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James Clerk Maxwell is often called one of the world's greatest physicists. He was also a major influence on other important scientists, like Albert Einstein. Maxwell's theories were essential in the development of technology we now take for granted, such as: radio, television ,mobile phones.
Group IV semiconductors (Si,Ge)
The band structure of these semiconductors is very similar because:
- They do crystallize in the same crystallographic structure (diamond)
- They have similar electronic outer orbitals
The structure of silicon is purely covalent. The last orbital of atomic silicon has the electronic configuration 3s2p2. There are therefore 4 electrons (2s et 2p) sharing an orbital that could contain 8 (2 for the s orbital, 6 for the p orbital). Silicon has therefore 4 valence bands. The band structure of silicon and germanium, two most important semiconductors formed using the column IV of the periodic table, is shown in Fig. 2.4.
Figure 2.4: Germanium (left), Silicon (center) and Gallium Arsenide (right) band structures.
The valence band maximum is at k = 0 and is degenerate with the heavy and light hole bands. A third important valence band is the “spin-split” called this way because it is splitby the spin-orbit interaction. Finally, most important in the band structure of Silicon and Germanium is the fact that the minimum of the conduction band does not coincide with the maximum of the valence band. The semiconductor is called “indirect”.
The conduction band minimum in silicon is in the direction  and, as a result, also in the directions , , , ,  for a total of six minima.
Figure 2.5: Minima of the conduction band of Si, Ge and GaAs
In Germanium, in contrast, the conduction band minimum is in the directions corresponding to the cube’s diagonal, and we have therefore 8 conduction band minima.
III-V Semiconductors (GaAs, InP, ..)
The band structure of III-V semiconductors is similar since the tetrahedral bonds have the same structure as the ones in Silicon or Germanium. In fact, the missing electron of the group III with the electron configuration 4s24p (for example Gallium) is provided by the column V element (for example Arsenic) of configuration 4s24p3 and these bonds have a low ionicity.
For a large number of III-V semiconductors, the bandgap is direct.
Figure 2.6: Structure de bande du GaAs
As an example, the computed band structure of GaAs is shown in Fig. 2.6.
The great German physicist, Heinrich Hertz made possible the development of radio, television, and radar by proving that electricity can be transmitted in electromagnetic waves. He explained and expanded the electromagnetic theory of light that had been put forth by Maxwell. He was the first person who successfully demonstrated the presence of electromagnetic waves, by building an apparatus that produced and detected the VHF/UHF radio waves.
The Hamiltonian of a semiconductor crystal has the translation symmetry
with being a reciprocal lattice vector.
The Bloch theorem states that the wavefunctions have two “good” quantum numbers, the band index n and a reciprocal vector such that the wavefunctions of the crystal may be written as:
where exhibits the periodicity of the crystal.
It may also be written as:
Proof of Bloch’s theorem
If R belongs to the Bravais lattice, the operator will commute with the Hamiltionian
These two operators form a common set of commuting observable, we can therefore write the wavefunctions using eigenfunctions of both:
Consid´erons maintenant les propri´et´es de c(R)
To simplify somewhat the notation, ai with i = 1, 2, 3 the basis vectors of the primitive Bravais lattice, and bi the basis vector of the reciprocal lattice. We can always define (because c(ai) is always normalized to unity:
If now R is a translation of the Bravais lattice, it can be written as
Using the orthogonality relations between the direct and reciprocal basis vectors, written as
Considering the product between the vector R of the real space and the vector k of the
reciprocal space, written in the basis of the bi:
It was in fact the result we had obtained for c(R), that was written as:
setting naturally xi = ki. The wavefunctions may well be written as:
that is one of the equivalent formulations of the Bloch theorem.
Electromagnetic interference (EMI) is an electromagnetic emission that causes a disturbance in another piece of electrical equipment. EMI can be attributed to a wide span of the electromagnetic spectrum including radio, DC and even microwave frequencies. Because anything that carries rapidly changing electrical currents gives off electromagnetic emissions, it is quite common for one object’s emissions to “interfere” with another’s.
Modern electrical motors are available in many different forms, such as single phase motors, three-phase motors, brake motors, synchronous motors, asynchronous motors, special customised motors, two speed motors, three speed motors, and so on, all with their own performance and characteristics.