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Why does the Fermi energy level lie closer to the conduction band than the valence band?
Wiki User
∙ 15y ago
Fermi energy levels can be anywhere. Anywhere. But can an electron actually be in a given energy level? There are specific Fermi energy levels associated with each atom where electrons might "hang out" or orbit. Certainly each electron in the atom occupies a given Fermi energy level. There are other Fermi energy levels where the electrons will go if they are given energy to go there. And there are yet other Fermi energy levels where the electron simply cannot be made to go because of quantum mechanical principles. That's in a single atom. There are other Fermi levels that electrons might occupy associated with collections of atoms that did not exist with just a single atom. Said another way, collections of atoms that make up a material cause other Fermi levels that didn't exist before (in the case of a single atom) to become possible places for electrons to be in the collection of atoms that is the material itself. In materials, the valence band is "here" and the conduction band is "here" and they either overlap (in conductive materials) or they don't. In insulators, the conduction band is above the valence band of the atoms and other bands that might be possible because of the macroatomic structure of the material. If the two bands do not overlap, then there is a band gap. The band gap is a "forbidden region" for electrons. They cannot exist there because the quantum mechanical properties of the electrons and the atoms of the material won't sustain their presence in that group of Fermi energy levels that make up the band gap. The question asks why the Fermi energy level lies closer to the conduction band than the valence band. Hopefully the information provided illuminates the situation and shows that Fermi energy levels don't lie closer to the conduction band than the valence band because Fermi energy levels can be anywhere. There is also the question of whether an electron can actually be allowed to be in a given Fermi energy level. Lastly, it's also a question of whether or not the conduction band is "low enough" that it overlaps the valence band where the valence electrons are hanging out.
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∙ 15y ago
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What is the working principle of semiconductor laser?
The principle of semiconductor laser is very different from CO2 and Nd:YAG lasers. It is based on "Recombination Radiation" The semiconductor materials have valence band V and conduction band C, the energy level of conduction band is Eg (Eg>0) higher than that of valence band. To make things simple, we start our analysis supposing the temperature to be 0 K. It can be proved that the conclusions we draw under 0 K applies to normal temperatures. Under this assumption for nondegenerate semiconductor, initially the conduction band is completely empty and the valence band is completely filled. Now we excite some electrons from valence band to conduction band, after about 1 ps, electrons in the conduction band drop to the lowest unoccupied levels of this band, we name the upper boundary of the electron energy levels in the conduction band the quasi-Fermi level Efc. Meanwhile holes appear in the valence band and electrons near the top of the valence band drop to the lowest energy levels of the unoccupied valence energy levels, leave on the top of the valence band an empty part. We call the new upper boundary energy level of the valence band quasi-Fermi level Efv. When electrons in the conduction band run into the valence band, they will combine with the holes, in the same time they emit photons. This is the recombination radiation. Our task is to make this recombination radiation to lase
How does copper conduct electricity?
Copper conducts electricity by "musical electrons" like other conductors do. Let's look at copper and see what's up. Copper atoms in a copper wire all form some kind of metallic crystal structure. Not all the electrons in the valence shells of the copper atoms are "locked in place" in this structure. They are free to move around, and are said to be "free electrons" in this application. As they are not "bound" in the structure, the electrons can be made to move fairly easily. They can contribute to current flow. If we apply a voltage across the wire from end to end, electrons will enter one end of the wire and electrons will emerge from the other. Not the same electrons, mind you. Put some in one end, some come out the other. It could also be said that some of the electrons of the copper are at Fermi energy levels that are in what is said to be the "conduction band" for copper. The conduction band is the minimum energy level necessary for electrons of a given material to be in to support conduction in that material. If the Fermi energy levels of the valence band electrons is up in the conduction band, then that material is a conductor. Copper is this way.
How would the shape of an electron shell affect the ability of the material to conduct electricity?
Basically, the Electron shells in an insulator are complete, they are not prone to accepting external electrons or donating any of theirs. As such they aren't waystations for electrons looking to move (conduct). There is a need to slip away from what's going on with individual atoms when looking at conductivity (which can be used to sort out insulators from conductors). When a whole bunch of atoms or molecules are put together, a number of other opportunities or places for electrons to exist are created. The valence band of a given atom is subordinated and another type of "valence band" is set up. This new valence band (we are assigning a new definition) does not have a given energy level (like it would for a given atom) but, rather, has a range of allowable energy levels. This is because the many different atoms and molecules when combined to make up whatever it is we are making provide other places (energy levels) in which electrons can hang out. (Let's give Fermi, Schrödinger, Bloch and Brillouin the day off to keep from running off the page.) We have our newly defined valence band as a range of energy levels which an electron can occupy. (These were not available in a single atom of the material.) In a conductor, the band of energies in which an electron must be to support current flow actually are so low that they overlap part of the valence band. That means electrons in the material can support conduction and play musical electrons. In an insulator, there is a gap between the valence band (that group of energy levels allowed by the material as a whole) and the conduction band. Electrons cannot support conduction because they cannot reach the higher energy bands necessary to support it.
Why does zener diode have negative temp coeff?
Zener Diodes like all other diodes use a semiconductor P-N junction. The focus of this topic is actually more on the semiconductor characteristics than the diode it self. Semiconductors are not conductors and do not exhibit the same behavior than conductors. In this section free electron band and conductive band are used synonymously and Fermi level is a quantum mechanical term I can roughly define in semiconductor physics for a basic understanding as the kinetic energy of the electron in the highest quantum level. Semiconductors do not have electrons in the conduction band. All electrons are bound in covalent bonds and their valance bands are nearly filled under normal condition. It is required for an external energy to increase the Fermi level of the electrons in the valance energy band to jump the energy gap (forbidden band) into the conduction band. Then the semiconductor becomes conductive. Thermal energy is one type of energy that can raise the Fermi level of the fermions (particularly electrons in this case) in the valance electron band to jump the energy gap into the conduction band. This will increase the probability for an electron to find it self in the conduction band resulting in the semiconductor to become more conductive. In this case we will see that the contribution that thermal energy makes to increase conductivity by raising the Fermi level is much more than the contribution thermal energy makes to increase resistance, by making the atoms vibrate more, increasing collusions between electrons. This cause the semiconductor to form NTC (negative temperature coefficients), The higher the temperature the better it conducts within normal boundaries. With conductors, they already have electrons in conductive bands and by adding energy to it will not make that much difference in the amount of free electrons (conductivity). This cause a very small contribution to increasing conductivity. But the contribution to increase resistances are far greater due to the vibration of the atom causing more collisions between electrons. This give conductors a PTC (positive temperature coefficient) in normal conditions and cause the electrical resistance to increase when temperature increases. One can also take note that not only silicon Si and germanium Ge are semiconductors, but carbon only appear to be a conductor because it conducts electricity and often misunderstood on "school science" level. It is actually a semiconductor and also have NTC (negative temperature coefficients). Graphite can conduct electricity due to the vast electron delocalization within the carbon layers, a phenomenon called aromaticity. These valence electrons are free to move, so are able to conduct electricity. However, the electricity is only conducted within the plane of the layers.
Why don't plastics conduct electricity?
In general, plastics are composed of many chains of complex molecules. In a lot of cases, all the valence electrons of all the atoms of the material are in Fermi energy levels below the conduction band. That is, the energy required to move electrons in plastics is "high" because the energy levels that electrons would have to be in within the structure of the plastic are well above where the electrons are actually hanging out. The conduction band is a term we apply to the energy band that electrons have to be in to support current flow. Remember that current flow is like musical chairs in that everyone has to "move over one" all along the current path for current to flow. It's isn't about one electron going "into" a circuit at one end and that same electron coming out the other end. The "willingness" of electrons to "move over" to support current flow is conductivity, and electrons that are in "too low" an energy level (because they are being "kept at home" by the chemical structure of the material - the plastic) won't help with conduction. Just as a quick contrast, in a metal, there are lots of electrons in energy levels high enough to support conduction. These are the so-called "free electrons" you hear about. Plastics don't have them.
Why in intrinsic semiconductors do you need to apply very high potential to transfer electrons from the valence band to the conduction band?
Semiconductors, in the absence of applied electric fields, act a lot like insulators. In these materials, the conduction band and the valence band do not overlap. That's why they insulate. And that's why you have to apply some serious voltage to them to shove the valence electrons across the gap between the valence and conduction bands of these semiconductor materials. Remember that in insulators, there is a "band gap" between the lowest Fermi energy level necessary to support conduction and the highest Fermi energy level of the valence electrons. Same with the semi's. In metals, the conduction band overlaps the valence band Fermi energy levels. Zap! Conductivity.
Effect of temperature on fermi level?
The Fermi level starts to change location when temperature reaches 300K as a room temperature and Fermi level will getting close to conduction band or valence band depending on energy band gap determines.
What are the conditions for the fermi level to be exactly in the middle of the energy band gap of a semiconductor material?
For intrinsic semiconductors like silicon and germanium, the Fermi level is essentially halfway between the valence and conduction bands. You don't have to do anything; just keep the semiconductor intrinsic!
What is the working principle of semiconductor laser?
The principle of semiconductor laser is very different from CO2 and Nd:YAG lasers. It is based on "Recombination Radiation" The semiconductor materials have valence band V and conduction band C, the energy level of conduction band is Eg (Eg>0) higher than that of valence band. To make things simple, we start our analysis supposing the temperature to be 0 K. It can be proved that the conclusions we draw under 0 K applies to normal temperatures. Under this assumption for nondegenerate semiconductor, initially the conduction band is completely empty and the valence band is completely filled. Now we excite some electrons from valence band to conduction band, after about 1 ps, electrons in the conduction band drop to the lowest unoccupied levels of this band, we name the upper boundary of the electron energy levels in the conduction band the quasi-Fermi level Efc. Meanwhile holes appear in the valence band and electrons near the top of the valence band drop to the lowest energy levels of the unoccupied valence energy levels, leave on the top of the valence band an empty part. We call the new upper boundary energy level of the valence band quasi-Fermi level Efv. When electrons in the conduction band run into the valence band, they will combine with the holes, in the same time they emit photons. This is the recombination radiation. Our task is to make this recombination radiation to lase
How many valence electrons does insulator have?
It is not the number of valence electrons that an insulator has that is important. It is the way the valence electrons are "arranged" in the structure of the material that matters. If not all the valence electrons of a substance are "involved" in the structure of the material, then these electrons are said to be free electrons. They move about in the substance, and are free to contribute to electron flow. The metals are examples. In contrast with this, if all the electrons are bound up in a material, they are not free to support current flow, and the material is said to be an insulator. Said another way, if the valence electrons in a material are in a Fermi energy level that overlaps the conduction band for that material, the material is a conductor. In an insulator, the valence electrons are all in Fermi energy levels that are below the conduction band for that material, and it is an insulator. Applying a voltage to an insulator will not "lift" the valence electrons up into the conduction band to allow them to support current flow.
Why fermi level is found in the energy gap region since this region is forbidden for electrons and how does its probability is half?
The Fermi level is also known as the electron chemical potential (μ), and is a constant appearing in the Fermi-Dirac distribution formula: F() = 1 / [1 + exp((-μ)/kT)] Even though the gap may not contain any electronic states, there may be some thermally excited holes in the valence band and electrons in the conduction band, with the occupancy given by the Fermi-Dirac (FD) function. By inspecting the FD function, it becomes clear that if a state existed at the Fermi level, it would have an occupancy of 1/[1 + exp(0)] = 1/[1+1] = 1/2. Lastly, do not confuse Fermi level with Fermi energy. One is the chemical potential of electrons, the other is the energy of the highest occupied state in a filled fermionic system. In semiconductor physics, the Fermi energy would coincide with the valence band maximum.
Why fermi energy level is midway between conduction band and valence band in semiconductors?
To be exact EF should be at the valence band edge (EV) at 0K because no energy state above EV are occupied at 0K; however, for intrinsic semiconductors there are no states in the band gap anyway, so placing the EF anywhere in the band gap including conduction band edge does not add any states as being occupied. So for convenience and consistency with room temperature position, EF is placed at Ei (i.e. room temperature intrinsic Fermi level position).
Why fermi level lie close to conduction band in p-type semiconductors?
The Fermi level moves to wherever it needs to be to assure that the overall system is charge-neutral. In an n-type semiconductor, we introduce fixed positive charges (donors), which must be balanced by mobile negative charges (electrons). The excess electrons must reside in the conduction bands, because the valence bands are full. To have excess electrons in the conduction band, the Fermi level (electrochemical potential for electrons) must lie near the conduction band. A similar argument can be made for p-type doping
Why is plastic a bad conductor of electricity?
Generally, the valence electrons in the atoms of the molecules in plastic hang out in Fermi energy levels lower than the energy bands that an electron would have to occupy to support conduction. The electrons are bound to parent atoms, and they also may have some mobility within the molecular matrix of the material, but any energy applied (like voltage) doesn't really get the attention of these electrons. They chill in energy levels too low to be jerked up into the conduction band with any fair amount of applied voltage. And if the electrons are not in or cannot easily reach the conduction band (an energy level high enough to support current flow), the material won't support conduction and is said to be a bad conductor.
Where does the Fermi level lie in p-type and n-type semiconductors?
Fermi levels are filled with electrons and lies very close to the conduction band.
1 Why you define Fermi level for semiconductor junction diodes?
Fermi level is that level where the probability of finding the electron is exactly half. it lies between the conduction and the valence band.. its helps in formation of extrinsic substance... also in finding the good recombination agent for a different combination's it is also used in various calculations and determining probability of finding electron
How does copper conduct electricity?
Copper conducts electricity by "musical electrons" like other conductors do. Let's look at copper and see what's up. Copper atoms in a copper wire all form some kind of metallic crystal structure. Not all the electrons in the valence shells of the copper atoms are "locked in place" in this structure. They are free to move around, and are said to be "free electrons" in this application. As they are not "bound" in the structure, the electrons can be made to move fairly easily. They can contribute to current flow. If we apply a voltage across the wire from end to end, electrons will enter one end of the wire and electrons will emerge from the other. Not the same electrons, mind you. Put some in one end, some come out the other. It could also be said that some of the electrons of the copper are at Fermi energy levels that are in what is said to be the "conduction band" for copper. The conduction band is the minimum energy level necessary for electrons of a given material to be in to support conduction in that material. If the Fermi energy levels of the valence band electrons is up in the conduction band, then that material is a conductor. Copper is this way.
