Optical absorption in Semiconductors

In summary, the chopped light source is used to measure small changes in light level due to absorption in the semiconductor. This is done by converting a dc light signal into an ac (pulsed) signal and extracting the relevant ac signal with a locked-in amplifier. Any changes in brightness of the source are due to interactions between the light and the semiconductor.
  • #1
Defennder
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[SOLVED] Optical absorption in Semiconductors

Homework Statement


I'm doing a lab report on the optical absorption of semiconductors. More specifically it was to determine the bandgap values of the semiconductor samples. This is the experimental setup.

http://img518.imageshack.us/img518/5184/apparatusgt4.png


One of the questions which was asked was why a chopped light source was used instead of a constant incident light source. I can't quite figure out why.



The Attempt at a Solution



I did some searching on the internet and found that chopping the light source allows one to convert a dc light signal into an ac (pulsed) signal, and the relevant ac signal can be extracted by using a locked-in amplifier. But I still don't understand why a dc light source can't be used instead.
 
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  • #2
You are trying to measure small changes in a light level due to absorption in the sample.
But you are also measuring changes in the brightness of the source and the response of the detector, a chopper let's you take these out.

By using a chopper you are taking the ratio of the signal with the light present and the light blocked a very short time before, any variations in the system slower than the chopper frequency are automatically removed.
You normally pick a chopper frequency at least 10-20x faster than any inteference you suspect - such as 60Hz line noise.
 
  • #3
Is the change in brightness of the light source caused by optical absorption of the semiconductor when the frequency of the source approaches the bandgap value of the semiconductor?
 
  • #4
Defennnder said:
Is the change in brightness of the light source caused by optical absorption of the semiconductor when the frequency of the source approaches the bandgap value of the semiconductor?
the change in brightness of the detected light is caused by interactions between the light and the semiconductor.
Changes in brightness of the source you try and avoid - there will be some just because lamps aren't perfect.
 
  • #5
I see, thanks a lot for your help.
 

What is optical absorption in semiconductors?

Optical absorption in semiconductors refers to the process by which light energy is absorbed by a semiconductor material, resulting in the promotion of electrons from the valence band to the conduction band. This absorption of light can lead to changes in the electrical and optical properties of the semiconductor.

What factors affect the optical absorption in semiconductors?

The optical absorption in semiconductors is affected by several factors, including the bandgap energy of the material, the type and concentration of impurities or defects, and the wavelength and intensity of the incident light. Additionally, the temperature and crystal structure of the semiconductor can also influence its absorption properties.

How is the optical absorption spectrum of a semiconductor measured?

The optical absorption spectrum of a semiconductor can be measured using various spectroscopic techniques, such as UV-Vis absorption spectroscopy, photoluminescence spectroscopy, and infrared absorption spectroscopy. These methods involve shining light of different wavelengths onto the semiconductor and measuring the amount of light absorbed at each wavelength.

What is the significance of optical absorption in semiconductors?

Optical absorption is a crucial phenomenon in semiconductors as it plays a vital role in various optoelectronic devices, such as solar cells, LEDs, and photodetectors. By controlling the absorption properties of a semiconductor, we can tune its light absorption and emission, making it useful for various applications in the fields of electronics and photonics.

What are some practical applications of optical absorption in semiconductors?

Optical absorption in semiconductors has numerous practical applications, including solar energy harvesting, optical communications, and sensing. It is also used in the production of various electronic devices, such as transistors, diodes, and lasers. Additionally, it is essential in scientific research for studying the electronic and optical properties of semiconductors.

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