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

Os sólidos são compostos de átomos com configurações eletrônicas e níveis de energia distintos.
Quando dois átomos isolados idênticos são aproximados, sua interação faz com que cada nível de energia se divida em dois níveis distintos.
Essa divisão é resultado do princípio de exclusão de Pauli, que afirma que dois elétrons em um átomo não podem ocupar o mesmo estado de energia ao mesmo tempo.
Se vários átomos são reunidos para formar um sólido, os níveis de energia se dividem para formar uma banda contínua. No espaçamento interatômico de equilíbrio, a banda se divide na banda de valência e na banda de condução.
Na temperatura do zero absoluto, a banda de valência compreende os níveis de energia preenchidos com elétrons.
Acima da banda de valência encontra-se a banda de condução, que contém níveis de energia vagos.
A lacuna de energia entre essas duas bandas é chamada de band gap. Representa a diferença de energia entre o topo da banda de valência e a parte inferior da banda de condução.

noções básicas de semicondutores

Energy Bands in Solids

Átomos isolados possuem níveis de energia discretos que são bem descritos pelo modelo de Bohr. E quantifica a energia de um elétron em um átomo de hidrogênio como En. Números quânticos 'n' mais altos produzem níveis de energia eletrônica menos negativos e mais próximos.
 Formação da banda:
Quando os átomos são aproximados, como em um sólido, esses níveis discretos de energia começam a se dividir devido à sobreposição de orbitais de elétrons de átomos adjacentes. Essa divisão ocorre devido ao princípio de exclusão de Pauli, que afirma que dois elétrons não podem ocupar simultaneamente o mesmo estado quântico. À medida que mais átomos são combinados, o número de níveis de energia discretos aumenta e a divisão torna-se tão fina que forma uma faixa de energia contínua. Num determinado ponto, conhecido como distância interatômica de equilíbrio, essas bandas tornam-se a banda de valência, preenchida com elétrons, e a banda de condução, que fica vazia na temperatura zero absoluto.
Bandas de energia em semicondutores:
A estrutura da banda de energia é mais complexa em semicondutores como o silício, que possui 14 elétrons. Os 10 elétrons internos ocupam os níveis de energia mais profundos e não contribuem para a ligação. Os quatro elétrons de valência restantes, que estão nas subcamadas 3s e 3p, determinam as propriedades químicas e elétricas do material. À medida que os átomos de silício formam uma rede cristalina, as subcamadas 3s e 3p se sobrepõem e formam bandas. Na distância de equilíbrio no cristal, essas bandas contêm 4N estados para a banda de valência e 4N estados para a banda de condução, onde N é o número de átomos de silício.
O tamanho do bandgap é crucial porque determina a facilidade com que os elétrons podem ser excitados para a banda de condução. Este bandgap é pequeno o suficiente para semicondutores para que a energia térmica ou a luz possam excitar os elétrons através do gap, resultando em condução elétrica.

Noções básicas de semicondutores

Os sólidos são compostos de átomos com configurações eletrônicas e níveis de energia distintos.
Quando dois átomos isolados idênticos são aproximados, ...

Education

JoVE Core

Electrical Engineering

Engenharia Elétrica

Unit 1

Unidade 1

Basics of Semiconductors

Fundamentos de Semicondutores

KEY TERMS AND CONCEPTS

TERMOS E CONCEITOS-CHAVE

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

Description

Transcript

Tags