Physics and Engineering of Radiation Detection

452 Chapter 7. Position Sensitive Detection and Imaging

7.3.C ChargedCoupledDevices

Most of the disadvantages of photographic films have been overcome through the
development of different kinds of electronic imaging devices. One such detection
system is the so called Charged Coupled Device or CCD. This semiconductor based
multichannel imaging system is used for two dimensional imaging and is now a
standard in many applications. The basic working principle of a CCD is the same
as any other semiconductor detector, that is conversion of incident radiation into
electron-hole pairs and then measuring the resulting signal. There are different
CCD design variants that have been developed over the years based on the specific
applications. However we can safely divide these devices into two broad categories:
direct imaging systems and indirect imaging systems.
It should be noted that CCDs are in no way limited to the detection of photons.
They can be easily configured to measure any type of radiation capable of creating
electron-hole pairs in their active detection media. In fact, as we will see later, CCDs
can also distinguish between different kinds of incident radiation. These qualities
make these devices so versatile that they are currently being successfully used in
very different applications ranging from digital cameras to medical radiation scans
to radiation monitoring in earth’s upper atmosphere.

7.3.D DirectImaging

A direct imaging CCD is used to convert the incident radiation directly into electron-
hole pairs by letting it interact with the semiconductor material. As in other semi-
conductor detectors that we visited in the previous Chapter, this conversion takes
place inside the depletion region (see Fig.7.3.1). The free electrons and holes move
towards opposite electrodes under the influence of the externally applied electric
field and induce charges on the readout electrodes. The resulting signal pulse is
processed by the electronics attached to the readout electrodes.

μm

μm

Pixels

~20

~500

Readout Electrodes Oxide Layer

−+ +−

Photons

Depletion Region

Bulk Semiconductor

Figure 7.3.1: Schematic of a typical CCD working in direct de-

tection mode with front illumination. The active medium in such

devices is generally silicon. An incident particle produces charge

pairs in the depletion region, which induce charges on respective

readout electrodes that are read out through the connected cir-

cuitry.