Si Valence Electrons

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Valence electrons periodic table

Valence-Band/Outer-Shell Electrons
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Jan 08, 2020 In the basic unit of crystalline silicon solid, a silicon atom shares each of its four valence electrons with each of four neighboring atoms. The solid silicon crystal is composed of a regular series of units of five silicon atoms. This regular and fixed arrangement of silicon atoms is known as the 'crystal lattice.' Valence electrons are the electrons in the outer energy level of an atom that can participate in interactions with other atoms. Valence electrons are generally the electrons that are farthest from the. The normal valance number of silicon is 4, so with Si 2- one knows that silicon now has 6 electrons in it's valence shell. Valence electrons are generally the electrons located in the outermost shell of an atom and can be gained or lost in a reaction. Valence electrons can be determined by looking at the periodic table; because titanium is four columns from the left, it has four valence electrons.

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Based on band theory, when a large number of atoms combine to form a large aggregate, molecular orbitals merge into an almost continuous band of energy levels. The bottom half (called valence band) of the band is composed of bonding M.O.s (molecular orbitals) and is filled with electrons. The upper half (called conduction band) of band is composed of antibonding M.O.s and is empty.

In general, those electrons in different orbitals form their own energy levels of valence and conduction bands as shown in Figure 2633a.

Figure 2633a. Schematic illustration of evolution of the atomic s and p orbitals into valence and conduction bands in some materials.

For instance, the schematic illustration in Figure 2633b shows the bonding levels for diamond and indicates the formation of valence band. When the hybrid carbon atoms bond, a second electron is contributed to the state by the other atom, and thus the interaction between the two electrons lowers the energy of the state. Therefore, the sp3 energy level splits into a series of four bonding and four antibonding orbitals. In an actual solid, there is a Coulomb interaction between the atom cores and the electrons, and then the bonding and anti-bonding energy levels split to form a continuous band structure, with the band of bonding states being the valence band and the anti-bonding states being the conduction band.

Figure 2633b. Schematic illustration showing the progression of the electronic structure for an sp3 bonded system.

The valence band electrons normally originate from the electrons in the incomplete outer shell of atoms, for instance, the valence band is formed for silicon (Si) crystals as shown in Figure 2633c. An isolated Si atom contains 14 electrons, which occupy 1s, 2s, 2p, 3s and 3p orbital in pairs. When atoms are far away from each other, the electrons in the out shell do not interact. As the distance between atoms is reduced to d1, there is an overlap of electron wavefunctions across adjacent atoms. This leads to a splitting of the energy levels consistent with Pauli exclusion principle, and forming energy bands. This splitting leads to 2N states in the 3s band and 6N states in the 3p band, where N is the number of Si atoms in the crystal. A further reduction of the lattice spacing causes the 3s and 3p energy bands to merge into a single band having 8N available states, and then split again into two bands containing 4N states each. At the temperature of zero Kelvin, the lower band is completely filled with electrons and named as the valence band. The upper band is empty and named as the conduction band. Note that in crystal Si (with lattice spacing d0), the core level electrons do not start yet to interact.

Figure 2633c. Detailed illustration of electronic structure of silicon as a function of distance between atoms. The left red circle is a zoom-in of the right red circle.

Those valence-band electrons are the only electrons:
i) Will combine with the electrons of other atoms to form compounds.
ii) Cause electric current to flow. For the same reason, the outer electrons of the atoms of a conductor are also called free electrons.

Figure 2633d shows the schematic illustration of the energy level diagrams for molecular structures with different number of atoms which are single atoms, dimers, clusters and bulk materials. Splitting of the atomic energy levels occurs when the single atoms form a diatomic molecule. As more atoms join the system, the levels split further until a quasi-continuous band structure is formed in the bulk material. In other words, quantum size effects occur when the quasi-continuous band structure of a solid state system begins to break down as more atoms are included.

Figure 2633d. Schematic illustration of the energy level diagrams for molecular structures with different number of atoms which are single atoms, dimers, clusters and bulk materials.

The gap between the top of valence band and the bottom of conduction band is defined as energy gap Eg. In insulators, the energy gap is very large and no vacant conduction band is available to the valence electrons. Otherwise, the energy gap is either zero due overlapping between valence and conduction bands for metals or is very small for semi-conductors so that conduction band is easily available to valence electrons.

For instance, the density (n) of valence electrons in Si3N4 is given by,
------------------------- [2633]
where,
ρ -- The Si3N4 atomic density,
NA -- The Avogadro number,
ASi and AN -- The atomic weights of silicon and nitrogen, respectively,
nSi and nN -- The numbers of valence electrons per silicon and per nitrogen atom taking part in the plasmon oscillation, respectively.

The sp2 bonded solids have both σ/σ* and π/π* states available to the electrons, while for sp3 bonded solids only the σ/ σ* states present. For the diamond and graphite, electron transitions to these states generate many of the characteristic features in EEL spectra. For instance, in core loss spectra from diamond, the excitation of the 1s electrons to the σ* states generates the carbon k edge peak. For graphite, the transitions of the inner shell electrons into unoccupied π* states give a peak prior to the edge onset. In the valence band, valence electron transitions into the π* states also produce a peak at around 6 eV.

Valence

Both silicon and germanium can be used as the intrinsic semiconductor when fabricating solid-state devices. In the Periodic Table of the Elements, germanium (atomic number 32) occupies the position directly below silicon (atomic number 14).

The Periodic Table of the Elements had been envisioned earlier, but its potential was more fully realized in the work of Dmitri Mendeleev, an unusual individual to put it mildly. Born in a remote village in Siberia, he became renowned for his discoveries and writings including the comprehensive two-volume Principles of Chemistry (1868–1870).

He was a hard worker, highly educated, with many accomplishments in chemical engineering and related fields. Like many of his time and place, he was an unstable genius with a turbulent inner life. Fourteen years after marrying Feozva Nikitichna Lechcheria in 1862, he became obsessed over Anna Ivanova Popova and threatened suicide if she did not marry him. The stratagem succeeded but the event cast a long shadow across his life and was probably the reason he did not receive the Nobel Prize for his innovative Periodic Table.

The Periodic Table’s horizontal rows are known as periods and its vertical columns are called groups. The square cells each have the symbol (e.g. Fe for iron) signifying an element along with its atomic number, which equals the number of protons in its nucleus.

The period number is the highest energy level of an unexcited electron. By examining the position of an element within the Table one can ascertain the electron configuration including the number of shells and number of electrons in each shell, especially the number of electrons in the valence or outer shell. Various editions of the Table contain additional information such as atomic weight.

Cells are color coded, indicating the type of element, i.e. alkali metals, alkaline earth metals, transition metals, post-transition metals, metalloid, lanthanides, actinides, nonmetals, halogens and noble gases. Elements located in the same column (group) have identical valence populations. They are therefore chemically similar as this determines the ways in which they react with other elements.

Since they are in the same column, we know that silicon and germanium have the same number of electrons in their outer or valence shell. Germanium atoms have one more shell than silicon atoms, but what makes for the interesting semiconductor properties is the fact that both have four electrons in the valence shell.

As a consequence, both materials readily constitute themselves as crystal lattices. Substituted atoms alter the electrical properties. The process of adding these atoms is known as doping. Doping may take place by passing a gas over the crystalline material, sometimes for several hours. If the dopant material is composed of atoms with five valence electrons, there will be extra free electrons and an n-type semiconductor is produced. If the dopant material is composed of atoms with three valence electrons, there is a deficiency of free electrons and a p-type semiconductor is produced.

Rather than saying there is a deficiency of free electrons, we can say the semiconductor has a surplus of holes. A hole is the absence of an electron. This may be simply a matter of semantics, but that is the customary terminology.

Si Valence Electrons Number

Dopants that have five valence electrons and make n-type semiconductors are antinomy, arsenic and phosphorous. Dopants that have three valence electrons and make p-type semiconductors are boron, aluminum and gallium. The same dopants are used for both silicon and germanium semiconductors.

Silicon is the principal component of common sand, and for this reason it is less expensive than other intrinsic semiconductor materials. But in such small quantities, raw material cost is not always decisive. Historically germanium was used as a semiconductor before silicon. The cat’s whisker RF detector could be found in early crystal sets. But in general, silicon is easier to process than germanium, able to handle higher power levels, has less reverse bias leakage and is more stable at higher temperatures.

Silicon and germanium can also be formed into an alloy of silicon-germanium with a molecular formula of the form Si1−xGex. Silicon-germanium serves as a semiconductor in integrated circuits for heterojunction bipolar transistors or as a strain-inducing layer for CMOS transistors.

Here heterojunction refers to the interface between two layers or regions of dissimilar crystalline semiconductors. The two semiconducting materials have unequal band gaps. (If their band gaps were equal, the interface would be a homojunction.)

SiGe lets CMOS logic integrate with heterojunction bipolar transistors. Heterojunction bipolar transistors have higher forward gain and lower reverse gain than typical homojunction bipolar transistors to help realize better low-current and high-frequency performance. Because it is a heterojunction technology with an adjustable band gap, SiGe enables more band-gap tuning than silicon-only technology.

List Of Valence Electrons For Each Element

SiGe-on-insulator (SGOI) is analogous to the Silicon-On-Insulator technology employed in computer chips. SGOI boosts the speed of transistors by straining the crystal lattice under the MOS transistor gate, improving electron mobility and raising drive currents. SiGe MOSFETs can also provide lower junction leakage because of the lower band-gap value of SiGe.