[Physics Class Notes] on Solid State Detector Pdf for Exam

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 I_e and the hole current is I_h. Here the formula is, I = I_e + I_h.

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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,

1. N-Type Detectors

2. 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

1. The solid detector is very small in size and light in weight.

2. They do not have a heating element and hence low power consumption. 

3. Detectors do not have warm up time.

4. They can operate on low voltage.

5. The solid detector is used in the medical field also as a solid state x-ray detector.

6. They have a high speed of operations. 

7. A complementary device is possible such as n-p-n and p-n-p transistors.