Physics and Engineering of Radiation Detection

270 Chapter 5. Solid State Detectors

problem since the general practice is to calibrate each detector module separately,
which accounts for the small differences in impurity levels and other factors.

Table 5.1.3: Common impurities found in silicon with their positions in the band gap
(47). HereEvandEcrepresent the highest valence band level and lowest conduction
band level respectively.

Impurity Symbol Type Position (eV)

Gold Au Donor Ev+0. 35

Acceptor Ec− 0. 55

Copper Cu Donor Ev+0. 24

Acceptor Ev+0. 37

Acceptor Ev+0. 52

Iron Fe Donor Ev+0. 39

Nickel Ni Acceptor Ec− 0. 35

Acceptor Ev+0. 23

Platinum Pt Donor Ev+0. 32

Acceptor Ev+0. 36

Acceptor Ec− 0. 25

Zinc Zn Acceptor Ev+0. 32

Acceptor Ec− 0. 5

Let us now turn our attention to the process of doping in silicon. Table.5.1.4 gives
the commonly used dopers with their ionization energies. Other doping agents such
as oxygen and copper are also sometimes used in detector technology, however boron
and phosphorus are perhaps the most common choices. For detector fabrication the
doping levels are kept very small so that the resistivity of the material remains high.
High resistivity is important to suppress noise and can be afforded in silicon since
its breakdown voltage is on the order of 10^5 V/cm.
Signal generation in a semiconductor depends on how the charges move in the
bulk of the material. We saw earlier that diffusion coefficient and mobility are the
two parameters than can be used to characterize the motion of electrons and holes in
semiconductors. Extensive research has gone into understanding these parameters