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Chapter 3 covers the special classification of Semiconductors. The first classification is Compound Semiconductors . Section 3.1 is devoted to Compound Semiconductors.

Chapter 3. Special Classification of Semiconductors.

Section 3.1. Compound - Semiconductors.

Compound Semiconductors are the basis of a whole new branch of Science and Technology known as Photonics. Light Sources and Light Detectors belong to this discipline. III-V elements give rise to Compound Semiconductors which are suitable for Light Generation or Light detection. These III-V elements form alloys across the whole range of concentration at their growth temperature. This wide miscibility range allows alloys to be grown with band structures adjusted for specific applications. This leads to Band structure manipulation according to our specific needs. This is known as Band-gap Engineering. The common Alloys used in Photonics are as given below:

  1. GaP(2.3eV, a = 5.42A°)____GaAs x P (1-x) ___________GaAs(1.42eV, 5.65A°): here x is the stoichometric coefficient and by adjusting ‘x’ , band-gap can be tailored from 1.42eV to 2eV.
  2. InP(1.3eV,5.85A°)__________InGaP_________________GaP(2.3eV, 5.42A°).
  3. GaAs(1.42eV, 5.65A°)____________GaAlAs__________________AlAs(2.2eV, 5.65A°).
  4. GaAs(1.42eV, 5.65A°)________GaAsSb__________GaSb(0.65eV, 6.1A°).
  5. GaAs(1.42eV, 5.65A°)_____GaInAs_________InAs(0.35eV, 6.05A°).
  6. InP(1.3eV,5.85A°)_______InPAs__________InAs(0.35eV,6.05A°).
  7. GaSb(0.7eV,6.1A°)______GaInSb_________InSb(0.15eV,6.5A°).

The three element alloys are TERNARY ALLOYS. Two from Group III and two from Group IV combine to form QUATERNARY ALLOYS.

Wide miscibility shown above has been translated into a topological diagram Figure 3.1 where Band-Gap versus Lattice Constant is plotted for the seven major Compound Semiconductors namely GaP, GaAs, GaSb, InAs, InSb , InP , AlAs, and AlSb and their derived Ternary Alloys.

As seen from Figure 3.1, none of the pure compounds listed have a direct band-gap more than 1.65μm for producing Visible Spectra Radiation. GaAs, InP, GaSb, InAs and InSb have direct band-gap but less than 1.65eV.

AlP, GaP, AlAs and AlSb all have Band-gap larger than 1.65eV but all are in-direct band-gap hence not suitable for optical generation. Hence for Optical LEDs we go for alloys of GaP and GaAs known as ternary alloy GaAs (1-x) P x . In Figure 3.2, the white light spectrum and the corresponding Band-gaps are shown.

Figure

Figure 3.1.Topoloigical diagram for Compound Semiconductors and their Ternary Compounds. The Solid lines indicate Direct Band Gap materials and dashed lines show In-direct Band Gap materials.

Figure 3.2. The plot of Band-Gap and the corresponding Wavelength versus Lattice Constant.

Figure 3.3. External Quantun Efficiency vs stoichiometric corefficient of Phosphorous in ternary alloy GaAs (1-x) P x .

For the manufacture of coloured LEDS we have to use GaAs (1-x) P x ternary alloy. This has a problem. Below x = 0.45 it is a direct band-gap material but at higher proportion of Phosphorous it becomes in-direct and its performance becomes very poor as shown in Figure 3.3. By doping with Nitrogen it can be restored to Direct Band-gap material and utilized for LED manufacturing. Table 3.1 tabulates the different ternary alloys used for manufacturing the spectrum coloured LEDs.

Questions & Answers

A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of 40° with an initial speed of 20 m/s, how close to the green does she come?
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can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?
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Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time
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Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing
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Source:  OpenStax, Electrical and electronic materials science. OpenStax CNX. May 01, 2014 Download for free at http://cnx.org/content/col11615/1.14
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