Physics and Engineering of Radiation Detection

5.1. Semiconductor Detectors 297

Supply

Reverse Bias

n−Type

n+ Layer −+ Material

p+ Layer

Guard Ring

Electrode Implant

Electrode Implant

Output

Figure 5.1.30: A simple but realistic semiconduc-

tor detector. Central light shaded region repre-

sents the sensitive volume that has been fully de-

pleted by applying reverse bias. The bulk of the

material is n-type, on which p+ and n+ regions

have been created by the process of doping. A

guard ring around the p+ layer guards against

discharges due to high bias voltage.

5.1.J DesirableProperties

The desired properties of semiconductor detectors are highly application dependent
as some of these have conflicting requirements. Semiconductor detectors are highly
versatile devices that can be used in very high to very low radiation environments
provided they are built for specifically for that radiation field. for example an
avalanche photodiode can be used to detect single photons but it will be completely
useless for detecting high intensity gamma rays.
Therefore we can safely divide semiconductor detectors in two categories depend-
ingonwhethertheyareusedinhighorlowradiationfields.

J.1 HighRadiationFields

There are three main characteristics of the detectors used in high radiation environ-
ments.

Charge collection efficiency: At high incident radiation rate, very large

number of charge carriers are produced, requiring efficient collection by elec-

trodes. Any non-linearity in charge collection would reflect as non-linearity in

the output signal. The deterioration in charge collection is caused by electron-

hole recombination and charge trapping.


Fast response: The associated electronics should be fast enough to avoid

pulse pileup. Generally the electronics should be able to distinguish between

pulses that are only a few tens of nanoseconds wide.


Radiation hardness: The detector must be least susceptible to radiation

damage. We will learn more about this later in this Chapter but at this time it

suffices to say that radiation hardness is perhaps the most actively researched

topics in semiconductor detector technology.

J.2 LowRadiationFields

For low radiation environments the requirements are very different.