Lifetime of the minority carriers

Good electrical properties of the epilayer (collection of the photogenerated carriers and generation of the current are linked to the diffusion length or lifetime of the minority carriers)... 

Khan and Bockris167 presented a model which accounts for charge transfer kinetics and surface recombination as well as potential drop in the Helmholtz layer. In their paper, they estimated the transit time across the space charge region and the lifetime of the minority carrier and showed that it is reasonable to neglect the... 

Obtaining information on a material s electronic band structure (related to the fundamental band gap) and analysis of luminescence centers Measurements of the dopant concentration and of the minority carrier diffusion length and lifetime... 

The increased lifetime of photogenerated minority carriers can be measured experimentally. This is shown for a single-crystal ZnO-electrode (Fig. 22). Both the stationary PMC peak and the potential-dependent lifetime in the depletion region, measured with transient microwave conductivity techniques are plotted.25 It is seen that the stationary PMC peak coincides with a peak in the lifetime of minority carriers. This... 

Other indirect methods for measuring lifetimes often involve device structures such as p-n junctions. The electron-beam-induced current (EBIC) technique, for example, measures the increase injunction current as an impinging electron beam moves close to the junction, i.e., within a few minority-carrier diffusion lengths. If a diffusion constant can be estimated, say by knowledge of the minority-carrier mobility, then the minority-carrier lifetime can be calculated. However, SI GaAs does not form good junctions, so such methods are really not applicable. 

The pn-junction formed between the photo-conductive detector and the substrate enhances the photo-conductive signal by essentially isolating the photogenerated minority carriers in the photo-conductive detector from the majority carriers. The minority carriers are swept across the junction while the majority carriers are allowed to flow in the photoconductive detector. This inhibits the recombination rate and extends the lifetime of the majority carriers. 

The time-resolved microwave reflectivity (TRMR) techiuque is well established for contactless characterisation of minority-carrier lifetimes in semiconductors. It can be applied to map surface recombination in this case the sample is moved by an X-Y stage to allow spatially resolved measurement of the minority-carrier lifetime. 

One may also interpret Ldiff as the diffusion length of the redox molecules. In principle this relation is identical to the diffusion length of minority carriers in semiconductor crystals (L = (Dt), see Eq. 2.26). The only difference is that the lifetime r of the minority carriers in a semiconductor is a material constant, whereas Ldiff depends on the time. In Fig. 7.6 the concentration profile in terms of Ldjff is illustrated. 

The actual compensation in a material is more complex than a simple balance between a majority impurity and a minority impurity as the material usually contains a combination of residual impurities, dopant and deep centres, whose concentrations must be estimated to determine the actual degree of compensation in the material. As mentioned before, compensation of the majority impurities by adding opposite type dopant leaves in the material charged ions, which reduce the lifetime of the free carriers. When the lifetime of the carriers in a given pure material is known, a lifetime measurement of an unknown sample of this material can determine the degree of compensation of the sample. 

Annealing in Gallium Arsenide. Gallium arsenide has a greater variety of defect interactions than silicon. Also, most gallium arsenide devices are based on majority carrier transport. This decreases the importance of the minority carrier lifetime. Therefore carrier activation is the primary purpose of the annealing process. 

In contrast to photoconductivity, the photovoltaic effect depends largely upon the minority carrier lifetime. This is because the presence of both the photoexcited electron and photoexdted hole is required for the intrinsic effect to be observed. Because the minority carrier lifetime is usually shorter than the majority carrier one, the photovoltaic signal terminates when the minority carrier recombines. The time dependent photosignal is given by (2.1S) and (2.16), but the appropriate lifetime is the minority carrier one. For this reason, photovoltaic detectors are usually faster than photoconductive ones made from the same material. 

In the limit where there is no nonradiative recombination, which means that 1/Tnonrad IS zero, the radiative efficiency becomes unity. The lifetime of excess minority carriers can be obtained by measuring the dynamic behaviors of optical emissions involved using time-resolved photoluminescence (TRPL). 

The situation is very different in indirect gap materials where phonons must be involved to conserve momentum. Radiative recombination is inefficient, resulting in long lifetimes. The minority carrier lifetimes in Si reach many ms, again in tire absence of defects. It should be noted tliat long minority carrier lifetimes imply long diffusion lengtlis. Minority carrier lifetime can be used as a convenient quality benchmark of a semiconductor. 

The equihbtium lever relation, np = can be regarded from a chemical kinetics perspective as the result of a balance between the generation and recombination of electrons and holes (21). In extrinsic semiconductors recombination is assisted by chemical defects, such as transition metals, which introduce new energy levels in the energy gap. The recombination rate in extrinsic semiconductors is limited by the lifetime of minority carriers which, according to the equihbtium lever relation, have much lower concentrations than majority carriers. Thus, for a -type semiconductor where electrons are the minority carrier, the recombination rate is /S n/z. An = n — is the increase of the electron concentration over its value in thermal equihbtium, and... 

This relation shows that the lifetime of PMC transients indeed follows the potential dependence of the stationary PMC signal as found in the experiment shown in Fig. 22. However, the lifetime decreases with increasingly positive electrode potential. This decrease with increasing positive potentials may be understood intuitively the higher the minority carrier extraction (via the photocurrent), the shorter the effective lifetime... 

Contamination of silicon wafers by heavy metals is a major cause of low yields in the manufacture of electronic devices. Concentrations in the order of 1011 cm-3 [Ha2] are sufficient to affect the device performance, because impurity atoms constitute recombination centers for minority carriers and thereby reduce their lifetime [Scl7]. In addition, precipitates caused by contaminants may affect gate oxide quality. Note that a contamination of 1011 cnT3 corresponds to a pinhead of iron (1 mm3) dissolved in a swimming pool of silicon (850 m3). Such minute contamination levels are far below the detection limit of the standard analytical techniques used in chemistry. The best way to detect such traces of contaminants is to measure the induced change in electronic properties itself, such as the oxide defect density or the minority carrier lifetime, respectively diffusion length. 

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