It has been developed a new theoretical model for the photosensitivity of porous silicon which takes into account the
recombination of photocarriers at the surfaces of spherical pores. An expression for the semiconductor photoconductivity
has been derived under assumption of uniform generation of photocarriers and diffusion character of their movement. The
photosensitivity of porous silicon has been shown to strongly depend on the velocity of photocarriers recombination at the
pore's surfaces, radius of pores and average distance between pores.
The paper theoretically investigates the peculiarities of graded-gap photodiodes in which the band gap linearly increases from the metallurgical boundary of p-n junction. On the basis of derived analytical expression for the photocurrent the spectral distribution of quantum efficiency of photoconversion and its dependence on the band gap gradient and surface recombination velocity are analysed. Numerical calculations are carried out as applied to a thin layer photodiode based on the CdHgTe graded-gap solid solution. The advantages of this type of graded-gap photodiodes over their homoband analogues are discussed.
The paper theoretically investigates the possibility of formation of negative differential resistance (NDR) region on the current-voltage characteristic (CVC) reverse branch of a diode with double layer heterojunction (DLHJ) in the diode’s base. The NDR formation is caused by decreasing the carrier thermal generation in the narrow gap part of the base with the thickness of the order of the Debye screening length at increase of the reverse bias. It is shown that the most favourable conditions for manifestation of the NDR are realized in the case when the carrier lifetime in the base region is determined by the Auger processes.
The paper theoretically examines the photocarrier extraction effect taking place in thin variable-gap photoresistors with linear profile of energy gap and Ohmic contacts. monochromatic light. Taking into account quasielectric built-in fields occurring in variable-gap semiconductors, it has been deduced analytical expressions for spatial dependencies of photocarriers, as well as for photocurrent in thin variable-gap photoresistors. When the applied external field has the direction opposite to quasi-electric one, the latter promotes pulling the carriers towards the Ohmic contact where they recombine with an infinite rate what leads to decrease in the carrier effective lifetime. In the case of opposite direction of external field the quasielectric field counteracts moving the photocarriers forward the Ohmic contact what gives rise to the increase in carrier effective lifetime. In these conditions the negative differential photoconductivity can arise with the magnitude of maximum photoconductivity considerably exceeding that of uniform semiconductor layers.
The paper theoretically examines the peculiarities of photoconductivity of thin graded-band-gap layers with linear profile of energy gap and Ohmic contacts. It is analyzed in detail the cases of illumination of the layers from wide gap side by polychromatic light with spatially constant carrier photogeneration function and by strongly absorbed monochromatic light. Carrier extraction effect is shown to cause strong nonmonotonous field dependence of effective lifetime in the case of thin layers. The maximum of carrier effective lifetime is reached when the total force acting on photocarriers is equal (in the case of bulk photoexcitation) or close (in the case of local photogeneration function) to zero. Such a field dependence of effective lifetime can lead to formation of negative differential photoconductivity of N- type. The photo-current peak occurs at the external field strength approximately equals to that of quasielectric field in the case of layer thickness comparable with the minority carrier length and at the substantially stronger fields in the case of very thin films.
An analytical expression for dark voltage-current characteristics (VCC) of double layer heterojunction (DLHJ) photodiode. It is shown that spatial inhomogeneity of energy band gap in photodiode base region allows to substantially increase the reverse current connected with thermal generation of carriers both in the base region and at the contacts. In the case when wide gap layer is remote from the metallurgical edge of p-n junction at the distance comparable with the thickness of the space charge region (SCR) a portion of negative differential resistance (NDR) may appear on the reverse branch of VCC.
The paper theoretically investigates the peculiarities of carrier redistribution taking place in periodic semiconductor variable-gap multilayer structures during current transfer in the direction perpendicular to the layers. It is studied in detail the cases of homogeneously doped symmetric and asymmetric structures with linear coordinate dependence of energy gap. Current transport in variable multilayer structures is shown to accompany with the substantial spatial redistribution of minority carriers resulting in deviation of carrier concentration from its equilibrium value. At strong fields the practically constant carrier concentration is set up in almost the whole volume of structure for the exception of thin regions in the vicinity of the interfaces. In the case of asymmetric structures carrier redistribution can give rise to changing the total number of carriers what is displayed on current- voltage characteristics.
By means of computer modeling and numerical analysis it has been shown that redistribution of charge carriers in non- homogeneous semiconductor structures has significant contribution to the Hall effect and conductivity of these samples and produce incorrect data when electrophysical measurements are used for determining the semiconductor layer parameters. For semiconductor structures on the basis of HgCdTe solid alloys this effect may manifest in the case of narrow gap layer with p-conductivity on the n-type wide gap base. Such structures are of interest for manufacturing IR photodetectors.SO for their characterization the differential Hall and conductivity method should be used with great caution.
New expression for the concentration dependence of diffusion coefficient of ionized impurity in semiconductors which is valid the whole range of carrier degeneracy is derived. It is shown that due to reduction of the impurity field screening by mobile carriers at degenerate conditions the diffusion coefficient is a monotonously increasing function of the impurity concentration. Taking the band gap narrowing into account is found to result in a reduction of the diffusion coefficient in comparison with that corresponding to the case of unperturbed band structure, and a decreasing concentration dependence of the diffusion coefficient can be realized at relatively low impurity concentrations.
Influence of interfacial recombination of charge carriers on photoconductivity of graded-band-gap semiconductor structures with extrinsic type of a conductivity and linear spatial dependence of band-gap is investigated theoretically. Effects of illumination of the structures from wide band gap side by strongly absorbed monochromatic light and by polychromatic light which uniformly generates photocarriers in the sample have been analyzed. Spectral dependences of the photoconductivity of the graded-band-gap semiconductor structures illuminated by monochromatic light are shown to have pronounced minimum occurring at photon energy equal to band-gap energy at the interface. Under conditions of coordinate-independent photocarrier generation rate the influence of interfacial recombination on the graded-band-gap semiconductor structure photosensitivity displays most substantially in the case when the sample thickness is of the order of minority carrier diffusion length.
Peculiarities of photovoltaic effect (PVE) in graded-band- gap (GBG) layers with intrinsic type of conductivity and linear coordinate dependence of energy gap in the conditions of layer illumination by strongly absorbed monochromatic light are investigated theoretically. It is established that sign reversal of the photoEMF takes place on the spectral dependencies of PVE in the case when light is incident on the wide-gap of the layers. The dependence of PVE reversal point on the energy gap gradient and surface recombination velocities of charge carriers is analyzed. The photoEMF of GBG layers illuminated from narrow-gap side is shown to essentially exceed the photovoltage of uniform samples with the same energy gap as at narrow-gap side of GBG layers.
Characteristic features of the photoelectric amplification of graded-band-gap photoresistors which energy gap is increased linearly towards contacts are considered theoretically. It has been shown that nonmonotonic field dependence of the photoelectric amplification coefficient is realized in such photoresistors. Maximum value the photoelectric amplification coefficient being increased under the increase of energy gap gradient can be much greater than that of the homogeneous samples.
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