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

载流子产生是指电子-空穴对的生成。
带间跃迁产生在直接带隙半导体中通过热激发或光子吸收发生。在这种半导体中，价带顶和导带底具有相同的动量。
载流子的间接产生是通过吸收声子的晶格能量实现的，在间接带隙半导体中占主导地位，这类半导体的价带顶和导带底位于不同的动量处。
在俄歇产生过程中，载流子通过消耗高能粒子的能量而产生。这些高能粒子在碰撞电离过程中，可利用存在的强电场产生多个电子-空穴对。
相比之下，复合是指电子与空穴相互湮灭的过程。
在带间复合过程中，电子从导带跃迁至价带，释放出一个光子。
在间接复合过程中，位于导带与价带之间的局域化能态（称为陷阱）被用于实现能带间的跃迁，并以热的形式释放能量。
俄歇复合涉及电子与空穴的复合，将产生的能量传递给另一个载流子。

半导体基础知识

Carrier Generation and Recombination

载流子的生成是电子-空穴对（EHP）在半导体中生成的过程。在砷化镓（GaAs）等直接带隙半导体中，当能量吸收能够促使价电子跃迁到导带并留下空穴时，这种情况就会发生。
由于价带最大值和导带最小值之间存在动量守恒，因此这一过可以用生成速率 G 来进行表示，并且这是高效的。
间接生成通常会涉及到一个中间步骤，在硅（Si）等间接带隙半导体中是很常见的。间接带隙半导体需要声子来提供额外的动量，并且还会使得载流子生成的效率降低。在强电场等高能环境中，俄歇电子的生成和碰撞电离会生成多个电子-空穴对。
重组是减少自由电荷中载流子数量的过程。直接的带间重组通常会发生在砷化镓等半导体中，其中电子和空穴会在没有中间态的情况下进行直接重组。
电子是多数载流子中的 n 型半导体，重组率可以由以下公式计算得出：
<img alt="Equation 1" src="/files/ftp_upload/15154/15154_Equation_1.svg" style="text-align: center; height: 20px;">
其中 B 为重组系数，n 和 p 分别为电子和空穴的浓度。间接重组通常还会存在陷阱：带隙内的局部能态。载流子会被这些状态暂时捕获，然后再进行重新结合，并以热量的形式来释放能量，这是一个非辐射过程。
生成和重组之间的平衡关系可以由以下公式来进行表示：
<img alt="Equation 2" src="/files/ftp_upload/15154/15154_Equation_2.svg" style="text-align: center; height: 20px;">
在非平衡条件下，过量载流子会导致净重组率 U 趋向于恢复平衡的状态。在低水平注入时，少数载流子浓度（Δp）会明显低于多数载流子浓度，此时的重组速率为： 
<img alt="Equation 3" src="/files/ftp_upload/15154/15154_Equation_3.svg" style="text-align: center; height: 18px;">
在热平衡状态下，载流子的生成和重组速率是平衡的。然而，当光或电场等外力来干扰这种平衡时，半导体就会进入到非平衡状态。恢复平衡的动力学过程通常会涉及到这些生成和重组机制之间的复杂相互作用。

载流子产生是指电子-空穴对的形成。  
带间产生通过热激发在直接带隙半导体中发生……

Education

教育

JoVE Core

Electrical Engineering

电气工程

Unit 1

第一单元

Basics of Semiconductors

KEY TERMS AND CONCEPTS

关键术语与概念

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...

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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...

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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 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

Semiconductor Charge Carriers: Creation and Loss

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