Semiconductor Radiation Detector
Solid state detector, is also known as Semiconductor Radiation Detector. The discovery of semiconductors and the invention of the transistor in 1947 has an impact on Electronics, Computer Technology, telecommunications, and Instrumentation. The materials can be classified as conductors, semiconductors, and insulators on the basis of their conductivity. Semiconductors include the materials having conductivity lying between the conductivity of conductors and that of insulators.
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A radiation detector in which the detecting medium is a solid state detector (semiconductor) material such as a silicon or germanium crystal. The solid state detector has conductivity in the range 104 to 10-6 Sm-1. As a beam of ionizing radiation passes through the device, it creates a p-n junction, which generates a current pulse. In a different device, the absorption of ionizing radiation generates pairs of charge carriers (current carries or electrons called holes) in a block of semiconducting material. The pulses created in this way are recorded, amplified, and analyzed to examine the energy, number, or identity of the incident charged particles. The sensitivity of solid state detectors can be improved by running them at low temperatures, such as 164°C (263°F), which suppresses the spontaneous forming of charge carriers due to thermal vibration. A semiconductor radiation detector in which a semiconductor material such as a silicon or germanium crystal constitutes the detecting medium.
The Intrinsic and Extrinsic Solid State Detector
Intrinsic Semiconductor
An extremely pure solid state detector is called an intrinsic semiconductor. An example of intrinsic semiconductors is silicon, germanium. Si (silicon) atom has 4 valence electrons. Silicon atoms share their four valence electrons with their four neighbour atoms and also take a share of 1 electron from each neighbour. At absolute zero temperature, the valence electron band is filled and the conduction band is empty. The departure of an electron from a valence bond creates a vacancy in the bond that is called a hole. That is, every thermally separated bond creates electron-hole pair. In intrinsic semiconductor total current is the sum of electronic current Ie and the hole current is Ih. Here the formula is, I = Ie + Ih.
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Extrinsic Semiconductor
The conductivity of an intrinsic semiconductor can permanently be increased, by adding suitable impurities. Hence the process of adding impurity to pure semiconductors called doping and the impurity atoms are called dopants. A doped solid state detector is called an extrinsic semiconductor. The Dopant atom should not distort the original semiconductor crystal structure.
Solid State Nuclear Track Detector
A solid state nuclear track detector (also known as a dielectric track detector, DTD) is a sample of a solid material (crystal, photographic emulsion, glass or plastic) exposed to a nuclear track detector (neutrons or charged particles), etched, and examined microscopically. Solid state nuclear track detector particles have a higher etching rate than bulk material and the shape and size of these tracks yield information about the charge, mass, energy and direction of motion of the particles. The precise knowledge available on individual particles is one of the key benefits over other solid state radiation detectors, the persistence of the tracks allowing measurements to be made over long periods of solid, and the simple, and robust construction of the detector.
Types of Semiconductor Detectors
There are Two Types of Detectors are as Follows,
N-Type Detectors
P-Type Detectors
N-Type Detector
The solid detector has a large number of electrons in the conduction band and the conductivity is due to negatively charged electrons it is called an n-type solid detector. The n-type semiconductor also has a few electrons and holes produced because of thermally broken bonds. Though n-type detectors have a large number of electrons, its net charge is neutral (zero). When Si or Ge crystals are doped with a pentavalent impurity such as Arsenic(As), Phosphorus (P), Antimony (Sb), we get an n-type semiconductor.
Therefore, valence orbit can hold a maximum of eight electrons, the fifth (extra) electron of the dopant atom is not part of covalent bonding and hence it is loosely bound with its core. Small energy is required to break the bound. It is 0.05 eV for Silicon and 0.01 eV for Germanium.
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P-Type Detector
The solid detector has a large number of holes and conductivity is because of positively charged holes, it is called a p-type semiconductor. The p-type solid detector has a large number of holes created by trivalent dopants and few electron-hole pairs because of thermally broken bonds. Though the p-types detector has a large number of holes, its net charge is neutral (zero). The p-type of detector has holes as majority carriers and electrons as minority carriers. When Si or Ge crystals are doped with trivalent impurities such as boron (B), aluminium (Al), indium (In), we get a p-type semiconductor. This trivalent atom has three electrons in a valence orbit.
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The Solid State Radiation Detector
The process which occurs during the detection of nuclear radiation in a solid-state device is considered in brief, and the advantages of the reverse-biased semiconductor junction in germanium or silicon are set out. The effects of radiation damage, as well as the factors that determine a detector's energy resolution, are investigated. The preparation of detectors is not discussed in detail, but the physical concepts on which the various types of detectors are based are briefly mentioned. The terminating section surveys the field of applications of solid state detectors in nuclear physics, radiochemical analysis, space research, medicine and biology. In the medical field, it is used as a solid state x-ray detector.
Solid state photomultipliers are called Silicon photomultipliers, often denoted "SiPM" in the literature. Although the device works in switching mode, most solid state photomultiplier (SiPM) is an analogue device because all the microcells are read in parallel and making it possible to generate signals within a dynamic range from 1 photon to 1000 photons for a device with just a square millimetre area.
Fun Facts
The solid detector is very small in size and light in weight.
They do not have a heating element and hence low power consumption.
Detectors do not have warm up time.
They can operate on low voltage.
The solid detector is used in the medical field also as a solid state x-ray detector.
They have a high speed of operations.
A complementary device is possible such as n-p-n and p-n-p transistors.
FAQs on Solid State Detector
1. What is a solid state detector?
A solid state detector, also known as a semiconductor radiation detector, is a device that uses a solid semiconductor material, typically silicon (Si) or germanium (Ge), to detect the presence of ionising radiation. When radiation passes through the semiconductor, it creates electron-hole pairs, which generate a measurable electrical signal proportional to the radiation's energy.
2. What is the basic working principle of a solid state detector?
The working principle is based on the interaction of radiation with a semiconductor. When an incident particle or photon strikes the detector material, it transfers energy to excite electrons from the valence band to the conduction band. This process creates pairs of charge carriers: a free electron and a positively charged hole. An applied electric field across the detector sweeps these carriers to their respective electrodes, producing a small electrical current pulse that is amplified and recorded.
3. What common materials are used to make solid state detectors?
The most common materials are high-purity, single-crystal semiconductors that are sensitive to radiation. The primary choices based on their properties are:
- Silicon (Si): Widely used for detecting charged particles like alpha and beta particles due to its excellent charge collection properties.
- Germanium (Ge): Preferred for gamma-ray spectroscopy because its higher atomic number increases the probability of gamma-ray interaction, providing better energy resolution for high-energy photons.
4. What are the main advantages of using a solid state detector?
Solid state detectors offer several key advantages over other types of radiation detectors:
- Compact Size: They are significantly smaller and lighter.
- High Energy Resolution: They can more accurately distinguish between radiations of very similar energies.
- Fast Response Time: The signal generation is very quick, allowing for the measurement of high radiation rates.
- Low Power Consumption: They operate efficiently on low voltages and do not require warm-up time.
5. How are solid state detectors used in medical fields like radiology?
In medical imaging, such as in modern CT scanners and digital X-ray systems, solid state detectors are essential. They function as solid state X-ray detectors that convert X-ray photons passing through the body into precise electrical signals. These signals are then processed by a computer to construct a detailed, high-resolution image of internal tissues and bones, contributing to clearer diagnostics at potentially lower radiation doses.
6. How does doping create n-type and p-type semiconductors for detectors?
Doping is the process of adding specific impurities to a pure semiconductor to alter its electrical properties, which is fundamental for creating detectors.
- N-type Semiconductor: This is formed by adding a pentavalent impurity (with 5 valence electrons), such as Phosphorus (P), to a Silicon (Si) crystal. The fifth electron is not needed for covalent bonding and becomes a free charge carrier, making electrons the majority carriers.
- P-type Semiconductor: This is formed by adding a trivalent impurity (with 3 valence electrons), such as Boron (B). This creates a vacancy or 'hole' in the crystal's covalent bond structure, which acts as a positive charge carrier, making holes the majority carriers.
7. Why is a reverse-biased p-n junction crucial for the operation of many solid state detectors?
A reverse-biased p-n junction is crucial because it creates a wide depletion region that is essentially free of mobile charge carriers. This region acts as the sensitive volume of the detector. When ionising radiation strikes this area, it generates electron-hole pairs. The strong electric field existing across the reverse-biased junction immediately separates these new charges and sweeps them to the electrodes, producing a sharp, well-defined current pulse. This efficient charge collection is vital for the detector's high resolution and sensitivity.
8. What is the difference between an intrinsic and an extrinsic semiconductor in the context of detectors?
The key difference is in purity and charge carrier concentration. An intrinsic semiconductor is an ultra-pure material where charge carriers (electrons and holes) are created solely by thermal energy. Its conductivity is low. An extrinsic semiconductor is one that has been doped with impurities to vastly increase the number of charge carriers. This process creates either an excess of electrons (n-type) or holes (p-type), making the material far more conductive and efficient for detecting radiation.
9. How does temperature affect the performance and sensitivity of a solid state detector?
Temperature significantly impacts a detector's performance by creating electronic noise. As temperature rises, thermal energy can spontaneously create electron-hole pairs, generating a 'dark current' that is not caused by radiation. This background noise can obscure the small signal from a genuine radiation event, which degrades the detector's energy resolution and overall sensitivity. For this reason, high-resolution germanium detectors are often cooled to liquid nitrogen temperatures to minimise this thermal noise.
10. What distinguishes a solid state detector from a gas-filled detector for radiation measurement?
The primary distinction is the detecting medium, which leads to major differences in performance. A solid state detector uses a solid semiconductor, whereas a gas-filled detector uses a gas. The consequences are:
- Density and Efficiency: Solids are thousands of times denser than gases, giving solid state detectors a much higher probability of stopping radiation. This makes them more efficient and allows for a more compact design.
- Energy Resolution: It requires about 10 times less energy to create an electron-hole pair in a semiconductor (~3 eV) than to ionise a gas atom (~30 eV). Therefore, for the same amount of deposited radiation energy, a solid state detector produces many more charge carriers, resulting in a stronger signal and superior energy resolution.
