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

La génération de porteurs fait référence à la création de paires électron-trou.
La génération de bande à bande se produit dans les semi-conducteurs à bande interdite directe par excitation thermique ou absorption de photons. Dans de tels semi-conducteurs, la bande de valence maximale et la bande de conduction minimale ont la même quantité de mouvement.
La génération indirecte de porteurs se produit en absorbant l’énergie du réseau des phonons et est dominante dans les semi-conducteurs à bande interdite indirecte, qui ont leur bande de valence maximale et leur bande de conduction minimale à différents moments.
Dans la génération à la perrière, les porteurs sont générés en consommant l’énergie d’une particule hautement énergétique. Ces mêmes particules à haute énergie peuvent générer plusieurs paires de trous d’électrons en utilisant le champ électrique élevé disponible lors de l’ionisation d’impact.
En revanche, la recombinaison est le processus d’annihilation des électrons et des trous.
Dans la recombinaison de bande à bande, l’électron passe de la conduction à la bande de valence, libérant un photon.
Lors de la recombinaison indirecte, des états d’énergie localisés appelés pièges sont utilisés pour les transitions entre les bandes de conduction et de valence, libérant de l’énergie sous forme de chaleur.
La recombinaison à l’autel implique la recombinaison d’un électron et d’un trou, transférant l’énergie résultante à un autre porteur.

bases des semi-conducteurs

Carrier Generation and Recombination

La génération de porteurs est le processus par lequel des paires électron-trou (EHP) sont créées dans le semi-conducteur. Dans les semi-conducteurs à bande interdite directe, tels que l'arséniure de gallium (GaAs), cela se produit efficacement lorsque l'absorption d'énergie incite les électrons de valence à sauter dans la bande de conduction, laissant derrière eux des trous.
Ce processus est donné par le taux de génération G et est efficace en raison de la conservation de la quantité de mouvement entre le maximum de la bande de valence et le minimum de la bande de conduction.
La génération indirecte implique une étape intermédiaire et est typique des semi-conducteurs à bande interdite indirecte comme le silicium (Si). Les semi-conducteurs à bande interdite indirecte nécessitent une impulsion supplémentaire de la part des phonons, ce qui rend la génération de porteurs moins efficace. La génération Auger et l'ionisation par impact produisent de multiples EHP dans des environnements à haute énergie, tels que des champs électriques puissants.
La recombinaison est le processus qui réduit le nombre de porteurs libres. La recombinaison directe bande à bande se produit dans les semi-conducteurs comme l'arséniure de gallium, où les électrons et les trous se recombinent directement sans états intermédiaires.
Le taux de recombinaison pour un semi-conducteur de type n, où les électrons sont les porteurs majoritaires, est donné par:
<img alt="Equation 1" src="/files/ftp_upload/15154/15154_Equation_1.svg" style="height: 20px;">
Où B est le coefficient de recombinaison, et n et p sont respectivement les concentrations d’électrons et de trous. La recombinaison indirecte implique des pièges: des états énergétiques localisés dans la bande interdite. Les porteurs sont temporairement capturés par ces états puis se recombinent, libérant de l'énergie sous forme de chaleur, un processus non radiatif.
L'équilibre entre génération et recombinaison est décrit par:
<img alt="Equation 2" src="/files/ftp_upload/15154/15154_Equation_2.svg" style="height: 20px;">
Dans des conditions de non-équilibre, les porteurs en excès provoquent un taux de recombinaison net U, qui tend à rétablir l'équilibre. Lors d'une injection à faible niveau, où la concentration en porteurs minoritaires (Δp) est nettement inférieure à la concentration en porteurs majoritaires, le taux est:
<img alt="Equation 3" src="/files/ftp_upload/15154/15154_Equation_3.svg" style="height: 18px;">
Les taux de génération de porteurs et de recombinaison sont équilibrés à l’équilibre thermique. Cependant, lorsque des forces externes telles que la lumière ou les champs électriques perturbent cet équilibre, le semi-conducteur entre dans un état de non-équilibre. La dynamique de retour à l’équilibre implique des interactions complexes entre ces mécanismes de génération et de recombinaison.

Bases des semi-conducteurs

La génération de porteurs fait référence à la création de paires électron-trou.
La génération de bande à bande se produit dans les semi-conducteurs à bande interdite directe via l’excitation thermique ...

Education

Enseignement

JoVE Core

Electrical Engineering

Génie électrique

Unit 1

Unité 1

Basics of Semiconductors

Les bases des semi-conducteurs

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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 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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