6.4 Applications (see also Callister, sections 21
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66 6.4 Applications (see also Callister, sections 21.11 - 21.13) In this part we will cover luminescence and photoconductivity and lasers (but leave out Callister section 21.14 on optical fibres). Luminescence This is the property of some materials (semiconductors) of absorbing incident energy and then re- emitting it as visible light. From previous discussion this requires for the emitted light 1.8 eV < hν < 3.1 eV The absorbed incident might be some other form of em radiation of similar or higher energy (e.g. uv radiation) or high-energy electrons or heat, etc. The emission process depends on the band-gap energy E g , as discussed, either through a direct or indirect process. Some more specific terms are:- Fluorescence: this is when the re-emission of energy (as visible light) occurs with a time delay but still takes place on a short time scale (<< 1 sec). Phosphorescence: this is when the time scale is of order 1 sec or more. Fluorescence and phosphorescence occur in only a few materials (e.g some sulphides, oxides, phosphors, etc) typically they are compounds and the controlled addition of impurities is important. Applications are to various kinds of optical coating (e.g. cathode-ray TV screens). Electroluminescence: this is when an electrical process (such as charge flow in a p-n semiconductor diode) can be used to generate visible light. Under conditions of forward bias in a p-n junction, the electrons and holes move towards each other in the recombination region. They annihilate to release energy: electron + hole → energy (ΔE) where ΔE is of order of the band gap energy E g . Therefore if E g is in the required range (1.8 to 3.1 eV) as in GaP or CdS, then visible light will be emitted. Such diodes that luminesce are the basis of light-emitting diodes (LEDs) used in digital displays. Photoconductivity The electrical conductivity of a doped semiconductor material depends on the number of charge carriers (n for electrons in the conduction band and p for holes in the valence band) and the mobilities: σ = n |e| μ e + p |e| μ h If incident light can be absorbed by the semiconductor, then this energy can be used to excite electrons from the valence band to the conduction band (so increasing n and p). This additional contribution to σ is known as photoconductivity.
Transcript of 6.4 Applications (see also Callister, sections 21
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6.4 Applications (see also Callister, sections 21.11 − 21.13)
In this part we will cover luminescence and photoconductivity and lasers
(but leave out Callister section 21.14 on optical fibres).
Luminescence
This is the property of some materials (semiconductors) of absorbing incident energy and then re-
emitting it as visible light.
From previous discussion this requires for the emitted light
1.8 eV < hν < 3.1 eV
The absorbed incident might be some other form of em radiation of similar or higher energy (e.g.
uv radiation) or high-energy electrons or heat, etc.
The emission process depends on the band-gap energy Eg, as discussed, either through a direct or
indirect process.
Some more specific terms are:-
Fluorescence: this is when the re-emission of energy (as visible light) occurs with a time delay
but still takes place on a short time scale (<< 1 sec).
Phosphorescence: this is when the time scale is of order 1 sec or more.
Fluorescence and phosphorescence occur in only a few materials (e.g some sulphides, oxides,
phosphors, etc) typically they are compounds and the controlled addition of impurities is
important. Applications are to various kinds of optical coating (e.g. cathode-ray TV screens).
Electroluminescence: this is when an electrical process (such as charge flow in a p-n
semiconductor diode) can be used to generate visible light. Under conditions of forward bias in a
p-n junction, the electrons and holes move towards each other in the recombination region. They
annihilate to release energy:
electron + hole → energy (∆E)
where ∆E is of order of the band gap energy Eg. Therefore if Eg is in the required range (1.8 to
3.1 eV) as in GaP or CdS, then visible light will be emitted. Such diodes that luminesce are the
basis of light-emitting diodes (LEDs) used in digital displays.
Photoconductivity
The electrical conductivity of a doped semiconductor material depends on the number of charge
carriers (n for electrons in the conduction band and p for holes in the valence band) and the
mobilities:
σ = n |e| µe + p |e| µh
If incident light can be absorbed by the semiconductor, then this energy can be used to excite
electrons from the valence band to the conduction band (so increasing n and p). This additional
contribution to σ is known as photoconductivity.
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Applications are as light meters (often using CdS), solar cells, etc.
Lasers
Two cases will be discussed:
Ruby laser
Semiconductor laser (e.g. GaAs)
(From Callister)
(From Callister)
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(From Callister)
(From Callister)
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(From Callister)
