The recombination in semiconductors eliminates free electrons and holes, setting up equilibrium in steady state.
Recombination in semiconductors can be categorized into two groups: radiative and non-radiative recombination.
Non-radiative recombination mechanisms in semiconductors are auger recombination, recombination by defects, and surface recombination.
Semiconductors are materials that are neither conductors nor insulators
Humans are able to stay well-connected as a result of innovations in the field of semiconductors, which date back several decades. Semiconductor technology has evolved from transistors, integrated circuits, and microprocessors to present-day, advanced-level computing gadgets that enable smartphones, personal computers, the internet, and even social networking.
Semiconductors are materials with conductivity greater than insulators and less than conductors. The presence of free electrons and holes in semiconductors are responsible for the controllable conductivity of the material, and the density of the free charge carriers influences the electron-hole recombination. Whether they are being used in the field of luminescence, photoconductivity, or semiconductor control devices, the recombination in semiconductors is of great concern to engineers. In this article, we will look at the different recombination mechanisms in semiconductors.
How Do Semiconductors Work?
Semiconductors are materials that are neither conductors nor insulators. They are characterized by two types of charge carriers, namely electrons in the conduction band and holes in the valence band. The conductivity property of semiconductors is exhibited when electrons from the conduction band jump to the valence band. At room temperature, the energy requirements for electrons to shift from the conduction band to the valence band are rarely met, which is why the conductivity of semiconductors is less than conductors and greater than insulators.
The lower the conductivity, the higher the resistivity and resistance of semiconductors. The resistance of semiconductors has a negative temperature coefficient and decreases with temperature, which is in contrast to conductors. Therefore, as temperature increases, semiconductors behave similarly to conductors.
The conductivity of semiconductors can be varied by increasing the number of free electrons and holes. As the density of the free charge carriers in the semiconductors increases, it influences the electron-hole recombination that occurs with the electron shift from the conduction band to the valence band. Next, we will look at recombination in semiconductors.
Recombination in Semiconductors
The conduction and valence band in semiconductors are separated by an energy gap. The electron in the conduction band is in a meta-stable state with higher energy compared to the valence band energy level. It can become stable in the valence band, as there is less energy there. When the electron in the conduction band surpasses the energy gap and reaches the valence band, it becomes stable and occupies the position of a hole in the valence band. With this action, the electron and hole disappear. This is called electron-hole pair recombination or simple recombination in semiconductors. Recombination in semiconductors eliminates the free electrons and holes, thus creating an equilibrium in steady state.
Types of Recombination in Semiconductors
There are different types of recombination mechanisms in a semiconductor. According to the application of semiconductors and the conditions prevailing, one of the recombination mechanisms becomes active and more efficient than other types. Recombination in semiconductors can be categorized into two groups: radiative and non-radiative recombination.
Radiative recombination occurs when an electron from the conduction band recombines with a hole in the valence band. As a result of electron-hole recombination, a photon is emitted. Radiative recombination is the radiative transition of the electron from the conduction band to the valence band, which involves optical processes: spontaneous emission, absorption or gain, and stimulated emission. This kind of recombination is prominent in direct-bandgap semiconductor lasers.
In non-radiative recombination, electron-hole recombination takes place non-radiatively. Non-radiative recombination mechanisms in semiconductors are:
- Auger recombination: In Auger recombination, the energy released during electron-hole recombination is transferred to excite another electron in the same band to higher energy levels. The excited electron achieves thermal equilibrium by dissipating the energy to lattice vibrations or phonons. Auger recombination is predominant non-radiative recombination in long-wavelength semiconductor lasers. As the doping density of the semiconductor increases, the Auger recombination lifetime decreases.
- Recombination through defects: Recombination through defects is a two-step process where electrons recombine with holes through defect energy levels in the bandgap. Defects are introduced in the crystal lattice intentionally or unintentionally. The defects in the semiconductor crystal lattice establish a forbidden region and an electron gets trapped by the energy state in that region. If a hole acquires the same energy state before the electron gets re-emitted into the conduction band, electron-hole recombination takes place. This type of recombination is also called Shockley-Read-Hall (SRH) recombination. This type of recombination is utilized in injection lasers.
- Surface recombination: A strong perturbation of crystal lattice can be called a surface. The surfaces in semiconductor crystal lattices create dangling bonds that are exposed to the ambient. Such exposed surfaces absorb impurities from the ambient and become localized centers of defects. The high concentration of defects in surfaces increases the probability of non-radiative recombination. Cleaved facets in injection lasers are introduced for enhancing recombination in semiconductors through surface recombination.
The type of recombination dominating direct bandgap semiconductors differs from that in indirect bandgap semiconductors. According to the application field, suitable recombination in semiconductors is activated by creating favorable conditions in the crystal lattice.
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