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

Уровень вакуума представляет собой энергетический порог для того, чтобы электрон покинул поверхность материала. Обычно он находится над зоной проводимости полупроводника и служит эталоном для энергий электронов.
В полупроводниках сродство к электронам — это энергетический разрыв между минимумом зоны проводимости и уровнем вакуума.
Рабочая функция представляет собой минимальную энергию, необходимую для высвобождения электрона с уровня Ферми на уровень вакуума, и зависит от материала.
Металлы демонстрируют высокую рабочую функцию, от 2 до 5 эВ, из-за их плотных состояний уровня Ферми.
Полупроводники демонстрируют переменные рабочие функции при изменении их уровня Ферми под влиянием легирования и температуры.
Металл и полупроводник в контакте достигают равновесия, создавая непрерывный уровень вакуума на переходе.
В то же время перенос заряда продолжается до тех пор, пока уровни Ферми не выровняются, вызывая изгиб энергетической полосы в полупроводнике и образование барьера Шоттки.
Высота барьера перехода определяется рабочей функцией металла и электронным сродством полупроводника.

Динамика уровня Ферми

Fermi Level Dynamics

Уровень вакуума обозначает энергетический порог, необходимый для того, чтобы электрон покинул поверхность материала. Обычно он расположен над зоной проводимости полупроводника и служит эталоном для сравнения энергий электронов в различных материалах.
Сродство к электрону в полупроводниках относится к энергетической щели между минимумом зоны проводимости проводника и уровнем вакуума и является критическим параметром, определяющим, насколько легко полупроводник может принимать дополнительные электроны.
Работа выхода представляет собой наименьшее количество энергии, необходимое для перемещения электрона с уровня Ферми (энергетического уровня, на котором вероятность присутствия электрона составляет 50%), на уровень вакуума. Это значение варьируется в зависимости от материала: металлы обычно демонстрируют высокие значения работы выхода в диапазоне от 2 до 5 эВ, что объясняется густонаселенностью их уровней Ферми.
Полупроводники, с другой стороны, демонстрируют динамические рабочие функции из-за различной природы их уровней Ферми, на которые влияют такие факторы, как легирование и изменения температуры.
Когда металл и полупроводник вступают в контакт, они стремятся к равновесию, что приводит к равномерному уровню вакуума на переходе за счет переноса заряда, продолжающемуся до тех пор, пока уровни Ферми обоих материалов не выровняются. Это вызывает изгиб энергетической зоны внутри полупроводника, что приводит к созданию барьера Шоттки — потенциального энергетического барьера для электронов, движущихся через переход металл-полупроводник.
Высота барьера Шоттки определяет проводимость перехода и определяется соотношением между работой выхода металла и сродством полупроводника к электрону.

Уровень вакуума представляет собой энергетический порог для того, чтобы электрон покинул поверхность материала. Обычно он располагается над ...

Education

JoVE Core

Electrical Engineering

Электротехника

Unit 1

Раздел 1

Basics of Semiconductors

Основы полупроводников

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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

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