Physics and Engineering of Radiation Detection

5.1. Semiconductor Detectors 283

Table 5.1.8: Mobilities (μe,μh,) velocities (ve,vh), and diffusion coefficients (De,Dh)
of electrons and holes in gallium arsenide (47).

Property Symbol Value

Electron Mobility μe ≤ 8500 cm^2 V−^1 s−^1

Hole Mobility μh ≤ 400 cm^2 V−^1 s−^1

Electron Thermal Velocity ve 4. 4 × 105 ms−^1

Hole Thermal Velocity vh 1. 8 × 105 ms−^1

Electron Diffusion Coefficient De ≤ 200 cm^2 s−^1

Hole Diffusion Coefficient Dh ≤ 10 cm^2 s−^1

G.4 Cadmium-Zinc-Tellurium (CdZnTe).............

Also referred to in short asCZT, this material has gained a lot of popularity in the
recent years. It has several properties that make it highly desirable for demanding
applications, such as spectroscopy. Its high detection efficiency, high resolution, low
cost, and good signal to noise ratio at room temperature make it a product of choice
for many applications. Its band gap is more than twice that of germanium at room
temperature, which drastically decreases the number of intrinsic charge carriers (see
Table.5.1.2). This makes it highly suitable for room temperature operation.
CdZnTeis actually a ternary alloy ofCdTeandZn. Its properties therefore
depend on the concentration of zinc in the bulk as well as on the surface of the
material. For radiation detection purposes the most important parameter is the
band gap. Fortunately the band gap has been found to be very lightly dependent
on the concentration of zinc. Fluctuations of a few percent in zinc concentration
change the band gap by only a fewmeV(46), which is insignificant for estimation
of most detector related parameters. In general the percentage of zinc in a typical
CZTbulk is less than 10%.
Another important property ofCZTmaterial is that it can be formed into dif-
ferent shapes and sizes. Large area and complicated geometryCZTdetectors can
therefore be practically fabricated.
CZTdetectors have fairly high stopping power and absorption efficiency due to
highZelements. This together with their high efficiency makes them well suited for
use in imaging applications, such as medical x-ray imaging.
There are two main disadvantages associated withCZTmaterials: their low hole
mobility (and hence low lifetime) and crystal defects. The later can be somewhat
controlled by using techniques that yield less crystal defects. The low hole mobility,
on the other hand, is caused by the hole trapping mechanisms and is more or less
intrinsic to the material. The only way to increase the hole lifetime is by increasing
the detector bias voltage. Note that crystal defects further deteriorate the hole
mobility and should therefore be controlled as much as possible. As a reminder to