CD and DVD

The CD player

The GaAs semiconductor gap is 1.43 eV, while the AlGaAs gap is bigger of 2.06 eV. Please find
a) The characteristic wavelength of photon absorbed by the GaAs semiconductor.
b) The characteristic wavelength of photon absorbed by the AlGaAs semiconductor.
c) The CDs and DVDs devices use laser diodes to read data from CD and DVD disks. A standard CD can hold 650 MB while a standard DVD can hold up to 8 GB,  respectively. Which semiconductor would you use in making the DVD laser reader? Explain.
d) The blue ray disk has a capacity of 25 GB. What is blue in the blue-ray device?

CD and DVD

Answer

The band gap energy is equal to the minimum energy of photon absorbed by the semiconductor.
$h*F \geq  Eg$
$h*c/\lambda \geq Eg$
$\lambda \leq h*c/Eg$

a)For GaAs the wavelength is

$\lambda1 = 6.62*10^{-34}*3*10^8/1.43/1.6*10^{-19} =8.6800*10^{-7} m =868 nm$
b) For AlGaAs the wavelength is

$\lambda2 =6.62*10^{-34}*3*10^8/2.06/1.6*10^{-19} =6.0255*10^{-7} m=602.55 nm$
c) Since both the CD and the DVD have the same area and there is more data on the DVD it means that the track width on the DVD will be smaller. Again, there will be more tracks on the DVD for the same diameter, which means the width of the DVD track is smaller. To read a smaller dimension track one will need light having smaller wavelength. The characteristic wavelength of the laser is given by the bandgap of the semiconductor. The wider the bandgap, the smaller is the wavelength of the light emitted. The aperture $d$ of an optical intrument is given by $d=1.22\lambda$ for good resolution.

Hence GaAlAs will need to be used for making the DVD laser.
d) The color of the laser reading the disc is blue. Thus it has a very short wavelength necessary to ready very narrow tracks.

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