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

真空度は、電子が材料の表面から逃げるためのエネルギーしきい値を表します。通常、半導体の伝導帯より上に位置し、電子エネルギーの基準として機能します。
半導体では、電子親和力は伝導帯の最小値と真空度との間のエネルギーギャップです。
仕事関数は、電子をフェルミ準位から真空準位に解放するために必要な最小エネルギーであり、材料に依存します。
金属は、密集したフェルミ準位状態により、約2〜5eVの高い仕事関数を示します。
半導体は、ドーピングや温度の影響を受けてフェルミ準位が変化すると、さまざまな仕事関数を示します。
接触している金属と半導体は平衡状態を達成し、接合部全体に連続的な真空レベルを作り出します。
同時に、フェルミ準位が整列するまで電荷移動が続き、半導体内でエネルギーバンドが曲がり、ショットキーバリアが形成されます。
接合部のバリア高さは、金属の仕事関数と半導体の電子親和力によって決まります。

フェルミ準位のダイナミクス

真空準位は、電子が物質表面から脱出するために必要なエネルギー閾値を表します。通常、半導体の伝導帯の上に位置し、さまざまな物質内の電子エネルギーを比較するためのベンチマークとして機能します。

半導体の電子親和力は、伝導帯の最小値と真空準位の間のエネルギーギャップを指し、半導体が追加の電子をどれだけ簡単に受け入れることができるかを決定する重要なパラメータです。
仕事関数は、電子をフェルミ準位 (電子が存在する可能性が 50% のエネルギー準位) から真空準位に移動するために必要な最小エネルギー量を表します。この値は物質によって異なり、金属は通常、フェルミ準位が密集しているため、2 ～ 5 eV の高い仕事関数を示します。
一方、半導体は、ドーピングや温度変化などの要因によって影響を受けるフェルミ準位の性質が変化するため、動的な仕事関数を示します。
金属と半導体が接触すると、両者は平衡状態を目指し、電荷移動によって接合部全体の真空レベルが均一になり、両方の材料のフェルミ準位が揃うまで続きます。これにより半導体内のエネルギーバンドが曲がり、ショットキー障壁が形成されます。ショットキー障壁は、金属半導体接合部を移動する電子に対する潜在的なエネルギー障壁です。
ショットキー障壁の高さは接合部の導電性を左右し、金属の仕事関数と半導体の電子親和力の関係によって決まります。

真空度は、電子が材料の表面から逃げるためのエネルギーしきい値を表します。通常、半導体の...

Education

JoVE Core

Electrical Engineering

電気工学

Unit 1

ユニット1

Basics of Semiconductors

半導体の基礎

KEY TERMS AND CONCEPTS

主要用語と概念

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 電気工学 は、電気回路の基本概念と回路解析テクニックを網羅したアニメーションビデオシリーズです。オペアンプと回路変換および 1 次・ 2 次回路の概念を網羅しています。実験を撮影した実写動画では、設計例や現代社会での応用が含まれており、複雑な概念も分かりやすく学習いただけます。

electrical engineering

Vacuum Level and Schottky Barriers

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