Questions tagged [electronic-band-theory]
Use this tag to ask about band-structure formation in solids, or about how electrons and holes behave inside them.
414
questions
3
votes
1answer
19 views
Derivation of Optical Absorption Coefficient in Semiconductors
I have been researching how to derive an expression for the absorption coefficient in semiconductors. I know the absorption coefficient can be expressed as such $$\alpha = A(hf-E_g)^{n}$$ with $n = \...
0
votes
2answers
42 views
Why electron-holes interpretation can't be effective in metals?
Help me out here,
If thought of as electron sea, there's is no need of holes in modeling conductivity in metals. But it's not that we can deny that there will be positive charge in the nucleus after ...
1
vote
1answer
22 views
Is “quaternary” adjective redundant while speaking of InGaAsP?
I'm reading an article. One sentence looks as following:
The 500 nm
thick waveguide layer consists of an InGaAsP quaternary layer
But since InGaAsP is quaternary compound, why do they write ...
1
vote
1answer
29 views
How can the oscillating states of many atoms in a lattice combine to give even a larger number of possible states?
Basically, I was studying the band theory and there was that thing that I couldn't understand, and it appeared to be connected to another question: how is the blackbody radiation appear continuous ...
0
votes
1answer
39 views
Energy Band of pn junction in thermal equilibrium (Zero bias)
Consider the following diagrams:
Question:
Why in thermal equilibrium—the Fermi energy level is constant throughout the entire
system? I know the following explanation
A gradient in the Fermi level ...
-1
votes
0answers
19 views
Why can't we choose a material with a low band gap to convert more photons into photo-electrons for a solar cell?
In Shockley–Queisser limit, how was the most efficient bandgap (1.5 eV) determined? Wouldn't you be able to generate more current with a lower bandgap since more photons are going to have enough ...
1
vote
0answers
55 views
Are there still no “simple analytical proofs” of Bloch's theorem in three dimensions?
Section 2.1.2 of Sydney G. Davison and Maria Stęślicka's 1992 Basic Theory of Surface States says:
2.1.2 Bloch theorem
The generalization of the Bloch-Floquet theorem to three dimensions is ...
0
votes
1answer
37 views
Why is only bandgap-energy usable? (Thermal Relaxation)
I know from solar cell physics, that in a first approximation, the maximum energy from an exited electron is the energy of the bandgap, no matter how high the electron was excited by a photon in the ...
0
votes
1answer
36 views
Confusion with Fermi energy in Quantum Hall effect
It is well-known, that when someone turn on magnetic field, and if magnetic field large enough, only few Landau levels are filled:
But I can't understand what going on with Fermi energy.
Fermi ...
3
votes
3answers
92 views
Is there any relation between turn on voltage of LED and the colour emitted?
It is evident from the above V-I graph that longer the wavelength of the light emitted lower is the turn on voltage. I know that turn on voltage is voltage required to have significant current that ...
1
vote
1answer
53 views
Why a photon cannot change the crystal momentum of an electron?
We know that absorption of a photon excites an electron from the valence band to the conduction band without changing its crystal momentum $\vec{k}$. Why can't a photon change the crystal momentum of ...
0
votes
2answers
77 views
Why are there electron and hole currents in semiconductor, but only electron current in metal?
Why are there electron and hole current in semiconductor, but only electron current in metal ?
I know that in metal there is an overlap between valence and conduction band,
while for semiconductor, ...
3
votes
2answers
72 views
Can we understand the strong reflectivity of metals from band theory?
I know that solids, including metals, have electronic bands and bandgaps. If we consider some typical metal such as copper, we know that it strongly reflects visible light. From the point of view of ...
0
votes
0answers
42 views
Is it true that the Fermi-Dirac distribution is a probability?
Is it true that the Fermi-Dirac distribution is a probability ?
I checked by doing the integral, and this does not give 1.
Also, for $T=0 K$, since it is a step function, it is easy to see that the ...
2
votes
1answer
85 views
How to draw different zone schemes in the Kronig-Penney model?
Kronig-Penney model does not give us an equation of how the electron energy $E$ depends on its wavenumber $k$. Then how do they draw the $E-k$ curves in different zone schemes such as extended, ...
0
votes
2answers
63 views
Why Fermi energy is in between band gap?
It may seems like a copied question Question
But It still didn't clear my doubt.Consider the following figure
As you can see the fermi energy (Fermi energy)
lies in between the bandgap and ...
0
votes
1answer
20 views
Effective Mass structure outside first band minimum
I wanted to ask: if I would have the electron dispersion function of the first band, which is not parabolic can it be used/(inserted) with the effective mass function, for $\frac{d^2E(k)}{dk^2}$:
$m^* ...
1
vote
0answers
23 views
Direct & Indirect (Kohn-Sham) band gap for $\rm MgS$
I recently performed a groundstate calculation for $\rm MgS$ using the self-consistent Kohn-Sham equation with LDA exchange-correlation. $\rm MgS$ has an FCC crystal structure with two atoms per (non-...
1
vote
1answer
36 views
Questions about Creation/Annhiliation Operators of electrons
I am trying to solve a question that asks me to find the two energy bands of a NaCl crystal, where "c" is the distance between Na and Cl where $a_{\vec{R}}^{\dagger}$ creates and electron centered on ...
0
votes
1answer
39 views
Wave function of $s$-band with odd parity
How does a wave function of a $s$-band state with odd parity looks like (in real space)? To keep it simple, the restriction to a linear chain and a state at the $\vec{k}=0$-point might be useful.
My ...
0
votes
2answers
22 views
Comparing energy band diagram for metals, semiconductors and insulators
The energy band diagram is a model for describing why an insulator is not a good conductor compared to a semiconductor: in the first case, electrons would need to pass a very large band gap in order ...
1
vote
1answer
68 views
Interpretation of electronic band structure diagram
I want to understand the electronic band structure diagram of the following image, corresponding to $\text{MoS}_2$ (TMD):
I read about DFT (density functional theory).
DFT is based on solving the ...
1
vote
2answers
59 views
Electrons involved in bonding cannot be involved in conduction?
I am currently studying the textbook Physics of Photonics Devices, Second Edition, by Shun Lien Chuang. In a section discussing the basic concepts of semiconductor band and bonding diagrams, the ...
0
votes
0answers
43 views
Conduction, conduction and valence bands, and energy bands
I am currently studying the textbook Physics of Photonics Devices, Second Edition, by Shun Lien Chuang. In a section discussing the basic concepts of semiconductor band and bonding diagrams, I was ...
1
vote
0answers
40 views
Why is density functional theory unreliable for prediction of band gap [closed]
Why is density functional theory unreliable for calculation of band gap?
0
votes
1answer
39 views
After a diode start conducting, can you increase voltage across the diode without current change in the diode?
Personal Background:
I am trying to reconcile my understanding of Fermi band diagram of diode with a diode's circuit behavior. I believe I have a point of misunderstanding somewhere that is causing me ...
1
vote
2answers
57 views
Landau levels degeneracy
I'm studying the Landau quantization of free electron systems and a question comes to my mind when I arrive to the degeneracy of the levels. This is given by the following expression:
$N=\frac{2e}{hc}...
0
votes
1answer
38 views
Flat bands in metals
When I look at the angular momentum resolved band structure of the sodium (Na), there are two flat band at very low energies (p at around -25 eV and s at around -50 eV). What should I understand from ...
0
votes
1answer
25 views
Semiconductor conduction band
What would happen in an hypothetical situation if we completely fill up the conduction band, will then also it will conduct?
I know we can't fill that more than half.
I think it will also not ...
1
vote
1answer
58 views
Effective mass as a consequence of energy band structure
The Wikipedia article on effective mass defines it as follows:
In solid state physics, a particle's effective mass (often denoted ${\textstyle m^{*}}$) is the mass that it seems to have when ...
3
votes
0answers
34 views
Reconciling electron configuration and valence electron distribution over separate energy levels in energy band theory
This seemed like more of a chemistry question to me, but I was told by the people at chemistry.stackexchange that it might be more appropriate to post here, since "it was the physics guys that did all ...
1
vote
1answer
77 views
Filling of energy bands
In the context of electron energy bands for crystals, I have heard the phrase
There are $x$ electrons per band, therefore the electrons fill $y$
bands.
I am trying to understand the 'therefore' ...
0
votes
1answer
95 views
electronic band structure calculation using plane wave expansion: what are the diagonal matrix elements for a coulomb potential?
I'm trying to do a plane wave basis expansion calculation for the band structure / wavefunctions of electrons in a periodic solid, using a coulomb potential for the nuclei. I'm working from Ashcroft ...
0
votes
1answer
21 views
Units in result for band gap photon momentum for GaAs
My textbook, Introductory Semiconductor Device Physics, by Parker, gives the following example:
For GaAs, calculate the typical (band-gap) photon energy and momentum, and compare this with a ...
1
vote
1answer
49 views
Nearly free electron model: Pauli principle
If I look at the bandstructure in the nearly free electron model in a diagram with energy over wave vector $\bf{k}$, there are some parts of parabolas in my first Brillouin zone. I understand, why ...
1
vote
1answer
124 views
How do flat bands imply superconductivity?
I'm just doing a small ugrad assignment where I have to present a talk on twisted bilayer graphene. I'm having trouble understanding the meaning of a flat band.
As far as I understand, a flat band ...
2
votes
3answers
170 views
Energy band theory: conduction of electrons and holes
My textbook, Introductory Semiconductor Device Physics, by Parker, says the following in a section on energy band theory:
We are aware that conduction in most metals is by electrons but conduction ...
1
vote
2answers
55 views
Band structure of a nanoparticle
Considering a crystal at such a small size (for example, a sphere with diameter about 20 atoms) that the periodic boundary condition assumed in Bloch Theorem is not really a good estimation, I wonder ...
1
vote
2answers
55 views
Why does the concept of effective mass fail for linear dispersion relations?
I had a discussion about graphene and stumbled about a conceptual problem of myself about the effective mass of electrons close to the Dirac point in graphene or for any linear dispersion relation in ...
0
votes
1answer
31 views
How objects reflect green light?
I'm currently interested in energy levels and I've learned that there are energy gaps between molecular orbitals that makes things appear transparent (for visible light).
Thus, objects like glass can ...
1
vote
1answer
40 views
Electron's excitement conditions for a Photon in conductors
Glass is a typical insulator: electrons inside their atoms has band gaps, which are setting limits for low-frequency photons and prohibits energy conveying for them.
Therefore, light just passes ...
1
vote
0answers
43 views
Functional form for pseudopotential
I have found this resource quite good for a brief summary of pseudopotentials, of which I am trying to learn more about.
The author shows that, once a functional form for the pseudo-wavefunctions has ...
0
votes
0answers
42 views
Hetero junction - similar work-functions
Why is it important that the two semiconductors in a hetero-junction, that is brought into contact, have similar work-functions? (If it is important). Obviously it's important for a metal-...
0
votes
2answers
60 views
Fermi level in a solid
I am confused as to how fermi level is defined within a crystal/ solid
It is normally situated midway between valence band and conduction band, is this how it is defined or is there other reasons this ...
0
votes
0answers
49 views
Berry curvature of a two band lattice Hamiltonian
Consider a general two band lattice Hamiltonian, which can be expressed as a $2\times2$ matrix:
$$\mathcal{H}=\vec{h}(\vec{k})\cdot \vec{\sigma}$$ where $\vec{\sigma}$ is the vector of Pauli matrices. ...
0
votes
0answers
34 views
Derivation of LCAO/Tight-binding method for energy bands
I'm having a bit of trouble properly understanding the LCAO/tight-binding method for calculating the band structure of metals. I'll try to go through the derivation/explanation given in our lecture ...
0
votes
1answer
61 views
What is the difference between HOMO-LUMO Energy gap and Forbidden (Energy) gap?
I have found a similar question on a different website upon searching on google but it didn't clear my confusion (which might be because I didn't understand the explanation).
(PS: I am a 12th STD ...
0
votes
1answer
87 views
Flattened band in band structure
What does a flattened band in a band structure plot imply? I've been reading this paper https://doi.org/10.1016/j.apsusc.2012.06.092 and one of the primary results is the flattening of bands however ...
0
votes
0answers
14 views
Magnetic field dependence of resistance
Experimentally, under the application of an external perpendicular magnetic field, we observe a change in the longitudinal resistance. However, in the theoretical development starting from the Drude ...
1
vote
2answers
178 views
Fermi level and chemical potential in doped and pure semiconductors
Recently I was studying about band theory of semiconductors and I have some questions.
I found two definitions of Fermi level
$1)$ The quantum state which has a probability of occupancy of $0.5$
(...
