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

Carrier generation refers to the creation of electron-hole pairs.
Band-to-band generation occurs in direct band gap semiconductors via thermal excitation or photon absorption. In such semiconductors, the valence band maximum and conduction band minimum have the same momentum.
Indirect generation of carriers occurs by absorbing lattice energy from phonons and is dominant in indirect-bandgap semiconductors, which have their valence band maximum and conduction band minimum at different momenta.
In Auger generation, carriers are generated by consuming the energy of a highly energetic particle. These same high energy particles can generate multiple electron-hole pairs using the available high electric field during impact ionization.
In contrast, Recombination is the process of the annihilation of electrons and holes.
In Band-to-band recombination, the electron transitions from the conduction to the valence band, releasing a photon.
During indirect recombination, localized energy states called traps are utilized for transitions between the conduction and valence bands, releasing energy as heat.
Auger recombination involves the recombination of an electron and a hole, transferring the resulting energy to another carrier.

carrier generation and recombination

Carrier generation is the process by which electron-hole pairs (EHPs) are created within the semiconductor. In direct-bandgap semiconductors, such as gallium arsenide (GaAs), this occurs efficiently when energy absorption prompts valence electrons to leap into the conduction band, leaving behind holes.
This process is given by the generation rate G and is efficient due to the conservation of momentum between the valence band maximum and conduction band minimum.
Indirect generation involves an intermediate step and is typical in indirect-bandgap semiconductors like silicon (Si). Indirect-bandgap semiconductors require additional momentum from phonons, making carrier generation less efficient. Auger generation and impact ionization produce multiple EHPs in high-energy environments, such as strong electric fields.
Recombination is the process that reduces the number of free-charge carriers. Direct band-to-band recombination occurs in semiconductors like Gallium Arsenide, where electrons and holes recombine directly without intermediary states.
The recombination rate for an n-type semiconductor, where electrons are the majority carriers, is given by:
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Where B is the recombination coefficient, and n and p, are the concentrations of electrons and holes, respectively. Indirect recombination involves traps: localized energy states within the bandgap. Carriers are temporarily captured by these states and then recombine, releasing energy as heat, a non-radiative process.
The equilibrium between generation and recombination is described by:
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In non-equilibrium conditions, excess carriers cause a net recombination rate U, which tends towards restoring equilibrium. At low-level injection, where the minority carrier concentration (&#916;p) is significantly lower than the majority carrier concentration, the rate is:
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Carrier generation and recombination rates are balanced at thermal equilibrium. However, when external forces such as light or electrical fields disturb this equilibrium, the semiconductor enters a non-equilibrium state. The dynamics of returning to equilibrium involve complex interactions between these generation and recombination mechanisms.

Carrier Generation and Recombination

Carrier generation refers to the creation of electron-hole pairs.
Band-to-band generation occurs in direct band gap semiconductors via thermal excitation ...

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Electrical Engineering

Unit 1

Basics of Semiconductors

KEY TERMS AND CONCEPTS

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

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Semiconductor Charge Carriers: Creation and Loss

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