Measurements of low-frequency noise in thick-film resistors at low temperatures are reported. Films were prepared in a standard “high temperature” process: 20 nm sized RuO2 powder was mixed with 0.5 μm granular lead–borosilicate glass and organic solvent to give a paste, which was then screen printed onto alumina substrates and fired in a tunnel furnace. Measurements below liquid helium temperature reveal that the low frequency (1/f) noise increases with decreasing temperature, approximately as T−α, α=2.1±0.1. Up to 4 T no dependence of noise intensity on magnetic field has been observed. Measurements of noise spectra in the range 4−300 K show that spectral and temperature slopes obey Dutta, Dimon, and Horn equation [Phys. Rev. Lett. 43, 646 (1979)] only in the range T>10 K. Below this temperature a gap of constant width opens between noise exponent calculated from the spectral slope and from temperature dependence of noise magnitude. This gap occurs due to the change of noise coupling mechanism that takes place at ≈10 K. At higher temperatures this coupling is temperature independent. At lower temperatures coupling becomes temperature dependent. It is shown that data agree quantitatively with the concept that noise sources modulate energies for thermally activated hops in the percolation network.

In this paper, we report the results of investigation of 9.5 µm AlGaAs/GaAs and strain compensated 4.7 µm AlInAs/InGaAs/InP QCLs. We also show the results for 9.5 µm lasers based on lattice matched AlInAs/InGaAs/InP structures. The developed GaAs/AlGaAs lasers show the record pulse powers of 6 W at 77 K and up to 50 mW at 300 K. This has been achieved by careful optimization of the MBE growth process and by applying a high reflectivity metallic coating to the back facet of the laser. The 9.5 µm AlInAs/InGaAs/InP lasers utilize AlInAs waveguide and were grown exclusively by MBE without MOCVD regrowth. The short wavelength, strain compensated QCLs were grown by MOCVD. They represent state‐of‐the‐art parameters for the devices of their design. For epitaxial process control, the atomic‐force microscopy (AFM), high resolution X‐ray diffraction (HR‐XRD) and transmission electron microscopy (TEM) were used to characterize the morphological and structural properties of the layers. The basic electro‐optical characterization of the lasers is provided. We also present results of Green's function modeling of mid‐IR QCLs and demonstrate the capability of non‐equilibrium Green's function (NEGF) approach for sophisticated but still computationally effective simulation of laser's characteristics.

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The behavior of 1/f noise effective intensity in two-phase percolation systems and percolationlike systems with an exponentially wide distribution of bond resistances is reviewed. Monte Carlo simulations on random resistor networks are performed. For a two-phase system the numerical values of noise critical exponents ϭ1.54Ϯ0.025, Јϭ0.61Ϯ0.02, wϭ6.31Ϯ0.25, and wЈϭ6.9Ϯ0.25 are found in agreement with theoretical analysis performed with the help of a hierarchical model of a two-phase percolation system. For a system with an exponentially wide spectrum of bond resistances, i.e., a system in which bonds take on resistances rϭr 0 exp͑Ϫx͒, where ӷ1 and x is a random variable, it is assumed that in the individual resistors the noise generating mechanism obeys the form ͕␦r 2 ͖ϳr 2ϩ . In this case the effective noise intensity C e ϵS⍀, where S is the relative power spectral density of system resistance fluctuations and ⍀ is the system volume, is given by C e ϳ m exp͑Ϫx c ͒, where 1Ϫx c is the percolation threshold. The exponent m is ''double universal,'' i.e., it is independent of lattice geometry and of the microscopic noise generating mechanism. Numerical simulations performed for ϭ1 and 0 give approximately mХ2.3 and confirm this ''double universality'' of the exponent m. The connections between 1/f noise effective intensity and effective susceptibility in a two-phase weakly nonlinear percolation system are also established. ͓S1063-651X͑96͒04405-4͔ PACS number͑s͒: 64.60.Ak, 64.60.Ht * Electronic address: snar@carrier.kiev.ua

Monte Carlo calculations of shot noise power S in one-and two-dimensional Anderson models of a disordered conductor are presented. For quasi-one-dimensional geometry all theoretical results derived from random matrix theory are confirmed in ballistic-to-diffusive, metallic, and weak localization regimes. For two dimensions in the weak localization regime the relation Sϭ 1 3 Gϩ␦S 2e 2 /h with ␦S ϭ0.123 74 is found. In the ballistic-to-metallic and strongly localized regimes both one-and two-dimensional geometries behave in the same manner.

1/f noise properties of carbon black/polyesterimide and (carbon black + graphite)/polyesterimide thick-film resistors are studied. In resistive inks either high-structure carbon black or medium-structure carbon black was used. The influence of the content p of carbon black and graphite in resistive inks and curing temperature on sheet resistance R and noise intensity C is considered. It is shown that a higher curing temperature causes a decrease in the resistor's volume and influences R and C indirectly, via changes of the parameter p. It is then found that an increase in p leads to a simultaneous decrease of sheet resistance R and noise intensity C . The relation C ∼ (R ) η is found in this case for all measured resistors. The values of noise exponent η are estimated and a comparison with the noise properties of conventional cermet thick-film resistors is made. It is shown that the noise intensities of carbon black/polyesterimide resistors are comparable to those of cermet resistors.