| Topic: |
Science > Physics |
| User: |
"electronician" |
| Date: |
16 Aug 2004 04:33:35 PM |
| Object: |
Photodiode sensitivity question |
The sensitivity of a photodiode over wave length is a peak-like
function: It has a maximum at some value and falls off to zero
for smaller and larger wave lengths.
For larger wave lengths, the reason is clear since the energy
(h * frequency) becomes smaller than the semiconductor band gap.
But what is the reason for the sensitivity decrese observed at
higher frequencies (i.e. smaller wave lengths)?
--
Electronician
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| User: "Old Man" |
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| Title: Re: Photodiode sensitivity question |
16 Aug 2004 07:22:00 PM |
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"electronician" <this_is_not_my_email_adr@hotmail.com> wrote in message
news:cfr97v$23i$04$1@news.t-online.com...
The sensitivity of a photodiode over wave length is a peak-like
function: It has a maximum at some value and falls off to zero
for smaller and larger wave lengths.
For larger wave lengths, the reason is clear since the energy
(h * frequency) becomes smaller than the semiconductor band gap.
But what is the reason for the sensitivity decrese observed at
higher frequencies (i.e. smaller wave lengths)?
--
Electronician
The ionization cross section decreases with increasing
photon energy. For photon energy, E, and ionization
energy, I, with E > I, the cross section for the photo-
electric effect is approximately proportional to (I / E)^4.
See: "Quantum Electrodynamics" Landau & Lifshitz
ISBN 0 7506 3371 9
[Old Man]
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| User: "electronician" |
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| Title: Re: Photodiode sensitivity question |
17 Aug 2004 11:52:29 AM |
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Old Man wrote:
The ionization cross section decreases with increasing
photon energy. For photon energy, E, and ionization
energy, I, with E > I, the cross section for the photo-
electric effect is approximately proportional to (I / E)^4.
See: "Quantum Electrodynamics" Landau & Lifshitz
ISBN 0 7506 3371 9
Thanks, Old Man :)
"Solid state physics" (Ch. Kittel) did not answer that question.
I'll have a look into Landau & Lifschitz QED.
--
electronician
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| User: "" |
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| Title: Re: Photodiode sensitivity question |
17 Aug 2004 04:22:39 PM |
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electronician <this_is_not_my_email_adr@hotmail.com> wrote in message news:<cfr97v$23i$04$1@news.t-online.com>...
The sensitivity of a photodiode over wave length is a peak-like
function: It has a maximum at some value and falls off to zero
for smaller and larger wave lengths.
For larger wave lengths, the reason is clear since the energy
(h * frequency) becomes smaller than the semiconductor band gap.
But what is the reason for the sensitivity decrese observed at
higher frequencies (i.e. smaller wave lengths)?
A common theory is the decrease in interaction crossection.
And in other theories the chance of interaction is unimportent.
Ask yourself why would the chance in order to predict the
sensitivity be importent? The data exists but the theory to
predict all such diodes and other bodies, is desirable.
An unknown diode exists. And so to predict the loss of
sensitivity, a person is left with a prediction of altered quanta
available for interaction.
It just may be the lower site solution and not the lower
chance solution in theory.
And then ask why all such radiators have the same characteristic
of the spectra?
Implying that the lower chance theory is incorrect.
And leaving to the solid state the structure of emission. A set
of states exists.
An inverted transision is therfor the cause of the decrease.
Douglas Eagleson
Gaitherbsurg, MD USA
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| User: "John Popelish" |
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| Title: Re: Photodiode sensitivity question |
16 Aug 2004 06:06:21 PM |
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electronician wrote:
The sensitivity of a photodiode over wave length is a peak-like
function: It has a maximum at some value and falls off to zero
for smaller and larger wave lengths.
For larger wave lengths, the reason is clear since the energy
(h * frequency) becomes smaller than the semiconductor band gap.
But what is the reason for the sensitivity decrese observed at
higher frequencies (i.e. smaller wave lengths)?
--
Electronician
There is a general (proportional) reduction in current per watt of
illumination) as the wavelength shortens below the peak efficiency
wavelength, because each photon creates, at most, one electron-hole
pair, and if there is more energy then this process needs, the extra
is wasted. And as the wavelength gets shorter, the absorption gets
stronger, so the photons penetrate to shallower depths. At some
point, the light is not getting deep enough to produce the charge
pairs inside the PN junction.
--
John Popelish
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| User: "electronician" |
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| Title: Re: Photodiode sensitivity question |
17 Aug 2004 11:50:52 AM |
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John Popelish wrote:
electronician wrote:
But what is the reason for the sensitivity decrese observed at
higher frequencies (i.e. smaller wave lengths)?
--
Electronician
There is a general (proportional) reduction in current per watt of
illumination) as the wavelength shortens below the peak efficiency
wavelength, because each photon creates, at most, one electron-hole
pair, and if there is more energy then this process needs, the extra
is wasted. And as the wavelength gets shorter, the absorption gets
stronger,
Are you really sure here?
Isn't absorption decreasing with shorter wave length?
At least metals get transparent when the light frequency is above
the plasma frequency.
--
electronician
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| User: "John Popelish" |
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| Title: Re: Photodiode sensitivity question |
18 Aug 2004 08:04:58 PM |
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electronician <this_is_not_my_email_adr@hotmail.com> wrote in message news:<cftd1l$rip$05$1@news.t-online.com>...
John Popelish wrote:
electronician wrote:
But what is the reason for the sensitivity decrese observed at
higher frequencies (i.e. smaller wave lengths)?
--
Electronician
There is a general (proportional) reduction in current per watt of
illumination) as the wavelength shortens below the peak efficiency
wavelength, because each photon creates, at most, one electron-hole
pair, and if there is more energy then this process needs, the extra
is wasted. And as the wavelength gets shorter, the absorption gets
stronger,
Are you really sure here?
Isn't absorption decreasing with shorter wave length?
At least metals get transparent when the light frequency is above
the plasma frequency.
With metals (that have free electrons) I think the property changes
from predominately reflective to transmissive at energies get higher
than the plasma frequency. But semiconductors are a bit different.
For photons with lower energy than the band gap, the semiconductor has
no way to absorbe energy, so either it transmits or reflects. At
energy a bit higher than the band gap, it absorbs by generating charge
pairs (holes and electrons) or reflects. I don't know what happens at
energies several times the band gap.
Take a look at this reference on the optical properties of silicon,
especially figure 5.
http://www.virginiasemi.com/pdf/Optical%20Properties%20of%20Silicon71502.pdf
--
John Popelish
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| User: "electronician" |
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| Title: Re: Photodiode sensitivity question |
21 Aug 2004 06:44:26 PM |
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John Popelish wrote:
With metals (that have free electrons) I think the property changes
from predominately reflective to transmissive at energies get higher
than the plasma frequency. But semiconductors are a bit different.
For photons with lower energy than the band gap, the semiconductor has
no way to absorbe energy, so either it transmits or reflects. At
energy a bit higher than the band gap, it absorbs by generating charge
pairs (holes and electrons) or reflects. I don't know what happens at
energies several times the band gap.
Take a look at this reference on the optical properties of silicon,
especially figure 5.
http://www.virginiasemi.com/pdf/Optical%20Properties%20of%20Silicon71502.pdf
Definitely an interesting paper; thank you for the link.
I need to read it first but I am not sure how well the optical properties of
silicon reflect those of a PN-transition (such as in a photo diode).
--
electronician
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| User: "John Popelish" |
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| Title: Re: Photodiode sensitivity question |
18 Aug 2004 08:04:58 PM |
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electronician <this_is_not_my_email_adr@hotmail.com> wrote in message news:<cftd1l$rip$05$1@news.t-online.com>...
John Popelish wrote:
electronician wrote:
But what is the reason for the sensitivity decrese observed at
higher frequencies (i.e. smaller wave lengths)?
--
Electronician
There is a general (proportional) reduction in current per watt of
illumination) as the wavelength shortens below the peak efficiency
wavelength, because each photon creates, at most, one electron-hole
pair, and if there is more energy then this process needs, the extra
is wasted. And as the wavelength gets shorter, the absorption gets
stronger,
Are you really sure here?
Isn't absorption decreasing with shorter wave length?
At least metals get transparent when the light frequency is above
the plasma frequency.
With metals (that have free electrons) I think the property changes
from predominately reflective to transmissive at energies get higher
than the plasma frequency. But semiconductors are a bit different.
For photons with lower energy than the band gap, the semiconductor has
no way to absorbe energy, so either it transmits or reflects. At
energy a bit higher than the band gap, it absorbs by generating charge
pairs (holes and electrons) or reflects. I don't know what happens at
energies several times the band gap.
Take a look at this reference on the optical properties of silicon,
especially figure 5.
http://www.virginiasemi.com/pdf/Optical%20Properties%20of%20Silicon71502.pdf
--
John Popelish
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