Purpose And Types Of Semiconductor Detector Engineering Essay

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A semiconductor detector is a device that uses a semiconductor to detect traversing charged particles or the absorption of photons. In the field of particle physics, these detectors are usually known as silicon detectors.

When their sensitive structures are based on a single dIode, they are called semiconductor diode detectors. When they contain many diodes with different functions, the more general term semiconductor detector is used.

Semiconductor detectors have found broad application during recent decades, in particular for gamma and X-ray spectrometry and as particle detectors..FIRST of all we will see…

WHAT IS SEMICONDUTOR…

A semiconductor is a material with electrical conductivity due to electron flow (as opposed to ionic conductivity ) intermediate in magnitude between that of a conductor and an insulator . This means a conductivity roughly in the range of 103 to 10−8 siemens  per centimeter. Semiconductor materials are the foundation of modern electronics, including radio, computers, telephones, and many other devices. Such devices include transistors , solar cells , many kinds of diodes  including the light-emitting diode, the silicon controlled rectifier, and digital and analog integrated circuits. Similarly, semiconductor solar photovoltaic panels directly convert light energy into electrical energy. In a metallic conductor, current is carried by the flow of electrons . In semiconductors, current is often schematized as being carried either by the flow of electrons or by the flow of positively charged "holes " in the electron structure of the material. Actually, however, in both cases only electron movements are involved.

CHARACTERSTICS OF SEMICONDUCTOR..

Common semiconducting materials are crystalline solids, but amorphous and liquid semiconductors are known. These include hydrogenated amorphous silicon  and mixtures of arsenic , selenium  and tellurium in a variety of proportions. Such compounds share with better known semiconductors intermediate conductivity and a rapid variation of conductivity with temperature, as well as occasional negative resistance . Such disordered materials lack the rigid crystalline structure of conventional semiconductors such as silicon and are generally used in thin film  structures, which are less demanding for as concerns the electronic quality of the material and thus are relatively insensitive to impurities and radiation damage. Organic semiconductors , that is, organic materials with properties resembling conventional semiconductors, are also known.

Silicon is used to create most semiconductors commercially. Dozens of other materials  are used, including germanium, gallium arsenide , and silicon carbide. A pure semiconductor is often called an "intrinsic" semiconductor.

BASED ON DOPING SEMICONDUCTORS ARE CLASSIFIED

WHAT IS DOPING….

The electronic properties and the conductivity of a semiconductor can be changed in a controlled manner by adding very small quantities of other elements, called dopants, to the intrinsic material. In crystalline silicon typically this is achieved by adding impurities of boron or phosphorus  to the melt and then allowing the melt to solidify into the crystal. This process is called doping.

Based on this semiconductor are classifies as..

. P type semiconductor

. N type semiconductor

P Type semiconductor..

he second type of impurity, when added to a semiconductor material, tends to compensate for itsdeficiency of 1 valence electron by acquiring an electron from its neighbor. Impurities of this type haveonly 3 valence electrons and are called TRIVALENT impurities. Aluminum, indium, gallium, and boronare trivalent impurities. Because these materials accept 1 electron from the doped material, they are alsocalled ACCEPTOR impurities.A trivalent (acceptor) impurity element can also be used to dope germanium. In this case, theimpurity is 1 electron short of the required amount of electrons needed to establish covalent bonds with 4neighboring atoms. Thus, in a single covalent bond, there will be only 1 electron instead of 2. Thisarrangement leaves a hole in that covalent bond. Following figure shows how an germanium is doped with an indium (In) atom.

N Type semiconductor…

The N-type impurity loses its extra valence electron easily when added to a semiconductor material,and in so doing, increases the conductivity of the material by contributing a free electron. This type ofimpurity has 5 valence electrons and is called a PENTAVALENT impurity. Arsenic, antimony, bismuth,and phosphorous are pentavalent impurities. Because these materials give or donate one electron to thedoped material, they are also called DONOR impurities.When a pentavalent (donor) impurity, like arsenic, is added to germanium, it will form covalentbonds with the germanium atoms. Following fig shows how an arsenic atom (AS) in agermanium (GE) lattice structure. Notice the arsenic atom in the center of the lattice. It has 5 valence electrons in its outer shell but uses only 4 of them to form covalent bonds with the germanium atoms,leaving 1 electron relatively free in the crystal structure. Pure germanium may be converted into an N-type semiconductor by "doping" it with any donor impurity having 5 valence electrons in its outershell. Since this type of semiconductor (N-type) has a surplus of electrons, the electrons are consideredMAJORITY carriers, while the holes, being few in number, are the MINORITY carriers.

APPLICATION OF SEMICONDUCTOR DETECTOR..

.SEMICONDUCTOR RADIATION DETECTOR

.SEMICONDUCTOR PARTICLE DETECTOR

(a)GERMANIUM DETECTOR

(b) SILICON DETECTOR

SEMICONDUCTOR PARTICLE DETECTOR..

GERMANIUM DETECTOR..

Germanium detectors are semiconductor diodes having a p-i-n structure in which the intrinsic (I) region is sensitive to ionizing radiation, particularly x rays and gamma rays. Under reverse bias, an electric field extends across the intrinsic or depleted region. When photons interact with the material within the depleted volume of a detector, charge carriers (holes and electrons) are produced and are swept by the electric field to the P and N electrodes. This charge, which is in proportion to the energy deposited in the detector by the incoming photon, is converted into a voltage pulse by an integral charge sensitive preamplifier.

Because germanium has relatively low band gap, these detectors must be cooled in order to reduce the thermal generation of charge carriers (thus reverse leakage current) to an acceptable level. Otherwise, leakage current induced noise destroys the energy resolution of the germanium detector. Liquid nitrogen, which has a temperature of 77 °K is the common cooling medium for such detectors. The germanium detector is mounted in a vacuum chamber which is attached to or inserted into an LN2Dewar. The sensitive detector surfaces are thus protected from moisture and condensible contaminants.

CRYOSTATS

The liquid nitrogen cryostat is the most important, and perhaps the least appreciated, component in assuring reliable long term performance of a Germanium detector system. CANBERRA manufactures its own cryostats to exacting quality standards to ensure long detector life under the harshest operating conditions.

The standard CANBERRA cryostat is our Slimline Design in which the detector chamber and preamplifier are packaged together in a compact cylinder.

Low energy detectors, such as the Ultra-LEGes and Si(Li)s use our flanged cryostats which are compatible with the small diameter (25 mm) end-caps associated with this type of detector. Flanged cryostats are available as an extra-cost option for other detector types.

For applications requiring liquid nitrogen . This electrically cooled cryostat uses a CFC-free refrigerant and is well suited for use in industrial and laboratory applications.

PREAMPLIFIERS

There are only two basic types of preamplifiers in use on Ge detectors. These are charge sensitive preamplifiers, which employ either dynamic charge restoration (RC feedback), or pulsed charge restoration (Pulsed optical or Transistor reset) methods to discharge the integrator. The following figure illustrates the energy rate limitation of dc-coupled RC feedback preamps, which is a function of the feedback resistor value and the dynamic output voltage range of the integrator, which is limited to about 20 volts.

The energy rate limit can be increased very substantially by choosing a lower value feedback resistor with, of course, an accompanying increase in noise. Actual performance data on a typical detector is given below:

Resolution vs. Feedback Resistor

Experimental Results with Detector

Resistor Value

57Co

(122 keV FWHM)

60Co

(1332 keV FWHM)

2 Gigohm

1.00

1.81

1 Gigohm

1.02

1.85

0.5 Gigohm

1.08

1.93

Pulsed-Optical Reset preamplifiers are widely used on low energy detectors where resolution is of utmost consideration. Eliminating the feedback resistor decreases noise without a serious impact on dead time, so long as the average energy per event is low to moderate. At 5.9 keV/event, a CANBERRA 2008 preamp may process almost 1000 pulses between resets. Since the reset recovery time is 2-3 amplifier pulse widths, little data is lost in this situation. Optical feedback systems can, however, exhibit long recovery times due to light activated surface states in the FET. Proper selection and treatment of components can minimize the problem, but it is generally present to some degree in pulsed-optical systems. With high energies, where resets necessarily occur very often, perhaps after as few as 10 events, this spurious response can be a serious problem. As a consequence, pulsed-optical feedback systems are not generally used with coaxial detectors.

The Transistor Reset Preamp was developed in an attempt to overcome the problems associated with Pulsed-Optical Reset Preamps in high energy, high rate systems. The feedback capacitor is discharged by means of a transistor switch connected to the FET gate. This transistor adds some capacitance and noise to the input circuit, but this is tolerable in most applications involving high count or energy rates. Compared to an RC preamplifier with selected feedback resistor for high rate performance, the Transistor Reset Preamplifier will exhibit less noise but will sacrifice dead time because the amplifier will require 2-3 pulse widths to recover from the periodic reset of the preamplifier. Thus, in applications demanding high throughput rates, the Transistor Reset Preamp is not a good choice. It can be used in situations where the energy rate is so high that an RC preamp might saturate - but the throughput rate may be diminishingly small in this case.

SILICON DETECTOR….

Most silicon particle detectors work, in principle, by doping narrow (usually around 100 micrometers wide) strips of silicon to make them into diodes, which are then reverse biased. As charged particles pass through these strips, they cause small ionization currents which can be detected and measured. Arranging thousands of these detectors around a collision point in aparticle accelerator can give an accurate picture of what paths particles take. Silicon detectors have a much higher resolution in tracking charged particles than older technologies such as cloud chambers or wire chambers. The drawback is that silicon detectors are much more expensive than these older technologies and require sophisticated cooling to reduce leakage currents (noise source) as well as suffer degradation over time from radiation..

SEMICONDUCTOR RADIATION DETECTOR..

In these detectors, radiation is measured by means of the number of charge carriers set free in the detector, which is arranged between two electrodes. Ionizing radiation produces free electrons and holes. The number of electron-hole pairs is proportional to the energy transmitted by the radiation to the semiconductor. As a result, a number of electrons are transferred from thevalence band to the conduction band, and an equal number of holes are created in the valence band. Under the influence of an electric field, electrons and holes travel to the electrodes, where they result in a pulse that can be measured in an outer circuit. The holes travel in the opposite direction and can also be measured. As the amount of energy required to create an electron-hole pair is known, and is independent of the energy of the incident radiation, measuring the number of electron-hole pairs allows the energy of the incident radiation to be found.

The energy required for production of electron-hole-pairs is very low compared to the energy required for production of paired ions in a gas detector. Consequently, in semiconductor detectors the statistical variation of the pulse height is smaller and the energy resolution is higher. As the electrons travel fast, the time resolution is also very good, and is dependent upon rise time. Compared with gaseous ionization detectors, the density of a semiconductor detector is very high, and charged particles of high energy can give off their energy in a semiconductor of relatively small dimensions.

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