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Short Answer:
Electrons are waves and waves in a confined geometry can exist only with specific allowed wavelengths. This is true for sound waves in Musical Instruments, water waves in a ripple tank, light waves and electron waves in solids. Bad gaps and allowed wave regimes exist for all waves and are the consequence of simple reflection, refraction and interference.
Long Answer:
Electrons in crystals (not all solids, but only regular periodic arrays of atoms) have energies that restricted to certain allowed values or ranges of values called "bands" or "energy bands" and not other values called "gaps" or "band gaps" for a simple reason. Electrons are waves and electron dynamics is described by a wave equation, specifically the Schroeding equation. Quantum waves must respect the same dynamics as all waves and that includes the processes of reflection, refraction, and interference. It is these processes that create the bands and band gaps.
If you are asked to explain why a musical instrument gives out specific notes and not all possible sounds, you explain it as a condition of wave dynamics. A guitar string, for instance, vibrates with waves that can exist with whole wavelengths that fit within the length of the string. That is completely analogous to the phenomena that forces the existence of bands and band gaps for electrons in solids.
In fact, if you scale things up to wavelengths appropriate to sound (Sound wave are about a trillion time larger than electron waves.) you will get bands of allowed sound and band gaps where sound is not allowed. You can create a volume of space with a regular array of objects that reflect and refract sound and in that space only certain bands of wavelengths of sound will be allowed. The phenomena is the same. The equations are the same. The cause is the same.
Though quantum mechanics is strange and complicated, the reason energy bands in crystals exist is simple and completely faithful to the analogy of notes from a musical instrument.
Caveat: Though the existence of bands follows from this simple explanation, the prediction of the behavior of electrons in particular solids requires a sophisticated description of the particular atomic and electronic interactions. One can spend a career as a physicist making such descriptions and explaining the properties of the various kinds of solids.
More Answers:
Electrons may only possess specific energies. Another way of saying this is that they can only occupy certain energy levels. A band gap is a continuous spectrum of energies where no such energy levels exist. The band gap is simply a range of energies that the electron cannot have.
If you want to know why there are energies the electron can have and why there is a gap in between these energy ranges then that's quite a bit more complicated. A short answer would be that there are gaps because there are asymmetries in the basic repeating unit of the crystal structure but I appreciate that that probably prompts a bunch of other questions.
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Why effective mass is infinite at the middle of the band gap?
It sounds like a math trick because the electron cannot exist in the middle of the band gap. The trick would be that it would take an infinite amount of energy to excite the electron to the middle of the band gap.
What is the forbidden energy gap?
it is the energy required to transfer an electron from valence band to conduction band, the external energy which is required is equal to the forbidden energy gap
Is ZnSe is direct or indirect band gap?
It is direct band gap material.
What is energy gap?
there will be only certain energy levels in which electrons get filled up. In valence orbitals there will be many such energy levels and the energy gap between conduction band and valence band is called energy band gap.
Why are Monovalent metal so conductive?
The single valence band electron can easily escape and become a conduction band electron.
Why effective mass is infinite at the middle of the band gap?
It sounds like a math trick because the electron cannot exist in the middle of the band gap. The trick would be that it would take an infinite amount of energy to excite the electron to the middle of the band gap.
What are the difference between direct and indirect band gap materials?
In a direct band gap the electron only needs energy to jump to the conduction band. In an indirect band an electron needs energy and momentum to jump to the conduction band
What are applications of direct and indirect band gap?
direct band gap means in e-k diagram valance bands are exactly below covalance band,in this band electron falls from the conduction band to valance band directly without going to metastable state and in indirect band gap the band electron falls from the conduction band to valance band by first going through the metastable state
Can extrinsic semiconductor conduct at 0 k?
Normally, no electron energy states exist in the band gap, the gap between the valence band and conduction band in a semiconductor. However, if we dope the semiconductor, i.e. add donor (n type) or acceptor (p type) atoms to it, we introduce new electron energy states in the band gap! Take for example silicon, in which we introduce phosphorus, which is a group V element and thus a donor atom. This will introduce extra filled electron states just below the conduction band. Now, this all happens at 0K, so no current can flow (this is logical as electrons don't move at this temperature, even with an electric field applied). But if we raise the temperature e.g. until room temperature at 300K, the electrons gain energy and can jump into the free energy states in the conduction band. These electrons in the conduction band can now conduct electricity.
Why is a rubber band an insulator?
The electron configuration of rubber (natural of artificial is such that there is a big gap between valance band and conduction band of electrons. Electrons has to make a transition from valence band to conduction band in order to conduct electricity.
When was The Gap Band created?
The Gap Band was created in 1967.
What is the difference between direct and indirect energy?
The band gap represents the minimum energy difference between the top of the valence band and the bottom of the conduction band, However, the top of the valence band and the bottom of the conduction band are not generally at the same value of the electron momentum. In a direct band gap semiconductor, the top of the valence band and the bottom of the conduction band occur at the same value of momentum.In an indirect band gap semiconductor, the maximum energy of the valence band occurs at a different value of momentum to the minimum in the conduction band energy
Why is rubber a good electrical insulator?
The electron configuration of rubber (natural of artificial is such that there is a big gap between valance band and conduction band of electrons. Electrons has to make a transition from valence band to conduction band in order to conduct electricity.
What is the forbidden energy gap?
it is the energy required to transfer an electron from valence band to conduction band, the external energy which is required is equal to the forbidden energy gap
What does gap stand for in the gap band?
greenwood archer and pine street band
When was The Gap Band II created?
The Gap Band II was created in 1979.
When was Gap Band IV created?
Gap Band IV was created in 1981.
