M7 - Solid State Physics

p type, n type

A single crystal can be grown with one half doped with a group III impurity and the other half doped with a group V impurity. Therefore, one half of the crystal will be a ___ semiconductor, and the other half will be an ____ semiconductor.

forward bias, reverse bias

A single crystal can be grown with one half doped with a group III impurity and the other half doped with a group V impurity. Therefore, one half of the crystal will be a p-type semiconductor, and the other half will be an n-type semiconductor. The crystal produced is a conductor in one direction (a direction known as the ___ ___) and an insulator in the ___ ___ direction. This is called a diode.

diode

A single crystal can be grown with one half doped with a group III impurity and the other half doped with a group V impurity. Therefore, one half of the crystal will be a p-type semiconductor, and the other half will be an n-type semiconductor. The crystal produced is a conductor in one direction (a direction known as the forward bias) and an insulator in the reverse bias direction. This is called a ____

impurity, energy levels, luminescent centres

An efficient phosphor contains a small amount of ____ which introduces extra ___ ___ into the energy band structure. The impurity sites in the crystal are known as ____ ___. The luminescent centres have discrete energy gaps (Ef) different from the crystal energy gap (Eg).

v

An n-type semiconductor is formed by adding a few parts per million of impurity from a group ____ element (i.e. an element with 5 electrons in its outermost shell).

low temperatures, insulators

At ___ ___, and in the absence of other energy sources, semiconductors act as _____ because no electrons are excited to the conduction band.

thermal energy, charge carriers, Photons

At room temperatures and above, ____ ___ excites some electrons to the conduction band. The resistance of the crystal decreases as temperature increases due to the increase in ___ ___. _____ can also excite electrons to the conduction band, increasing conductivity.

electrons, empty sites, energy gap

Conduction cannot occur in insulators for 3 reasons: 1) There are no ____ in the conduction band 2) There are no ___ ___ in the valence bond 3) The ____ ___ b/w the valence band and the conduction band is too large.

increases

Conductivity _____ with temperature.

a (there are no valence bands in conductors)

Conductors can carry current because a) There are both electrons and empty sites in the conduction band b) There is a small gap between the valence band and the conduction band c) There are holes in the valence band d) all of the above

traps, average time delay, temperature, structure, impurity

Crystals exhibiting phosphorescence contain impurities that act as ____, as well as impurities that act as luminescent centres. An electron falling from the conduction band may be trapped by an impurity region in the crystal. Once trapped, electrons are released with an ___ ___ __ that can vary from a fraction of a second to hours or decades. The time delay depends on the crystal ____ , _____ and _____ forming the trapping centre.

bonds

Each silicon atom (except the ones at the surface) shares its four electrons with four other silicon atoms, forming ____.

atom, energy

Electrons exist at precise energy levels in the ____. In a crystal, however, no two electrons can have exactly the same ____.

bands, crystal

Energy ____ in a ____

levels, atom

Energy _____ in a Single ____

empty site, energy

For an electron to move: 1) There must be an ___ ___ for the electron to move to. 2) The electron must have enough _____ to occupy that empty site.

four, four, conduction, jumps

For example, the silicon atom shown in red has ____ valence electrons also shown in red, and the silicon atom shown in green has ____ valence electrons also shown in green. Energy is required to break a bond and release an electron for _____. When a bond is broken, the electron ____ from the valence band to the conduction band.

Optically Stimulated Luminescence, Photostimulable Phosphor Plates

Give two examples of Applications of Phosphorescence in Radiography:

Valence band

Identify A

conduction band

Identify A Mechanisms of Fluorescence

valence band

Identify B Mechanisms of Fluorescence

luminescent centre

Identify C Mechanisms of Fluorescence

conduction band

Identify the band that the arrow is pointing to

n type, p type

Impurities can form two types of doped semiconductor material:

negative

In "n" in n-type semiconductors stands for ______- as the major source of current is ______ electrons in the conduction band.

conduction band, electrons

In crystalline solids, conductors are produced when the upper occupied energy band (called the ___ ____) is only PARTIALLY filled with _____.

positive, negative, pn junction,

In forward bias configuration, the electrons on the n side are attracted to the ____ terminal, the holes on the p side are attracted to the ____ terminal, electrons and holes meet at the ___ ___ and recombine. There is a new hole and electron provided by EMF = current flows

valence band, empty

In insulators, the upper occupied band (called the ___ ___) is COMPLETELY FILLED with electrons, and the band above the valence band (called the conduction band) is completely ____.

postive, negative,

In reverse bias configuration, electrons on the n side move toward the ___ terminal (away from pn junction) and holes on the p side move toward the ____ terminal (away from pn junction), no recombination at the pn junction so no current.

electric field, hole, positive charge carrier

In semiconductors; when an electron jumps from the valence band to the conduction band, two things happen: 1) The electron in the conduction band can move within the conduction band in response to an ___ ___. 2) The electron that left the valence band leaves behind an empty state, called a "____", or a ___ ___ __ that can also move in response to an electric field.

move

In the valence band, there are no empty states for an electron to move to. Therefore, electrons cannot ____ in the valence band.

a

Is this a: a) Insulator b) conductor c) Semiconductor

b

Is this a: a) Insulator b) conductor c) Semiconductor

c

Is this a: a) Insulator b) conductor c) Semiconductor

easily excited

Making n-type semiconductors: For example, arsenic can be added to a silicon semiconductor: Four of the arsenic valence electrons form a double bond with a neighbouring silicon atom, but the fifth valence electron from arsenic does not form a double bond. Being in a higher energy state, it is ____ ___ to the conduction band.

primary source, majority charge, minor charge, minor source

Making n-type semiconductors: These extra electrons that are easily excited to the conduction band are the ____ ___ of current in an n-type semiconductor. Thus, we say that electrons in the conduction band are the ___ ___ carrier. There will also be some thermal excitation of electrons from the valence band, producing some holes in the valence band. Since the holes are free to move around in the valence band, we say that holes in the valence band are the ___ ___ carrier (meaning a ____ ___ of current).

double bonds, single, extra electron

Making p-type semiconductors: Because group III elements have only 3 electrons in their outermost shell, the gallium forms ___ ___ with 3 of the neighbouring silicon atoms, but forms a ____ bond with the fourth neighbouring silicon atom. The single electron bond between gallium and silicon can accept an ___ __. Electrons occupying this fourth double bond leave behind holes, or empty states in the valence band.

fluorescence

Materials exhibit ____ if they emit visible light when irradiated by ultraviolet or visible light.

visible light

Mechanisms of Fluorescence: Electrons raised to the conduction band will fall back down to the valence band. When they fall through the luminescent centre, ___ __ is emitted.

thermal vibrations, heat, trap, photon

Mechanisms of Phosphorescence: To escape a trap, an electron must absorb sufficient energy from ___ ___ (heat) to overcome the trap binding energy and rise back up to the conduction band. An electron released from a trap may then: 1) fall to the valence band directly producing ___ (most probable), or 2) fall back into a ___ (least probable), or 3) fall through a luminescent centre producing a ____ (probability of 5% to 20%).

x-rays, absorbed dose, green light, blue light

OSL's : Electrons are elevated to trapping sites by ____ or other high or low energy electromagnetic radiation or beta particles. The number of trapped electrons is proportional to the ___ __ of the person wearing the OSL. When the OSL is read, it is exposed to ___ __which released the trapped electrons. As the trapped electrons fall from the conduction band to valence band, ___ __ is emitted. The amount of blue light emitted is directly proportional to the radiation incident on the OSL before it was read.

rectifiers, radiation detectors

One of the main applications of diodes in radiography is as ______ in the x-ray generator, but diodes are also used as ___ ___, which will be discussed later.

irradiation, time delay

Phosphorescence is similar to fluorescence, except the emission of visible light continues after ____ has ceased. That is, phosphorescence involves a ___ ___, fluorescence does not.

trapping sites, incident xray intensity, laser, blue photon

Photostimulable Phosphor Plates: During an x-ray exposure, electrons are elevated to ___ __. The number of trapped electrons per unit area of the plate is proportional to ___ ___ ___. Therefore, trapped electron density forms a latent image. To develop the latent image, the plate is scanned, line by line, with a very fine ___ beam. A helium-neon red light photon has sufficient energy to lift the trapped electron out of the trap. As the trapped electron falls from conduction band to valence band, a ___ __ is emitted. The number of blue photons is proportional to the incident x-ray intensity on that small area of the image (a pixel).

IV, electrons, outer shell

Pure semiconductors are formed from group ___ elements of the periodic table - there are four _____ in the ___ ___. A two dimensional representation of the crystal structure of silicon is shown

electrons, empty states

Pure semiconductors generally do not conduct electricity very well at room temperature, because, like insulators, there are no _____ in the conduction band, and no ___ ___ in the valence band.

current, holes

Semiconductors can carry some ____ due to both electrons in the conduction band and ____ in the valence band.

energy band

Since there are many electrons in a crystal, the energy levels of the single atom change slightly to form an ___ __ of many closely spaced energy levels.

rectifiers, detectors, phosphors

Solid state technology has many applications in x-ray imaging such as:

positive

The "p" in p-type semiconductor stands for "____". The major source of current is (_____) holes in the valence band.

spacing, filling, room temperature

The _____ and _____ of these energy bands dictates whether a crystal will be a conductor, an insulator or a semiconductor at ____ ___.

smaller

The band structure of pure semiconductors is similar to insulators except the energy gap is ____.

impurities

The conductivity of pure semiconductor materials (silicon, germanium) is increased by adding small amounts of ____ during crystal growth.

conductor, insulator, semiconductor

The spacing and filling of these energy bands dictates whether a crystal will be a _____, an _____ or a _____ at room temperature.

diode

The symbol for a ____ is a triangular arrow pointing in the only direction current can flow:

move around

There are empty states to move to in the conduction band, and electrons have enough energy to fill those states, therefore they may ____ ___ in the conduction band.

toward, electric field, electron, current flows

Under forward bias, holes and electrons move____ the crystal junction in response to the ___ ___, and combine at the p-n junction. For every recombination at the junction, a new ____ is injected into the n-type at the right terminal and a new hole is injected at the left terminal (in the p-type). Thus, ___ __.

away, current flows

Under reverse bias, holes and electrons move ____ from the crystal junction in response to the electric field, and do not recombine. Thus, no ___ ___.

c

What is the best at conducting electricity? a) insulator b) semi insulator c) conductor

extra electron (that are easily excited to conduction band)

What is the main source of current (majority charge carrier) in a n type semiconductor?

holes (in the valence band)

What is the main source of current (majority charge carrier) in a p type semiconductor?

reverse bias

When the P side of the diode is connected to the negative terminal of the emf, and the n side is connected to the positive terminal, the diode is under ___ __

forward bias

When the P side of the diode is connected to the positive terminal of the emf, and the n side is connected to the negative terminal, the diode is under ___ __

reverse bias

When the p side of the diode is connected to the negative terminal of the emf, and the n side is connected to the positive terminal, the diode is under ____ ___. In this configuration, holes and electrons move away from the crystal junction in response to the electric field, and do not recombine. Thus, no current flows.

forward bias

When the p side of the diode is connected to the positive terminal of the emf, and the n side is connected to the negative terminal, the diode is under ____ ___

holes, majority charge, minority charge carriers

____ in the valence band are the ____ ____ carriers (main source of current) in p-type semiconductors. Some electrons will be thermally excited to the conduction band, and will also contribute to current, and are the ___ ___ __.

semiconductor

a material that conducts electricity better than an insulator, but not as well as a conductor.

Photostimulable Phosphor Plates

are constructed of Barium Fluoride crystals doped with Eu2+. Impurities added to the BaF crystal form trapping sites that hold electrons almost indefinitely at room temperature.

phosphors

are crystals that exhibit fluorescence or phosphorescence.

OSL

constructed of aluminum oxide crystals, and are used to measure absorbed dose.

solid state devices

electronic components constructed using crystals (usually silicon with impurities added).

p type semiconductors

formed by adding a few parts per million of a group III element such as gallium to a silicon semiconductor crystal.

n type semiconductos

is formed by adding a few parts per million of impurity from a group V element (i.e. an element with 5 electrons in its outermost shell).

conductors

materials with charge carriers (usually electrons) free to move in response to an electric field.

insulators

materials with no charge carriers that are free to move in response to an electric field.

conduction

occurs when electrons in the conduction band move from energy state to energy state in response to an electric field.