Стандартизация государственного финансового контроля

We investigate two types of GaAs microwave detector: point contact diode with n-n+ junction and planar one, which involves asymmetrically necked thin semiconductor film containing n-n+ junction in its narrowest part. The voltage sensitivity of the GaAs point contact diode, in contrast to those made from Ge or Si, increases with the strength of the microwave electric field due to the increase of both the electron energy relaxation time and the diffusion coefficient at room temperature. Investigating the voltage-power characteristic of the planar diode we have determined experimentally that the voltage sensitivity of the planar detector weakly depends on frequency within the 26 - 140 GHz range

GaAs/AlGaAs heterojunction: a promising detector for infrared radiation

We report our results on experimental study of photovoltage, induced by pulsed CO2 laser in GaAs/AlGaAs heterojunctions. We show that photoemission of hot carriers across the potential barrier is the dominant mechanism in formation of the photovoltage. The analysis of the current-voltage characteristic reveals that the photocurrent has maximum at bias voltage related to the potential barrier height of p-n heterojunction. Moreover, we demostrate that the use of heterojunction has an advantage over homojunction in the infrared detection

Excitonic photoluminescence quenching by impact ionization of excitons and donors in GaAs/Al(0.35)Ga(0.65)As quantum wells with an in-plane electric field

We present a detailed experimental study on photoluminescence quenching due to exciton and donor impact ionization by accelerated electrons under an in-plane nanosecond duration electric field created in GaAs/Al(0.35)Ga(0.65)As quantum wells. From the photoluminescence transients measured by the time-correlated single-photon counting technique, we have determined the experimental conditions under which donor impact ionization can have an influence on quenching of the excitonic photoluminescence. The coefficient of twodimensional exciton impact ionization has been estimated; its dependences on the applied electric field, lattice temperature, and width of the quantum wells are given

Xenopus H-RasV12 promotes entry into meiotic M phase and cdc2 activation independently of Mos and p42MAPK

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Analysis of CRP-CytR interactions at the Escherichia coli udp promoter.

Multiprotein complexes regulate the transcription of certain bacterial genes in a sensitive, physiologically responsive manner. In particular, the transcription of genes needed for utilization of nucleosides in Escherichia coli is regulated by a repressor protein, CytR, in concert with the cyclic AMP (cAMP) activated form of cAMP receptor protein (CRP). We studied this regulation by selecting and characterizing spontaneous constitutive mutations in the promoter of the udp (uridine phosphorylase) gene, one of the genes most strongly regulated by CytR. We found deletions, duplications, and point mutations that affect key regulatory sites in the udp promoter, insertion sequence element insertions that activated cryptic internal promoters or provided new promoters, and large duplications that may have increased expression by udp gene amplification. Unusual duplications and deletions that resulted in constitutive udp expression that depended on the presence of CytR were also found. Our results support the model in which repression normally involves the binding of CytR to cAMP-CRP to form a complex which binds to specific sites in the udp promoter, without direct interaction between CytR protein and a specific operator DNA sequence, and in which induction by specific inducer cytidine involves dissociation of CytR from cAMP-CRP and the RNA polymerase interaction with cAMP-CRP bound to a site upstream of then transcription start point. The stimulation of udp expression by CytR in certain mutants may reflect its stabilization of cAMP-CRP binding to target DNA and illustrates that only modest evolutionary changes could allow particular multiprotein complexes to serve as either repressors or transcriptional activators

Microwave sensor based on modulation-doped GaAs/AlGaAs structure

We propose a microwave diode based on a modulation-doped GaAs/Al(0.25)Ga(0.75)As structure. The principle of the diode operation relies on a non-uniform heating of the two-dimensional electron gas in microwave electric fields arising due to the asymmetric shape of the device. The voltage sensitivity of the diode at room temperature is dose to 0.3 V W(exp -1) at 10 GHz, which is comparable to the value obtained using similarly shaped and sized diodes based on bulk n-GaAs. At liquid nitrogen temperature, the voltage sensitivity strongly increases reaching a value of 20 V W(exp -1) due to the high mobility of the two-dimensional electron gas. The detected signal depends linearly on power over 20 dB, until hot-electron real-space-transfer effects begin to predominate. We discuss noise temperature measurements at 10 GHz, consider the frequency dependence of the voltage sensitivity in the microwave range and compare the performance data of the proposed device and the asymmetrically shaped bulk GaAs diode within the 10 GHz-2.5 THz frequency range