Carrier lifetime, mobility and diffusion length

Carrier lifetime refers to the average survival time of non-equilibrium carriers before recombination, which is the abbreviation for non-equilibrium carrier lifetime. In the case of thermal equilibrium, the generation rate of electrons and holes is equal to the recombination rate, and the concentration of the two maintains a balance. Under external conditions (such as light), additional non-equilibrium carriers will be generated, namely electron-hole pairs; after the external conditions are removed, because the recombination rate is greater than the production rate, the non-equilibrium carriers will gradually recombine and disappear, returning to the thermal equilibrium state. The decay law of non-equilibrium carrier concentration with time generally obeys an exponential relationship. In semiconductor devices, the non-equilibrium minority carrier lifetime is referred to as the minority carrier lifetime.

The recombination process can be roughly divided into two types: the direct transition of electrons between the conduction band and the valence band causes the disappearance of a pair of electrons and holes, which is called direct recombination; the electron-hole pair may also recombine through the energy level (recombination center) in the band gap, which is called indirect recombination. The minority carrier lifetime of each semiconductor is not a fixed value, it will vary greatly depending on the chemical composition and crystal structure. Mobility refers to the average drift speed of carriers (electrons and holes) under the action of a unit electric field. That is, a measure of the speed of the carrier moving under the action of an electric field. The faster the movement, the greater the mobility; the slower the movement, the smaller the mobility. In the same semiconductor material, different types of carriers have different mobility. Generally, the mobility of electrons is higher than that of holes. Under the action of a constant electric field, the average drift speed of carriers can only take a certain value, which means that the carriers in the semiconductor are not free from any resistance and are continuously accelerated. In fact, in the process of thermal movement, carriers continuously collide with crystal lattices, impurities, defects, etc., and change their moving direction irregularly, that is, scattering occurs. Inorganic crystals are not ideal crystals, while organic semiconductors are essentially amorphous, so there are lattice scattering, ionized impurity scattering, etc., so the carrier mobility can only have a certain value.

Because the minority carrier has a certain lifetime, that is, the minority carrier lifetime. Therefore, in the process of diffusion, the minority carriers will inevitably recombine while diffusing. After a certain distance, the minority carriers will disappear, and the distance covered is the so-called diffusion length.

Semiconductor light absorption. The absorption of light by a semiconductor is mainly determined by the forbidden band width of the semiconductor material. For a semiconductor with a certain forbidden band width, low-energy photons with a small frequency, the semiconductor has a small degree of light absorption to it, and most of the light can penetrate; as the frequency becomes higher, the ability to absorb light increases sharply. In fact, the light absorption of semiconductors is determined by various factors. Here, only the transition between electronic energy bands used in solar cells is considered. Generally, the wider the band gap, the smaller the absorption coefficient for a certain wavelength. In addition, the absorption of light also depends on the density of states of the conduction band and valence band.

When contact between different types of semiconductors (to form a PN junction) or between a semiconductor and a metal, diffusion occurs due to the difference in electron (or hole) concentration, forming a barrier at the contact, so this type of contact has unidirectional conductivity. Utilizing the unidirectional conductivity of PN junctions, semiconductor devices with different functions, such as diodes, triodes, and thyristors, can be made. PN junction also has many other important basic properties, including current-voltage characteristics, capacitance effect, tunneling effect, avalanche effect, switching characteristics, and photovoltaic effect. Among them, the current-voltage characteristic is also called the rectification characteristic or the volt-ampere characteristic, which is the most basic characteristic of the PN junction, and the solar photovoltaic conversion is the photovoltaic effect generated by the built-in electric field of the PN junction.

Carrier lifetime, mobility and diffusion length
Holes and electrons

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