Physics and Engineering of Radiation Detection

304 Chapter 5. Solid State Detectors

Example:

A 300μmthick silicon detector having a surface area of 8× 10 −^4 cm^2 is placed

in a 1MeVneutrons field having intensity of 3× 106 cm−^2 s−^1. Compute the

change in leakage current relative to the initial value of 10nAafter one year

of constant irradiation. Take the damage coefficient to be 2× 10 −^17 A/cm

and assume that the detector was operated in fully depleted mode.

Solution:

In fully depleted mode we can assume that the active region is equal to the

volume of the detector. HenceV= (300× 10 −^4 )(8× 10 −^4 )=2. 4 × 10 −^5 cm^3.

The total fluence to which the detector remained exposes for one year is give

by

Φ=(10^6 )(365× 24 ×3600) = 3. 15 × 1013 cm−^2.

According to equation 5.1.87, the relative change in leakage current can be

calculated as follows.

δi =

ir−i 0

i 0

= α

ΦV

i 0

=2× 10 −^17

(3. 15 × 1013 )(2. 4 × 10 −^5 )

100 × 10 −^9

=0.15 or

δi = 15%. (5.1.89)

Hence after a year of irradiation the current will change by 15%. Whether this

change is observed and attributed to the radiation damage or not depends on

how well the detector is being monitored and calibrated.

L.3 TypeInversion

A special type of damage caused by high intensity radiation is the inversion of the
material type, in which an n-type material may change into a p-type and vice versa
after prolonged irradiation. The effect can be easily understood by noting that radi-
ation damage has the overall effect of changing the effective dopant concentration in
the bulk of the material by increasing the charge carriers of the opposite sign. With
increase in integrated radiation dose, the original effective dopant concentration may
be overcome by the charges of opposite sign. This effect has been observed specially
in silicon detectors in hostile radiation environments. What happens is that the
radiation creates acceptor sites, which capture electrons resulting in a decrease in
effective doping level. This continues until the donor level becomes equal to the ac-
ceptor level, changing the material into intrinsic type. Further irradiation increases
the acceptor level and consequently the types of material changes (see Fig.5.1.33).