What is the difference between conduction band and valance band?
Valence band is the highest band or energy level in a substance which is partially or fully filled. In case of inert gases, the valence band is full whereas for other materials, it is partially filled. Normally in an atom, valence band has the electrons of highest energy.Conduction band is the lowest energy band in a substance which is either empty or partially filled by some of the valence electrons at room temperature. In certain materials, such as metals, the valence electrons are loosely attached to their nuclei.Even at ordinary temperature, some of them can jump to conduction band. In fact, this is the process responsible for the conduction of current in a conductor. Moreover, if a substance has empty conduction band , it might be an insulator. On the other hand, conduction band is partially filled for conductors.
When graphite forms covelent bond and all of the carbon's valence electrons are bonded why is it a good conductor of electricity?
If you have studied about structure of graphite than u should have known thatCarbon has four spare/ valenceelectrons. In graphite structure, two of them make two single (sigma) bonds while the other pair form a double bond (consists of one sigma and one pi bond). Electron that forms pi bond is freer to travel between atoms compared to the others and is theoretically associated with all adjacent atoms (imagine the 3d structure of graphite) rather than a single atom. Because of that, it is called delocalized electron. In other words, electrons that form pi bonds continously replace each other causing temporary, local crystallizations within the structure.According to Wikipedia "It is increasingly appreciated that electrons in sigma bonding levels are also delocalized". But it is different topic.Electric current is basically the flow of charged particles (ions and electrons, but electric currents in everyday life can simply be defined as flow of electrons). If graphite is exposed to an electric current in a circuit, "the pi electrons" which are already traveling between carbon atoms that forms the graphite structure begin to flow through the circuit and leave the structure from one point of connection between graphite and the circuit. At the same time, other electrons come via the other connection point to replace them as new delocalized electrons. So, the new delocalized electrons simply come from the current.And in this way graphite conducts electricity because it has movable delacalised pi electrons where as there is no such provision in case of any other allotrope of carbon because they basically form structure having four sigma bonds using all 4 spare or valence electrons so there is no movable electrons available so no conductivity of current.For example: Diamond etc.I hope it can be helpful for you..,
Why do conduction band and valence band overlap in conductors but not in semi-conductors?
It all goes back to atomic bonding. Solids form because their total energy is lower than the energy of the sum total of atoms of which they are made.The way metal atoms lower energy on solidification is different from the way semiconductor atoms lower energy when they come together in a solid.Semiconductors form perfect bonds with 4 nearest neighbor atoms. Energy is lowered by sharing one electron with each adjacent atom. There are just the right number of these valence electrons so that all of them are used in bonding. That means the solid has no spare electrons for electronic conduction at absolute zero.At slightly elevated temperature some of the shared electrons in bonds can gain enough thermal energy, about 1 eV, to free themselves from their bond. This energy is the band gap. The electron must gain all of this energy, it cannot gain some fraction of this energy because its energy in the bond is a fixed quantity. So it’s either bound or it’s free.Once the electron is free, it moves about in the lattice until it finds a missing bond to drop back into. It has no way to lose energy in this condition except by dropping into a slot. The free electron is in the conduction band while the empty space it left is a hole in the valence band. The hole can also hop to an adjacent atom with much lower energy input than it takes to break a bond, meV instead of eV. In reality, an electron from an adjacent atom hops into the hole and now it has left a hole behind.This process of generation and recombination produces a steady supply of electrons in the conduction band and holes in the valence band.Metals don’t solidify like this. The metal atoms come together closer and closer lowering the overall energy of the solid. The valence electrons of the metal are free to move about among the atoms because they are not involved in nearest neighbor bonding. Instead, the electron cloud plus the lattice of positive ions interact in such a way that the overall energy of the system is lowered. There is no mechanism for a band gap and thus we say that the bands overlap. Electrons can gain any amount of energy from meV to eV.Metal atoms pack as tightly as they can into close packed structures while semiconductors form into sparse crystals with bonds between adjacent atoms of a very specific length and very specific orientation.It’s all about the bonding!
How do electrons move inside the conductor when there is a potential difference applied across the conductor? Does every electron collide with atoms and leave them to collide with other atoms and so on or what happens?
To understand this we have to go through metal and its bonding inside.Electron Sea ModelMetals make up most of the elements in the periodic table (around 80%), and they are special. When metals bond with themselves, they bond in a different way than when they bond with other elements. It isn't ionic or molecular or covalent. It is its own metal bond.When metals are together, the electrons float around the atoms.Metallic Bonds Electrons Float AroundMost metals have very few electrons in their outermost energy shells, and some have vacant outer electron orbitals. What this means for the metal is that its valence electrons are decentralized and free to move around. Remember that in ionic bonds, the electrons transfer from one atom to another atom. In covalent bonds, the electrons are shared between atoms. In metal bonds, the electrons wander around and aren't transferred or shared. It's more of a communal thing where they belong to all the metal atoms around them.Metals form compact and orderly crystalline structures. They look like this. When metals are next to each other, the valence electrons don't just stay on their own atom; they roam around the whole metal complex. They float free as though floating through a sea of electrons, much like an individual water molecule floats free in the sea. This is why it is called the electron sea model.Each metal atom allows its electrons to roam freely, so these atoms become positively charged cations. These cations are kind of like a positively charged island and are surrounded by a sea of negatively charged electrons. It looks a bit like this. The attraction between the mobile electrons and positive centers is a metal bond.Properties of MetalsThe electron sea model explains many of the physical properties of metals. They are good electrical conductors because the electrons flow freely in them. They are malleable because of the drifting electrons and because the cations slide easily past each other. They reflect light because of the free electrons.
What is covalent bonds?
A covalent bond is one in which the two atoms involved share the bonding electrons relatively equally, thus there is little or no dipole moment. This is contrasted with an ionic bond, where on atom is significantly more electronegative, thus the bonding electrons spend a significantly greater time in the orbitals of that atom, resulting in a significant dipole moment.
Why does graphite conduct electricity whereas diamond does not?
A material’s ability to conduct electricity is determined by the number and mobility of its free electrons.In diamond, each carbon atom uses all four of its valence electrons to bond with neighboring atoms. That doesn’t leave any electrons free for conducting electricity.But in graphite, each carbon atom is only bonded to three other atoms, leaving one valence electron free to carry an electric current.
Who invented semiconductor?
Karl Braun, in Germany, 1874, discovered the galena chrstal rectified alternating current. This was the basis of the cat's whisker detector in crystal radio sets in the early 1900s. Perhaps this was the first semiconductor used for electrical purposes. On the other hand, carborundum is another 19th century semiconductor device but I don't know who invented that use or when. Selenium rectifiers were in common use in the 1950s Noyce is credited with a development of the integrated circuit in 1961---Kilby filed the patent for the first integrated circuit in 1959