Name: valence and conduction bands in silicon
Uploaded: 2024-07-08T13:20:08.000Z
Duration: 1 min 1 s
Channel: JoVE

I solidi sono composti da atomi con configurazioni elettroniche e livelli di energia distinti.
Quando due atomi isolati identici vengono avvicinati, la loro interazione fa sì che ogni livello di energia si divida in due livelli distinti.
Questa scissione è il risultato del principio di esclusione di Pauli, che afferma che due elettroni in un atomo non possono occupare lo stesso stato energetico allo stesso tempo.
Se più atomi vengono riuniti per formare un solido, i livelli di energia si dividono per formare una banda continua. Alla spaziatura interatomica di equilibrio, la banda si divide nella banda di valenza e nella banda di conduzione.
Alla temperatura dello zero assoluto, la banda di valenza comprende i livelli di energia riempiti di elettroni.
Al di sopra della banda di valenza si trova la banda di conduzione, che contiene i livelli di energia vacanti.
Il gap di energia tra queste due bande è chiamato band gap. Rappresenta la differenza di energia tra la parte superiore della banda di valenza e la parte inferiore della banda di conduzione.

nozioni di base sui semiconduttori

Energy Bands in Solids

Gli atomi isolati hanno livelli energetici discreti che sono ben descritti dal modello di Bohr. Questo quantifica l'energia di un elettrone in un atomo di idrogeno come E_N. Numeri quantici “n” più alti, producono livelli energetici degli elettroni meno negativi e più vicini.

Formazione della banda:

Quando gli atomi vengono avvicinati, come in un solido, questi livelli energetici discreti iniziano a dividersi a causa della sovrapposizione degli orbitali elettronici degli atomi adiacenti. Questa divisione avviene a causa del principio di esclusione di Pauli, secondo il quale due elettroni non possono occupare contemporaneamente lo stesso stato quantico. Man mano che più atomi vengono combinati, il numero di livelli energetici discreti aumenta e la scissione diventa così fine da formare una banda energetica continua. Ad un certo punto, noto come distanza interatomica di equilibrio, queste bande diventano la banda di valenza, piena di elettroni, e la banda di conduzione, che è vuota allo zero assoluto.

Bande energetiche nei semiconduttori:

La struttura delle bande energetiche è più complessa nei semiconduttori come il silicio, che ha 14 elettroni. I 10 elettroni interni occupano i livelli energetici più profondi e non contribuiscono al legame. I restanti quattro elettroni di valenza, che si trovano nei sottolivelli 3s e 3p, determinano le proprietà chimiche ed elettriche del materiale. Poiché gli atomi di silicio formano un reticolo cristallino, gli orbitali 3s e 3p si sovrappongono e formano bande. Alla distanza di equilibrio nel cristallo, queste bande contengono 4N stati per la banda di valenza e 4N stati per la banda di conduzione, dove N è il numero di atomi di silicio.

La dimensione della banda proibita è cruciale, perché determina la facilità con cui gli elettroni possono essere eccitati nella banda di conduzione. Questo gap di banda per i semiconduttori è sufficientemente piccolo affinchè l'energia termica o la luce possano eccitare gli elettroni attraverso il gap, determinando conduzione elettrica.

Nozioni di Base sui Semiconduttori

I solidi sono composti da atomi con configurazioni elettroniche e livelli di energia distinti.
Quando due atomi isolati identici vengono avvicinati, ...

Education

Didattica

JoVE Core

Electrical Engineering

Ingegneria elettrica

Unit 1

Unità 1

Basics of Semiconductors

Fondamenti dei semiconduttori

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

valence and conduction bands in silicon

Valence and conduction bands explain how electrons behave in solids. In isolated atoms, electrons occupy discrete energy levels that the Bohr model describes well. The hydrogen atom, for example, has specific electron energies labeled by the quantum number n, and higher n values give levels that are less negative and closer together.

When atoms move close together in a solid, those discrete levels start to split. This happens because electron orbitals from neighboring atoms overlap. The Pauli exclusion principle also applies, since no two electrons can share the same quantum state. As more atoms are added, the splitting becomes so small that it appears as a continuous energy band.

At the equilibrium interatomic distance, these bands form the valence band and the conduction band. The valence band is filled with electrons at this point. The conduction band is empty at absolute zero temperature. This separation helps explain why some solids conduct electricity and others do not.

Silicon shows how band formation works in a semiconductor. A silicon atom has 14 electrons, but the inner 10 electrons stay in deep energy levels and do not take part in bonding. The remaining four valence electrons, which occupy the 3s and 3p subshells, control the material’s chemical and electrical behavior. As silicon atoms arrange into a crystal lattice, the 3s and 3p subshells overlap and form bands.

In the crystal, the valence band and conduction band each contain 4N states, where N is the number of silicon atoms. The bandgap, or energy gap between the bands, is important because it controls how easily electrons can move into the conduction band. In semiconductors, this gap is small enough that thermal energy or light can excite electrons across it, which allows electrical conduction.

Valence and Conduction Bands in Silicon

energy bands in solids

Valence and conduction bands explain how electrons behave in solids. In isolated atoms, electrons occupy discrete energy levels that the Bohr model describes well. The hydrogen atom, for example, has specific electron energies labeled by the quantum number n, and higher n values give levels that are less negative and closer together. When atoms move close together in a solid, those discrete levels start to split. This happens because electron orbitals from neighboring atoms overlap. The Pauli...

semiconductors

Energy Bands in Metals and Insulators

Materials conduct electricity in different ways because their energy bands are arranged differently. Metals, semiconductors, and insulators can be compared with energy band diagrams. These diagrams show how easily electrons can move when an electric field is applied. Metals such as copper, zinc, and lead have low resistivity. Their conduction band is not fully occupied, or it overlaps with the valence band, so a band gap is essentially absent. Electrons in the highest valence energy levels can...

types of semiconductors

Intrinsic and Doped Semiconductors

Intrinsic semiconductors are very pure materials with no impurities. At absolute zero, they act as perfect insulators because the valence electrons are bound and the conduction band is empty. That means electrical conduction cannot occur under those conditions. The Fermi level helps describe the chance that an energy level is occupied by an electron at thermal equilibrium. In an intrinsic semiconductor, the Fermi level sits near the middle of the band gap at absolute zero. As temperature...

carrier generation and recombination

Semiconductor Charge Carriers: Creation and Loss

Carrier generation and recombination control how free charge carriers change inside a semiconductor. Carrier generation creates electron-hole pairs, or EHPs, when electrons move from the valence band into the conduction band and leave holes behind. Carrier recombination does the opposite and lowers the number of free carriers. In direct-bandgap semiconductors such as gallium arsenide, generation happens efficiently because energy absorption can move a valence electron straight to the...

carrier transport

Drift and Diffusion in Semiconductors

Drift and diffusion are the two main ways electric current forms in semiconductors. These charge-carrier motions help determine how semiconductor devices work and how well they perform. The carriers involved are electrons and holes. Drift current begins when an external electric field acts on charge carriers. Electrons and holes move between collisions with lattice atoms, and this motion creates drift velocity. For electrons, drift velocity depends on electron mobility and the electric field...

p-n junction

P-N Junction Depletion Region

A p-n junction forms when p-type and n-type semiconductor materials are joined together. At the boundary, holes from the p-side and electrons from the n-side diffuse into the opposite side because of the concentration gradient. This movement leaves a region with no free charge carriers near the junction, called the depletion region. The charge density in the depletion region can be described on the n-side and p-side using the donor and acceptor doping concentrations, ND and NA. The fixed...

biasing of p-n junction

P-N Junction Diode Under Bias

P-N junction diodes change behavior under equilibrium, forward bias, and reverse bias. In equilibrium, no external voltage is applied across the junction. Carriers diffuse at the boundary and leave behind charged dopants. Acceptors remain on the p-side, and donors remain on the n-side. These fixed charges form a depletion region and an electric field that blocks further diffusion. The band diagram at equilibrium shows the Fermi levels on both sides aligned. That alignment means the junction is...

metal-semiconductor junctions

Schottky and Ohmic Metal Contacts

Metal contacts with semiconductors can form either Schottky or Ohmic junctions. The result depends on the work function of the metal, the work function of the semiconductor, and how electrons move when the two materials touch. At equilibrium, the Fermi levels align, so charge shifts until both sides match in energy. Schottky barriers form when a metal with work function Φm contacts a semiconductor with a different work function Φs. If Φm is greater than Φs, the semiconductor Fermi level starts...

biasing of metal-semiconductor junctions

How Bias Changes Metal-Semiconductor Current

Biasing metal-semiconductor junctions means applying a voltage across the junction. The metal connects to the voltage source, and the semiconductor is grounded. This setup helps control the direction and size of current in devices such as diodes, transistors, and photovoltaic cells. In a Schottky junction with an n-type semiconductor, a positive voltage on the metal lowers its Fermi level. A Fermi level is the energy level that helps describe how electrons are distributed. Lowering this level...

fermi level

Fermi Level in Semiconductors

The Fermi level is a key energy level in semiconductors. It sits between the lower-energy valence band and the higher-energy conduction band. The Fermi-Dirac function, which follows an S-shaped curve, shows the chance that an energy state is filled by an electron at a given temperature. At absolute zero, electrons occupy every energy state up to the Fermi level. States above it stay empty. As temperature rises, electrons gain energy and can move into vacant states above the Fermi level. In an...

fermi level dynamics

Vacuum Level and Schottky Barriers

Vacuum level and Schottky barriers help explain how electrons move between materials. The vacuum level is the energy needed for an electron to escape a material surface. In a semiconductor, it usually sits above the conduction band and serves as a reference for comparing electron energy. Electron affinity is the energy gap between the conduction band minimum and the vacuum level. It shows how easily a semiconductor can accept extra electrons. The work function is the least energy needed to...

key terms and concepts

Learn how atomic energy levels split into valence and conduction bands, and why silicon’s small bandgap lets heat or light drive conduction.

Valence and Conduction Bands in Silicon | JoVE K12

JoVE Core Electrical Engineering offers easy-to-understand animated videos that cover fundamental concepts of electric circuits along with circuit analysis techniques. It incorporates first-order and second-order circuits using operational amplifiers and circuit transformation concepts. The inclusion of practical design examples and real-world applications through scientist-in-action videos adds significant educational value, making complex concepts accessible.

electrical engineering

Valence and Conduction Bands in Silicon

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