0.12 Chapter 3. special classification of semiconductors.sec3.1  (Page 2/2)

Table 3.1.The Ternary Alloys (GaAs (1-x) P x ) used in the manufacture of the whole range of spectrum coloured LED. Here E g =1.424+1.15x+0.176x 2 .

3.1.1. The Crystal Structure of Compound Semiconductors and its Dopent.

GaP, GaAs, GaSb, InAs, InSb , InP , AlAs, and AlSb have the the same crystalline structure as Diamond but now it is called Zinc-Blende. It is two interpenetrating FCC sublattices with one sublattice made of Group III element and the other FCC sublattice made of Group V element and one sub-lattice is displaced with respect to the other along the diagonal of the cube by a quarter of the diagonal length i.e. by a√3/4. The net result is that every Group III has FOUR Group V atoms as neighbours and similarly Group V has for Group III atoms as neighbours. This completes the co-valent bond requirement.

Table 3.2 tabulates the dopents of III-V Compound semiconductors.

Table 3.2. P and N type dopents for III-V Compunds.

Silicon and Germanium can either be Donor or Acceptor depending upon what they substitute. If Group III is substitute then they become DONOR and if they substitute Group V then they become acceptor. But since Si is smaller in size hence energetically it is favourable to replace Ga hence Si is Donor in GaAs and bigger Ge substitutes the bigger As atoms hence Ge is acceptor.

3.1.2. Band-gap Engineering.

The manipulation of Band Structure required for different kinds of applications is Band-gap Engineering. There are three techniques of Band-gap Enginering:

1. Alloying;
2. Use of Heterojunctions and
3. Built-in strain via mismatched epitaxy.

The Aim of Band-gap Engineering is to tailor/customize the band-gap according to the wavelength at which we want to operate.

The second objective is to tailor the lattice constant according to our matching or our mis-matching requirements.

The wide miscibility range allows alloys to be grown with Band structures adjusted and finally tuned for specific applications.

3.1.3. Properties of Alloys.

In Alloys we have Lattice Parameter (a)Law called Vegard’s Law. If we have two solid mixture A _X B _(1-X) then the Alloy’s Lattice Parameter is given as follows:

Alloys are not perfect crystals even if they have perfect lattice structure. This because in solid mixture atoms donot have periodic placement.

By virtual crystal approximation:

We have quadratic approximation also:

Equation 3.3 is the same as the Equation given in Table 3.1.

Alloying induced dis-order causes a BOWING Parameter in compound semiconductor Wafer. Equations 3.2 and 3.3 are valid only if the alloy is a good mixture i.e. perfectly random mixing.

In an alloy A _X B _(1-X) a good mixing results into the fact that the probability that A is surrounded by B is (1-X) and B is surrounded by A is X. If proportion is different from the stichiometric corfficient then it is clustered or phase repeated.

Test of Eq (3.2):

AlAs has E _g = 2.75eV and GaAs has E _g = 1.43eV therefore in Al _0.3 Ga _0.7 As has

E _g = 0.3×2.75+ 0.7×1.43 = 1.826eV.