Physics and Engineering of Radiation Detection

644 Chapter 11. Dosimetry and Radiation Protection

p+(Source) p+(Drain)

Silicon Substrate

Al Al

SiO 2 SiO 2

Oxide

Polysilicon

SiO 2

Metal

Kapton

Figure 11.4.4: Simplified

schematic of a p-channel MOS-

FET dosimeter.

The working principle of the device shown in Fig.11.4.4 is fairly simple. Appli-
cation of a high voltage to the polysilicon forces a large number of holes to move
from the surrounding regions into the oxide layer and the adjacent silicon substrate.
If a large number of holes are gathered in that area, they form a current channel
between the source and the drainSiO 2 regions. A small voltage, called thethreshold
voltage, can then initiate the current flow. The radiation passing through the oxide
region produces electron hole pairs. The holes move towards the interface of silicon
andSiO 2 , where they get trapped. This excess positive charge induces current in
the channel between the source and the drain. Consequently the threshold voltage
shifts to an extent that is proportional to the positive charge buildup. And since
this charge buildup is proportional to the energy deposited by the incident radiation,
the change in the threshold voltage is a measure of the radiation dose delivered to
the material.
Note that, in principle, it is possible to operate a MOSFET dosimeter without
any applied bias. However in this situation the recombination of electrons and holes
is significantly large, resulting in a nonlinear signal loss. Hence, in most instances
MOSFET dosimeters are operated with a negative bias applied to the polysilicon.
MOSFET dosimetry has many advantages over conventional ion chamber and
even silicon diode dosimetry. Some of its advantages are listed below.

Small Size:MOSFET dosimeters are very small in size with typical dimension

of less than a millimeter. This makes them highly suitable forin vivodosimetry.

Good Spatial Resolution: Due to their small size, MOSFET dosimeters

offer excellent spatial resolution.

Good Isotropy:The axial anisotropy exhibited by typical MOSFET dosime-

ters is±2% for 4π, which is acceptable for most applications.

Large Dynamic Range:Because of their large dynamic range they can be

used in very low to very high radiation environments.

Radiation Type Sensitivity:Since MOSFET dosimeters work on the prin-

ciple of electron hole production by the incident radiation, therefore they can

be used for dosimetry of any type of radiation. Their use in photon, electron,

and proton dosimetry is very common.