What is Band gap: Definition and 85 Discussions

In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. It is the energy required to promote a valence electron bound to an atom to become a conduction electron, which is free to move within the crystal lattice and serve as a charge carrier to conduct electric current. It is closely related to the HOMO/LUMO gap in chemistry. If the valence band is completely full and the conduction band is completely empty, then electrons cannot move in the solid; however, if some electrons transfer from the valence to the conduction band, then current can flow (see carrier generation and recombination). Therefore, the band gap is a major factor determining the electrical conductivity of a solid. Substances with large band gaps are generally insulators, those with smaller band gaps are semiconductors, while conductors either have very small band gaps or none, because the valence and conduction bands overlap.

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  1. prox777

    I Why diode with larger band gap has larger onset potential?

    I don't understand why voltage increases with band gap. I understand why from the equations, but not conceptually. From my understanding, the onset voltage is what is needed to overcome the contact potential that forms when the p-type and n-type samples are joined together in a p-n junction. But...
  2. Janusz Bednarski

    How Does Voltage Affect LED Color Through Band Gap Adjustments?

    Hi! This question is translated so that's why it might sound a bit weird. I know that the answer should mention the band diagram of the LED, and I think that the answer is to introduce an electrical contact connected to a voltage source. Then when you increase the voltage, it should attract the...
  3. yecko

    Engineering Can the efficiency of a solar cell with a single band gap reach 100%?

    I have tried looking up online with the photoelectric effect, it just mentioned the energy of the photon must have to equal to band-gap energy. Yet, it didn't mention if 100% can be reached in that case, or if a single bandgap makes any effect...
  4. M

    An accurate method to determine the band gap

    Summary:: What is an accurate method to determine the band gap? What is an accurate method to determine the band gap?
  5. T

    How to Measure and Calculate Band Gap of a Photovoltaic Device

    Hi there! I bought a photovoltaic cell. I would like to learn its band gap at 300 K and 0 K. How can i measure band gap or calculate? I have already measured Temperature to Vmax values. I couldn't find any good information about this topic. Thanks.
  6. jisbon

    Energy band gap when there is an electric field

    So I have just been taught this topic but this question seems to be one of a kind and I can't seem to figure it out. What I've learnt: When there is a positive electric field applied to the right, for example, the electrons that are free moving in a crystal (aka conducting band) will oppose...
  7. E

    Why is the band gap in alloys such as GaAs less?

    In pure crystals,such as,in silicon and germanium,the band gap is more than that in compound semiconductors,such as,GaAs.Why is that so.
  8. N

    A Band Gap Vs Pressure: Proskite Compounds

    why band gap reduces when pressure increased for provskite compounds ? Nawzad
  9. T

    A Criticality, phase transitions and closing bandgap

    At critical points, when correlation lengths diverge near a phase transition, people often say that the spectral gap closes, which is to my understanding just the energy difference between the ground state and the first excited state (the first two eigenvalues of the hamiltonian). How should we...
  10. H

    Understanding solar cell band diagram

    Hi everyone, I have been studying the physics of solar cells for a long time. The only thing that I can not completely understand is the physical operation of solar cell based on band diagram. Can anyone briefly explain the movements of carriers from one electrode to the another based on band...
  11. G

    I Relationship between the depletion region and the band gap of a diode?

    Hi, Lately I've been reading about a lab exercise in which Planck's constant is estimated using LEDs. Every procedure I've encountered states that the energy of an emitted photon (and by extension the potential across the semiconductor's band gap) is equal to the threshold voltage multiplied by...
  12. V

    A Nanowire with charge neutrality in band gap

    I'm working on a finite element simulation of the electrostatic potential V(r) in and around semiconductor nanowires, based on solving Poisson's equation. While the details of the problem will follow shortly, the crux of where I run into trouble is that for nanowires it is important to include...
  13. S

    A What is the charge neutral point?

    I have a question about definition of charge neutral point. I`m reading some article about trilayer graphene, and this term, charge neutral point, is used in the article. I googling this word, but I can`t find something which can tell me what it is. In monolayer graphene, what i found by...
  14. Kara386

    I Why does LED band gap decrease as temperature increases?

    I've tried to summarise the explanation my professor gave. Is it correct or have I misunderstood? It's a highly simplified view of things anyway, but here goes: Taking the simplest 1D case, there are two possibiities for an electron in the lattice: it may be scattered and move back the way it...
  15. Nikhil Rajagopalan

    I Photosensitive properties of Molybdenum Disulphide

    Dear Experts, I am studying about a Stanford University innovation, using Molybdenum Disulphide and solar radiation to disinfect water at a very quick rate. I am unable to find details about photo sensitive properties of Molybdenum Disulphide. What is the work function of molybdenum...
  16. A

    I Phonon energy in absorption of indirect band gap

    Does phonon required a minimum energy in the absorption process of an indirect band gap, and why?
  17. M

    I Opening of a band gap - Diffraction at the edge of BZ zones?

    I have a hazy idea that comes from a few video lectures (see below for example) where the following is stated: The condition for constructive interference (Bragg condition) in reciprocal space is that the wave vector should touch a Brillouin zone boundary. In this case, the standing wave that...
  18. M

    A Can Doping Concentration Affect the Band Gap in Graphene?

    if we solve elliptic integrals related to doped graphene, band gap does not occur. But actually it occures with p doping. How can I solve this puzzle? Thanks for any help.
  19. T

    Do all electrons orbit in the ground state, when the band ga

    Do all electrons orbit in the ground state, when the band gap, is increased, or decreased, as atoms come together to form molecules.? Like in glass, transparent liquids, and plastics. As atoms come together to form glass, like silicon, sodium, and calcium. Do the electrons orbital permanently...
  20. sounouhid

    Optical band gap of a cobalt ferrite

    Hi everyone How can I determine the optical band gap of a cobalt ferrite film deposited on a glass substrate using its absorption, transmittance and diffuse reflectance spectrum (knowing that I don't know the thickness of this film)
  21. Telemachus

    Nearly free electron model: band gap

    I was trying to determine the bandgap in the nearly free electron model. I'm having trouble to determine the band gap bewteen the second and the third band. Its a one dimensional problem. So, the central equation reads: ##\displaystyle \left [ \frac{\hbar}{2m} (k-G)^2-E \right ]c_{k-G}+...
  22. W

    Why is the band offset of an LED equal to the band gap?

    I am doing an experiment where we have to use the threshold voltage of LEDs to determine Planck's constant given the following relation $$ eV_0 = E_g = \frac{hc}{\lambda} $$ where ## V_0 ## is the threshold voltage, ## \lambda ## is the wavelength of emitted light and ## E_g ## is the band...
  23. J

    Surface States within band gap STM/STS

    Hi there people! So my question is why you can see localized surface states within the band gap of the material with an STM. How is a tunneling circuit being established?
  24. H

    Proving direct band gap property of a semiconductor

    Hi, How can I prove direct band gap property of a Transition Metal Dichalcogenide electrically and optically through experimentation? I can use PL measurements to prove this for optical band gap. Is there any other equipment or technique to prove this? I can not use ARPES because channel length...
  25. M

    Finding Band Gaps for Dirac Comb Potential

    Homework Statement Find band gaps for Dirac Comb potential $$V = \sum_n aV_0(x-na) $$ Homework Equations Bloch Theorem $$\psi(x+a) = e^{ika}\psi(x)$$ The Attempt at a Solution I can solve exactly up to $$\cos(k a) = \cos(\kappa a) + \frac{2ma^2V_0}{\hbar^2}\frac{\sin(\kappa a)}{\kappa a} =...
  26. vino

    How choose k-points for 5*5*4 slab for band gap calculations

    Is there any method for how to choose k-points in band structure calculations for aluminium slab 5*5*4 can somebody help in this question? Thanks
  27. N

    How does Band Gap and Refractive index relate to Wavelength?

    Homework Statement Nobel prize for physics for blue laser diode using Gallium Nitride with band gap of 3.4 eV and a refractive index of 2.429. Explain how these parameters determine at what wavelength a Gallium Nitride semiconductor will laser at. Homework Equations E=\frac{hc}{\lambda}...
  28. A

    Electron affinity, work function, band gap

    What exactly is the relation between these 3 quantities? As far as I can tell the work function is the energy needed to bring an electron from the fermi level out into vacuum, while affinity is from the bottom of the conduction band. Does this then mean that they can be used to calculate the...
  29. Benevito

    Photoluminescence at a heterojunction

    Do I understand it correctly that photoluminescence at a semiconductor heterojunction occurs because of intralayer recombination? If so, why can't photoluminescence occur because of interlayer recombination? Is it because momentum cannot be conserved in such a process ( like in the indirect gap...
  30. A

    What Causes the Forbidden Gap in Semiconductors?

    Can anybody explain my what is the origin of this forbidden gap? I mean how it was created and what is the physics behind this issue?
  31. E

    Explaining the Absence of Band Gaps in Superconductors

    Hi all, I am currently writing a report about superconductors, and am currently reading about how the band gap shows that single electrons are not the charge carriers responsible for superconductivity. However, I was confused when I read that electrons are fermions and as such there are no band...
  32. U

    Energy gap between brillouin zones?

    Homework Statement Consider a monovalent 2D crystal with a rectangular lattice constants ##a## and ##b##. Find expressions for the fermi energy and fermi wavevector in 2D. Show that the fermi surface extends beyond first zone if ## 2a > b\pi##. If the crystal is now divalent, estimate the...
  33. baby_1

    Connect different material with different Band Gap

    Hello As you know AlGaAs and GaAs have different band gap).I want to know what will happen when we connect both of them to each other? In this figure I don't know how can we draw the jump in their junction. Could you tell me what does fermi level do? Thanks
  34. U

    Nearly free electron model - band gap

    For a wavefunction at the Brillouin boundary we have: \langle k|H|k\rangle = \epsilon_0 (\vec k) \langle k'|H|k'\rangle = \epsilon_0 (\vec k+\vec G) \langle k'|H|k\rangle = V_G = \frac{1}{L^3} \int e^{i(\vec k - \vec k') \cdot \vec r} V(r) d\vec r \langle k|H|k'\rangle = V_G^* Using...
  35. Laplas

    DOS calculation in a photonic crystal

    I have calculated the band structure of an infinite photonic crystal structure by COMSOL. So, I have an array of meanings of K-vector and different values of frequency V. I need to calculate density of states (DOS). How can I calculate it? Thanks.
  36. NaOH

    Significance of band gap in k-space

    I am struggling in understanding what the band structure (E vs. k) diagram tells me. Let's say I have the following material: http://www2.warwick.ac.uk/fac/sci/physics/current/postgraduate/regs/mpags/ex5/phonons/indirect_absorption/indirect_absorption.bmp...
  37. G

    Silvaco TCAD Energy band gap issue

    Is there any possibility in silvaco TCAD to introduce Band gap in material. i want to use different energy band gap for different transport direction of a material. i.e.transverse direction one energy gap value and longitudinal directon different energy x-axis one energy value and y direction...
  38. R

    Band Gap of Semiconductor Materials

    Hi Guys, i need your help, where can i download free articles about the band gap of semiconductor materials (like copper (II) oxide). don't mention sciencedirect and arxiv, because i already explore that, and they provide limited research articles. thank you.
  39. C

    Free electron concentration range between semiconductors and metals

    A structure with free electron density around 10^26 m^-3 is considered as a highly doped semiconductor or a metal? Or in other words, what is the lowest possible free electron concentration for a metal and what is the highest possible free electron concentration for a doped semiconductor?
  40. G

    Why do doped semiconductors have a decreased band gap?

    Could anyone please explain why doped semiconductors show decrease of the bang gap in comparison to the pure semiconductors. Web/literature links to relevant information will be appreciated. Thanks.
  41. A

    Why doesn't Graphene have a band gap?

    Is there any simple justification about graphene having no band gap? How bout its linear E-K? Why bilayer graphene has a quadratic E-K and electric field can open a band gap there? I do not completely understand the broken symmetry argument? Also Why MoS2 which has similar structure, do not...
  42. H

    Can we have band gap anywhere?

    According to solid state texts in Brillouin zone borders where diffraction condition satisfies we have a band gap. However I usually see Band gaps which are located in the center of Brillouin zone. Please correct me if I am wrong.
  43. N

    How ionization energy can be lower than band gap?

    I am reading about the ionization energy in semiconductors and came across this thing that for Silicon, the ionization energy is lower than its band gap energy. I don't understand how can this be?
  44. A

    What is the Connection Between Bragg Reflection and Energy Band Gaps in Solids?

    My book has a discussion of how the Bragg model can explain the origin of energy band gaps in solid. I have attached the relevant part of this discussion. I would very much like if someone could help me understand what it's basically trying to say. Facts: 1) In the free electron model, the...
  45. A

    Why Does Band Gap Energy Increase Higher Up the Periodic Table?

    Hi, Which of ZnS and CdSe will have the larger band gap energy, explain. The answer is given as Zinc sulfide will have a larger band gap energy than cadmium selenide. Both are II-VI compounds, and Zn and S are both higher vertically in the periodic table than Cd and Se. In moving from...
  46. A

    What Factors Influence the Band Gap in Band Structures?

    What are the parameters affects the band gap in a band structure?
  47. S

    Spin-dependent band gap calculations

    Dear colleagues, I am trying to deal with quantum-chemical calculations of metal-containing coordination polymers, and I have a question which turned out to be difficult to me: If a system includes copper atoms, which implies that it has 'unpaired' electron, how to describe a band gap in...
  48. D

    Direct and Indirect Optical Band Gap

    I've been looking on journals and textbook, as well as internet. Does anybody know if there is an equation to find out the value of direct optical band gap and indirect optical band gap? What is the difference of optical band gap, direct optical bandgap and indirect optical bandgap? also...
  49. W

    Band gap calculation,how to choose the Kpoints?

    hello, is there anyone who can help me in the band gap calculation. my situation is that: I can calculate the band gap of sigle crystalline graphene, because i know the high symmetrical pionts of the first Brillouin zone of an hexagonal crystalline(M-G-K-M). But the problem is that...
  50. M

    Interpretation of 1D Band Gap in Metallic Systems

    In a metallic system, the Fermi level is crossed either from the conducting zone into the non-conducting zone or vice versa. Is there an interpretation one can give to the direction of the crossing? In other words, if the 1D band gap diagram shows the fermi line is crossed from the...