Physics and Engineering of Radiation Detection

296 Chapter 5. Solid State Detectors

of radiation is minimal. When radiation traverses through this region and produces
a large number of charge pairs, the reverse current increases. The magnitude of
this current is proportional to the energy deposited by the incident radiation. This
proportionality is the most desirable factor for any detector since it enables one to
derive meaningful quantities from the measurements about the incident radiation.
The extremely low leakage current in a well designed semiconductor detector ensures
that this proportionality remains a reality throughout the dynamic range of the
detector. To get an idea, the typical leakage current is of the order of a few nano
amperes, while the radiation induced current can be several orders of magnitude
higher than this.
Fig.5.1.29 shows how the current increases with respect to increase in flux of
incident radiation (the higher the flux the higher the energy deposited). Since the
leakage current does not depend on the energy deposited by the radiation therefore
its effect can be easily subtracted from the detector response, provided all other
conditions including temperature remain constant.

+

φ 1

φ 3

φ 2

φ 1 φ 2 φ 3

i

−

−

−

−

−

−

−

−

+ −

+

+

+

+

p n

i

V

(a)

Reverse

− +

Depletion Region

(b)

<<

Bias

ForwardBias

+

+

+

Figure 5.1.29: (a) Production of

electron-hole pairs in the depletion

region of a reverse biased pn-diode.

(b) Typical current-voltage curve of

a reverse biased pn-junction diode in

the presence of radiation. As the flux

of incident radiation increases, the

number of electron-hole pairs in the

depletion region increases, which in

effect increases the measured current.

Since only the charge pairs produced in the depletion region contribute to the
signal, therefore to make the active region large the depletion region is stretched
as much as possible. This can be done in two ways: by increasing the reverse bias
and by introducing an intrinsic semiconductor between the p and n materials. The
former has already been discussed and the later will be explored later in this chapter.
Fig.5.1.30 shows a simple but realistic detector built by doping p- and n-type
impurities on a bulk n-type semiconductor material such as silicon.