Name: vacuum level and schottky barriers
Uploaded: 2024-07-08T13:20:09.000Z
Duration: 1 min 12 s
Channel: JoVE

Il livello di vuoto rappresenta la soglia di energia per un elettrone per sfuggire alla superficie di un materiale. In genere si trova al di sopra della banda di conduzione di un semiconduttore e funge da riferimento per le energie elettroniche.
Nei semiconduttori, l'affinità elettronica è il divario di energia tra il minimo della banda di conduzione e il livello di vuoto.
La funzione di lavoro è l'energia minima richiesta per liberare un elettrone dal livello di Fermi al livello di vuoto ed è dipendente dal materiale.
I metalli mostrano elevate funzioni di lavoro, da 2 a 5 eV, a causa dei loro densi stati di livello di Fermi.
I semiconduttori mostrano funzioni di lavoro variabili al variare dei loro livelli di Fermi, influenzati dal drogaggio e dalla temperatura.
Un metallo e un semiconduttore a contatto raggiungono l'equilibrio, creando un livello di vuoto continuo attraverso la giunzione.
Contemporaneamente, il trasferimento di carica continua fino a quando i livelli di Fermi si allineano, causando la flessione della banda di energia nel semiconduttore e formando una barriera Schottky.
L'altezza della barriera della giunzione è determinata dalla funzione di lavoro del metallo e dall'affinità elettronica del semiconduttore.

dinamica del livello di fermi

Il livello di vuoto indica la soglia di energia richiesta affinché un elettrone possa fuoriuscire da una superficie materiale. Di solito è posizionato sopra la banda di conduzione di un semiconduttore e funge da punto di riferimento per confrontare le energie degli elettroni all'interno di vari materiali.

L'affinità elettronica nei semiconduttori, indica il divario energetico tra il minimo della sua banda di conduzione e il livello di vuoto, ed è un parametro critico nel determinare con quanta facilità un semiconduttore può accettare elettroni aggiuntivi.

La funzione lavoro, rappresenta la minima quantità di energia richiesta per spostare un elettrone dal livello di Fermi, che è il livello energetico in cui un elettrone ha una probabilità del 50% di essere presente, al livello di vuoto. Questo valore varia a seconda dei materiali, con i metalli che tipicamente mostrano funzioni di lavoro elevate, che vanno da 2 a 5 eV, attribuite ai loro livelli di Fermi densamente popolati.

I semiconduttori, d'altro canto, mostrano funzioni di lavoro dinamiche a causa della natura variabile dei loro livelli di Fermi, influenzati da fattori quali il drogaggio e le variazioni di temperatura.

Quando un metallo e un semiconduttore entrano in contatto, cercano l'equilibrio, portando ad un livello di vuoto uniforme attraverso la giunzione mediante il trasferimento di carica, continuando fino all'allineamento dei livelli di Fermi di entrambi i materiali. Ciò innesca la flessione della banda energetica all’interno del semiconduttore, portando alla creazione di una barriera Schottky, una potenziale barriera energetica per gli elettroni che si muovono attraverso la giunzione metallo-semiconduttore.

L'altezza della barriera Schottky governa la conduttività della giunzione ed è determinata dalla relazione tra la funzione lavoro del metallo e l'affinità elettronica del semiconduttore.

Dinamica del Livello di Fermi

Il livello di vuoto rappresenta la soglia di energia per un elettrone per sfuggire alla superficie di un materiale. In genere si trova al di sopra ...

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

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 move an electron from the Fermi level, the level where an electron has a 50% chance of being present, to the vacuum level.

Work function values are not the same for every material. Metals usually have high work functions, often between 2 and 5 eV, because their Fermi levels are densely filled. Semiconductors have work functions that can change. Doping and temperature both affect their Fermi levels.

When a metal touches a semiconductor, the two materials move toward equilibrium. Charge transfer continues until their Fermi levels line up and the vacuum level becomes uniform across the junction. This process bends the energy bands in the semiconductor and forms a Schottky barrier, which is a potential energy barrier for electrons crossing the metal-semiconductor junction.

The height of the Schottky barrier controls how well the junction conducts. It depends on the metal’s work function and the semiconductor’s electron affinity. This relationship is key to understanding electron flow at metal-semiconductor contacts.

Vacuum Level and Schottky Barriers

energy bands in solids

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...

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 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

Explore vacuum level, electron affinity, work function, and Schottky barriers. Learn how metal-semiconductor contacts align energy levels and affect conductivity.

Vacuum Level and Schottky Barriers | 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

Vacuum Level and Schottky Barriers

Description

Transcript

Tags