Name: semiconductor charge carriers: creation and loss
Uploaded: 2024-07-08T13:20:08.000Z
Duration: 1 min 22 s
Channel: JoVE

キャリア生成とは、電子と正孔のペアを作成することを指します。
バンド間生成は、直接バンドギャップ半導体で熱励起または光子吸収を介して行われます。このような半導体では、価電子帯の最大値と伝導帯の最小値は同じ運動量を持ちます。
キャリアの間接生成は、フォノンから格子エネルギーを吸収することによって発生し、異なるモーメントで価電子バンドの最大と伝導バンドの最小を持つ間接バンドギャップ半導体で支配的です。
オージェ世代では、高エネルギーの粒子のエネルギーを消費してキャリアが生成されます。これらの同じ高エネルギー粒子は、衝撃イオン化中に利用可能な高電場を使用して複数の電子-正孔ペアを生成できます。
対照的に、組換えは電子と正孔を消滅させるプロセスです。
バンド間再結合では、電子が伝導から価電子バンドに遷移し、光子を放出します。
間接的な再結合では、トラップと呼ばれる局所的なエネルギー状態が伝導バンドと価電子バンドの間の遷移に利用され、エネルギーが熱として放出されます。
オージェ再結合には、電子と正孔の再結合が含まれ、結果として生じるエネルギーを別のキャリアに伝達します。

半導体の基礎

Carrier Generation and Recombination

キャリア生成は、半導体内で電子正孔対 (EHP) が生成されるプロセスです。ガリウムヒ素 (GaAs) などの直接バンドギャップ半導体では、エネルギー吸収によって価電子が伝導帯に飛び込み、正孔が残るときに、キャリア生成が効率的に発生します。

このプロセスは生成率 G で表され、価電子帯の最大値と伝導帯の最小値の間の運動量が保存されるため効率的です。

間接生成には中間ステップが含まれ、シリコン (Si) などの間接バンドギャップ半導体で一般的です。間接バンドギャップ半導体ではフォノンから追加の運動量が必要になるため、キャリア生成の効率が低下します。オージェ生成と衝撃イオン化により、高エネルギー環境 (強電界など) で複数の EHP が生成されます。

再結合は、自由電荷キャリアの数を減らすプロセスです。バンド間の直接再結合は、ガリウムヒ素などの半導体で発生し、電子と正孔は中間状態を介さずに直接再結合します。

電子が多数キャリアである n 型半導体の再結合率は、次の式で与えられます。

<img alt="Equation 1" src="/files/ftp_upload/15154/15154_Equation_1.svg" style="height: 20px;">

ここで、B は再結合係数、n と p はそれぞれ電子と正孔の濃度です。間接再結合には、トラップ (バンドギャップ内の局所的なエネルギー状態) が関係します。キャリアはこれらの状態によって一時的に捕獲され、その後再結合して、非放射プロセスとして熱としてエネルギーを放出します。

生成と再結合の間の平衡は、次の式で表されます。

<img alt="Equation 2" src="/files/ftp_upload/15154/15154_Equation_2.svg" style="height: 20px;">

非平衡状態では、過剰キャリアによって再結合率 U が発生し、平衡を回復する傾向があります。少数キャリア濃度 (Δp) が多数キャリア濃度よりも大幅に低い低レベル注入では、速度は次のようになります。

<img alt="Equation 3" src="/files/ftp_upload/15154/15154_Equation_3.svg" style="height: 18px;">

キャリア生成速度と再結合速度は熱平衡状態でバランスが取れています。ただし、光や電界などの外部力がこの平衡を乱すと、半導体は非平衡状態になります。平衡に戻るダイナミクスには、これらの生成メカニズムと再結合メカニズムの間の複雑な相互作用が関係します。

キャリア生成とは、電子と正孔のペアを作成することを指します。
バンドツーバンドの生成は、直接バンドギャップ半導体で熱励起を介して発生します...

Education

JoVE Core

Electrical Engineering

電気工学

Unit 1

ユニット1

Basics of Semiconductors

KEY TERMS AND CONCEPTS

主要用語と概念

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 conduction band. This works well when momentum is conserved between the valence band maximum and the conduction band minimum. Indirect generation is different and is common in indirect-bandgap semiconductors like silicon. It uses an intermediate step and needs extra momentum from phonons, so it is less efficient.

Some high-energy conditions can create more than one EHP. Auger generation and impact ionization both produce multiple electron-hole pairs, especially in strong electric fields. These processes show how carrier populations can rise quickly under extreme energy input.

Recombination removes free charge carriers. In direct band-to-band recombination, seen in materials like gallium arsenide, electrons and holes recombine directly without a middle state. For an n-type semiconductor, where electrons are the majority carriers, the recombination rate depends on the recombination coefficient B and on the electron and hole concentrations, n and p.

Indirect recombination uses traps, which are localized energy states inside the bandgap. Carriers can be captured by these states and then recombine later. This process releases energy as heat, so it is non-radiative.

At thermal equilibrium, generation and recombination are balanced. When light or an electric field pushes the semiconductor out of equilibrium, excess carriers create a net recombination rate U that works to restore balance. At low-level injection, where the minority carrier change Δp is much smaller than the majority carrier concentration, the recombination rate follows a simpler form. These interactions describe how semiconductors return to equilibrium after disturbance.

Semiconductor Charge Carriers: Creation and Loss

semiconductor-charge-carriers-creation-and-loss

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

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 semiconductors create electron-hole pairs, lose carriers through recombination, and return to equilibrium after light or electric field changes.

Semiconductor Charge Carriers: Creation and Loss | 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

Semiconductor Charge Carriers: Creation and Loss

Description

Transcript