semiconductor Detectors 301
K.4 AvalanchePhotodiode.....................
Avalanche photodiodes utilize the concept of multiplying the number of charge car-
riers to increase the signal height. These detectors are highly efficient in detecting
low light levels and in principle are capable of detecting single photons. We will
have a look at APDs in the next chapter when we discuss the photodetectors.
K.5 SurfaceBarrierDetector....................
A surface barrier detector is usually made by evaporating a thin gold layer on an
n-type highly pure silicon crystal. The contact on the other side is generally made by
evaporating a thing layer of aluminum. The depletion region is created by applying
reverse bias across the aluminum and the gold contacts, which produces a wide
depletion region near the gold side and a thin dead region near the aluminum side.
Since the gold thickness is very small, there is minimal parasitic absorption. The
detector can therefore be used in low level radiation fields. The advantage of surface
barrier detectors include high stability and superior energy resolution.
K.6 PositionSensitiveDetectors..................
The onset of aggressive research and development in semiconductor detector tech-
nology can be traced back to the realization of the potential to develop position
sensitive detectors of extremely high resolution and sensitivity. With today’s tech-
nology, detectors having position sensitivity of better than 10μmcan be built. The
main idea behind such detectors involves doping a wafer of semiconductor material
by selectively masking arrays of desired geometry, thus developing an array of de-
tectors on the wafer. The details of how this is done and how this array is read out
depends on the particular design. Although we will discuss such detectors in the
Chapter on position sensitive detectors but it is worthwhile to briefly describe some
of the commonly used position sensitive detectors here as well.
Microstrip Detectors
Microstrip detectors are made by doping a large semiconductor wafer into very
thin (typically 10μm) parallel segments. The segments generally have a pitch (cen-
ter to center distance between two consecutive segments) of around 50μm.Each
of these segments then behaves as a separate detector. The main advantage of such
a geometry is high position sensitivity, which is not achievable from other conven-
tional position sensitive detectors, such as wire proportional counters. With today’s
technology a position resolution of better than 10μmis possible. The main com-
plication with building such detectors is the incorporation of readout electronics.
In a typical few centimeter wide detector module there are several hundred strips
acting as individual detectors. Each of these detectors has to be read out through a
unique analog electronics channel having its own preamplifier. Fortunately, there is
technology available now to build electronic chips that can handle a large number
of channels at the same time. These chips are installed on boards that are densely
populated with interconnects and other electronic components, hence called High
Density Interconnects or simply HDIs. The detectors are connected to the pads
available on HDIs through wire bonds, a complicated process that is done through