7.5 Effect of Radiation on the p−n Junction
127
tion was exposed to these radiations gives information about the energy of interacting
radiation, provided the potential developed across the p−n junction is calibrated in
terms of energy (MeV). In other words, one single radiation interacting with p−n
junction produces one pair of electron/hole. The number of such electron/hole pairs
produced is equal to the number of potential pulses produced. Therefore, measurement of the number of such potential pulses corresponds to the number of radiations
interacting with the p−n junction. Therefore, with the help of a pulse height analyzer and an amplifier, one can identify and measure the activity of the sample.
This description is an oversimplified version of what actually happens at the p−n
junction after illuminating it with such electromagnetic radiation. In reality, we do
not measure the potential, which is developed across the junction. In fact, the junction
is formed due to the difference in the Fermi levels of the two semiconductors. This
produces a contact potential (φ volt). The generation of electron/hole pairs decreases
the magnitude of this contact potential. Hence, as in ionization counters, we actually
measure the decrease in contact potential (like decrease in anode potential of a
proportional counter) when the radiations interact at the p−n junction. As ionization
counters, this decrease in the contact potential appears as a negative pulse.
The efficiency of generation of electron/hole pairs by radiation largely depends on
the feasibility of their absorption within the space charge width. Therefore, thickness
of the p−n junction, especially the side through which radiation has to interact with
the junction, should be kept to a minimum value. Thin material allows the radiation
to penetrate through the material to reach the space charge region. Thickness of the
semiconductor which is exposed to the radiation should, therefore, be kept to a few
microns, so that even weak β-particle or a particle can penetrate to reach the space
charge region of the p−n junction (Fig. 7.3).
7.5.1 Design of a Semiconductor Detector
Considering these factors, a typical design of a p−n junction and its electronic
circuit are shown in Fig. 7.3. This circuit resembles a proportional counting system.
The p−n junction (Fig. 7.3) is made of p-type (shown by slanting lines), on which
the n-type semiconductor is kept such that there is no air gap in between. There are
many techniques to achieve this configuration. In order to collect electrons (from ntype) and holes from the back of p-type, a thin layer of conducting material, normally
gold, is deposited over both semiconductors. But one of the main considerations is
that the Fermi energy of the metal must match with the Fermi energy of the nand p-type materials, so that one can achieve an ohmic contact and not a junctiontype contact. The entire thickness of the p−n junction (d) is kept to approximately
0.5 cm or less. The entire system is sealed in a metal enclosure, with n-type being left
open for the radiation to penetrate. The p-type is connected to the positive terminal
of a battery (normally 2.0 V DC battery). A capacitor and a resistance are joined in
parallel to avoid counting of any spurious pulses. The negative pulses are initially fed
through a diode for measurement, followed by feeding through an amplifier, pulse
127
tion was exposed to these radiations gives information about the energy of interacting
radiation, provided the potential developed across the p−n junction is calibrated in
terms of energy (MeV). In other words, one single radiation interacting with p−n
junction produces one pair of electron/hole. The number of such electron/hole pairs
produced is equal to the number of potential pulses produced. Therefore, measurement of the number of such potential pulses corresponds to the number of radiations
interacting with the p−n junction. Therefore, with the help of a pulse height analyzer and an amplifier, one can identify and measure the activity of the sample.
This description is an oversimplified version of what actually happens at the p−n
junction after illuminating it with such electromagnetic radiation. In reality, we do
not measure the potential, which is developed across the junction. In fact, the junction
is formed due to the difference in the Fermi levels of the two semiconductors. This
produces a contact potential (φ volt). The generation of electron/hole pairs decreases
the magnitude of this contact potential. Hence, as in ionization counters, we actually
measure the decrease in contact potential (like decrease in anode potential of a
proportional counter) when the radiations interact at the p−n junction. As ionization
counters, this decrease in the contact potential appears as a negative pulse.
The efficiency of generation of electron/hole pairs by radiation largely depends on
the feasibility of their absorption within the space charge width. Therefore, thickness
of the p−n junction, especially the side through which radiation has to interact with
the junction, should be kept to a minimum value. Thin material allows the radiation
to penetrate through the material to reach the space charge region. Thickness of the
semiconductor which is exposed to the radiation should, therefore, be kept to a few
microns, so that even weak β-particle or a particle can penetrate to reach the space
charge region of the p−n junction (Fig. 7.3).
7.5.1 Design of a Semiconductor Detector
Considering these factors, a typical design of a p−n junction and its electronic
circuit are shown in Fig. 7.3. This circuit resembles a proportional counting system.
The p−n junction (Fig. 7.3) is made of p-type (shown by slanting lines), on which
the n-type semiconductor is kept such that there is no air gap in between. There are
many techniques to achieve this configuration. In order to collect electrons (from ntype) and holes from the back of p-type, a thin layer of conducting material, normally
gold, is deposited over both semiconductors. But one of the main considerations is
that the Fermi energy of the metal must match with the Fermi energy of the nand p-type materials, so that one can achieve an ohmic contact and not a junctiontype contact. The entire thickness of the p−n junction (d) is kept to approximately
0.5 cm or less. The entire system is sealed in a metal enclosure, with n-type being left
open for the radiation to penetrate. The p-type is connected to the positive terminal
of a battery (normally 2.0 V DC battery). A capacitor and a resistance are joined in
parallel to avoid counting of any spurious pulses. The negative pulses are initially fed
through a diode for measurement, followed by feeding through an amplifier, pulse