We study current-voltages and low-frequency noise characteristics of the metal--porous silicon--silicon single crystal--metal structure with 50% and 73% porosity of porous silicon. The study is performed in dry air and in a mix of dry air
with carbon monoxide of different concentrations. The Hooge noise parameter α<i><sub>H</sub></i>
and the parameter γ in the
frequency dependence of the noise voltage spectral density
S<i><sub>U</sub></i>(ƒ) were determined from experimental data. High
sensitivity of spectral dependence of noise to gas concentration allows offering powerful method for determination of gas
concentration in the air or environment.
The influence of the surface and interfaces of semiconductor-metal Al/n<sup>+</sup>Si-nSi/Al and Ag/n<sup>+</sup>Si-nSi/Ag structures with different aluminum and silver contact coating layers on the level and behavior of the low frequency current noise spectra was experimentally investigated. It is shown that the level of low frequency noise strongly depends on the material and form of the contact coating layers. At the room temperature, at the frequency 10 Hz, the noise level for Al/n<sup>+</sup>Si-nSi/Al structures is equal to ~ 10<sup>-15</sup> A<sup>2</sup>/Hz, for the Ag/n<sup>+</sup>Si-nSi/Ag structures is equal to ~ 10<sup>-17</sup>-10<sup>-18</sup> A<sup>2</sup>/Hz. It seems possible that the interface conditions modification, which in its turn mirrors at the processes of surface reflection and refraction of electrons and phonons, affects on the relaxation processes of longer-wavelength electron distribution function fluctuation and thus on mobility fluctuation and 1/f noise level as well. On the base of noise spectral characteristics of the mentioned structures the peculiarities of the acoustic phonons refraction on the semiconductor-metal flat interface are experimentally investigated. Several practical aspects related with so-called "refraction points" are discussed. It is proposed that, by manipulation of those phonons "refraction points" at hetero-interface, it will be possible to suppress the part of 1/f noise level, which arises in the volume of semiconductor. It is supposed that the hetero-interface by itself is not the source ("generator") of the 1/f noise, but probably is a factor of the volume 1/f -noise "reduction".