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Threshold Current (threshold + current)

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Physics and Astronomy61%

Terms modified by Threshold Current

  • threshold current density
    		•

    Selected Abstracts

    Low dose cocaine self-administration transiently increases but high dose cocaine persistently decreases brain reward function in rats

    EUROPEAN JOURNAL OF NEUROSCIENCE, Issue 1 2003
    Paul J. Kenny
    Abstract This study investigated the effects of self-administered cocaine on brain reward function, measured by intracranial self-stimulation (ICSS) reward thresholds in rats. Self-administration of 10 and 20 cocaine injections (0.25 mg per injection, equivalent to 4.94 ± 0.23 and 9.88 ± 0.46 mg/kg, self-administered over 40 ± 6.9 and 99 ± 11.9 min, respectively) lowered reward thresholds 15 min later, indicating a facilitation of rewarding ICSS, but had no effect at 2, 24 or 48 h after administration. Thus, self-administration of low cocaine doses did not cause persistent changes in brain reward function. Forty cocaine injections (19.64 ± 0.94 mg/kg; self-administered over 185 ± 10.9 min) also transiently lowered reward thresholds 15 min later, while significant threshold current elevations were observed at 2 and 24 h after administration, indicating persistent withdrawal-like reward deficits. Finally, 80 cocaine injections (39.53 ± 1.84 mg/kg, self-administered over 376 ± 19.9 min) significantly elevated thresholds 2 and 48 h after self-administration, but not at 24 h. Threshold currents also tended to be elevated 15 min after self-administration. Overall, these data suggest that as the amount of self-administered cocaine increases the motivation to consume further cocaine may be shifted, from obtaining the rewarding actions of cocaine to avoidance and alleviation of a cocaine-induced negative affective state. [source]

    Resistance to Propagation of Amygdaloid Kindling Seizures in Rats with Genetic Absence Epilepsy

    EPILEPSIA, Issue 10 2002
    Esat E, kazan
    Summary: ,Purpose: The existence of absence epilepsy and temporal partial seizure pattern in the same patient is an uncommon state. In the present study, we aimed to evaluate whether the process of kindling as a model of complex partial seizures with secondary generalization is altered in rats with genetic absence epilepsy. Methods: Six- to 12-month-old nonepileptic control Wistar rats and genetic absence epileptic rats from Strasbourg (GAERS) were used in the experiments. One week before the experiments, bilateral stimulation and recording electrodes were implanted stereotaxically into the basolateral amygdala and cortex, respectively. Animals were stimulated at their afterdischarge threshold current twice daily for the process of kindling and accepted as fully kindled after the occurrence of five grade 5 seizures. Bilateral EEGs from amygdala and cortex were recorded continuously during 20 min before and 40 min after each stimulus. Results: All control Wistar rats were fully kindled after stimulus 12 to 15. Although the maximal number of stimulations had been applied, GAERS remained at stage 2, and no motor seizures were observed. The afterdischarge duration in bilateral amygdala and the cortex after the kindling stimulus was shorter in GAERS when compared with control rats. Conclusions: Occurrence of only grade 2 seizures and no observation of grade 3,5 seizures in GAERS with the maximal number of stimulations would suggest that the generalized absence seizures may be the reason of the resistance in the secondary generalization of limbic seizures during amygdala kindling. [source]

    Saccadic eye movements evoked by microstimulation of striate cortex

    EUROPEAN JOURNAL OF NEUROSCIENCE, Issue 4 2003
    Edward J. Tehovnik
    Abstract Experiments were performed to assess the excitability of neural elements activated while inducing saccadic eye movements electrically from different cortical layers of striate cortex (area V1) in rhesus monkeys. Excitability was assessed by measuring current thresholds, saccadic latencies, chronaxies, and the effectiveness of anode-first vs. cathode-first pulses. Minimum current thresholds for the evocation of saccades (i.e. less than 5 µA) were observed when the deepest layers of V1 were stimulated. The shortest saccadic latencies were also observed at these depths. The shortest latency at 10 times the threshold current was 49 ms on average. The chronaxies of the elements mediating saccades were less in deep V1 (i.e. 0.17 ms) than in superficial V1 (i.e. 0.23 ms). Anode-first pulses were more effective at evoking saccades from superficial V1, whereas cathode-first pulses were more effective at evoking saccades from deep V1. These results indicate that the excitability properties of superficial and deep V1 are distinct for the generation of saccades. Moreover, the excitability of elements mediating saccades in V1 of monkeys is comparable to that of elements mediating phosphenes in human V1. [source]

    Optimization of Bragg reflectors in AlGaAs/GaAs VCSELs

    LASER PHYSICS LETTERS, Issue 5 2005
    V. M. N. Passaro
    Abstract In this paper a detailed investigation of the distributed Bragg reflectors in GaAs-based vertical cavity surface emitting lasers is presented. The influence of layer doping concentration, number of periods, oxide aperture and AlxGa1,xAs alloy composition on output emission power and threshold current has been found. Both oxidized and non oxidized structures have been considered. A number of interpolation curves are extracted and presented for design and fabrication purposes. Although the results are presented for GaAs-based structures, the theoretical approach is very general. (© 2005 by Astro, Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source]

    Testing procedure to obtain reliable potentiodynamic polarization curves on type 310S stainless steel in alkali carbonate melts

    MATERIALS AND CORROSION/WERKSTOFFE UND KORROSION, Issue 4 2006
    S. Frangini
    Abstract Potentiodynamic polarization measurements have been employed to evaluate the anodic behavior of a type 310S stainless steel in the eutectic Li/K molten carbonate. In general, the electrochemical tests yield useful information to predict the stability of the oxide films formed on the surface at the initial period of corrosion, although some precaution is required in the testing procedure as the reproducibility of results is seen to be adversely affected by the passage of large currents. Especially when the steel is in a passive state, erratic results are easily observed if the corrosion layer is being damaged by uncontrolled large currents. This is because the acid-base properties of the melt are susceptible to deep changes by applied currents in the milli-ampere range resulting in hysteresis phenomena in the polarization plot. Hysteresis is caused, on one hand, by acidic dissolution of the passive layer at high anodic currents and, on the other hand, by increased melt basicity due to oxide ion build-up at high cathodic currents. An optimized testing procedure is therefore suggested that minimizes these effects by imposing a 2 mA/cm2 threshold current during polarization measurements. Moreover, the conditions for the applicability of the linear polarization technique to estimate kinetic parameters have been discussed in relationship with the corrosion mechanisms analysed by impedance spectra. It is concluded that the presence of diffusional impedance terms and formation of surface resistive films in molten carbonates may result in not reliable polarization resistance values obtained with the linear polarization. [source]

    Degradation of InGaN-based laser diodes due to increased non-radiative recombination rate

    PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2010
    N. Trivellin
    Abstract With this paper we analyze the correlation between the degradation of InGaN-based laser diodes (LDs) and the increase in the non-radiative recombination rate in the active region. Several 405,nm MOCVD LDs have been submitted to CW stress, for 2000,h (stress current in the range 40,100,mA, case temperature,=,75,°C). During stress, we extensively evaluated the optical characteristics of the LDs: a technique for the evaluation of the non-radiative recombination lifetime (,nr) in the active material was developed and used for the analysis of the stress effects. We demonstrate the following: (1) degradation determines the increase in LDs threshold current (Ith) and the decrease in the ,nr; (2) degradation of Ith and ,nr have similar kinetics; and (3) the degradation rate of the LDs is almost linearly related to the stress current level. The degradation process is therefore ascribed to the decrease of internal quantum efficiency caused by the increase of the non-radiative recombination rate in the active region. [source]

    Low thermal resistance, high-speed 980 nm asymmetric intracavity-contacted oxide-aperture VCSELs

    PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 7 2009
    Y. M. Song
    Abstract We demonstrated high-speed characteristics of an oxide-aperture vertical-cavity surface-emitting laser (VCSEL) with intracavity structures for both p- and n-contacts, based on InGaAs/GaAs multiple quantum wells operating at , , 980 nm, indicating a low thermal resistance (Rth). The asymmetric current injection scheme is employed for reducing current crowding around the rim of the oxide aperture. A high aluminium content undoped Al0.88Ga0.12As and GaAs distributed Bragg reflector (DBR) mirror is used for efficient heat dissipation. The VCSEL with a 7 ,m oxide aperture exhibited an output power of 2.5 mW and a threshold current of 0.8 mA with a slope efficiency of 0.39 mW/mA at 20 °C under continuous-wave operation and it still worked with 1.3 mW at 90 °C. The temperature tuning coefficient of 0.081 nm/°C and dissipated electrical power tuning coefficient of 0.104 nm/mW were observed, leading to a low Rth of 1.28 °C/mW. A high modulation bandwidth up to 13 GHz with a modulation current efficiency factor of 6.1 GHz/mA1/2 was achieved. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    High-performance UV emitter grown on high-crystalline-quality AlGaN underlying layer

    PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 6 2009
    Hirotoshi Tsuzuki
    Abstract Al0.25Ga0.75N films were grown on a grooved-Al0.25Ga0.75N/ AlN/sapphire template by MOVPE. The dislocation density on the grooved areas was as low as 1 × 108 cm,2. We fabricated a UVA light-emitting diode grown on such an AlGaN underlying layer exhibiting an output power of 12 mW at a DC current of 50 mA with a peak emission wavelength of 345 nm, which corresponds to an external quantum efficiency of 6.7%. This efficiency is the highest reported to date in this wavelength region. We also fabricated a 358 nm UVA laser diode (LD) using a GaN/AlGaN MQW active layer grown on an AlGaN underlying layer. This UV LD exhibits a threshold current of 73 mA and a corresponding current density of 3.8 kA/cm2 at 7 °C. The characteristic temperature T0 was 174 K in the temperature range of 7,27 °C. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Temperature and pressure dependence of carrier recombination processes in GaAsSb/GaAs quantum well lasers

    PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2007
    Konstanze Hild
    Abstract We investigated the temperature and pressure dependence of carrier recombination processes occurring in GaAsSb edge-emitting lasers operating near 1.3 ,m. Below ,100 K, the threshold current, Ith, is dominated by the radiative current, Irad, and is proportional to temperature, T. However, above 100 K, non-radiative recombination increases abruptly such that by 125 K it accounts for 40% of Ith. From high pressure measurements at this temperature, we find that the non-radiative current decreases with increasing pressure, consistent with the presence of Auger recombination. At room temperature, non-radiative recombination accounts for ,90% Ith and gives rise to a super-linear temperature dependence of Ith, in spite of the fact that Irad , T. At room temperature the non-radiative current increases with increasing pressure, indicating that under ambient operating conditions, the devices are also limited by carrier leakage into the ,-minimum of the GaAs barriers and possibly also into the X-minima of the GaAsP confining layers. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Carrier recombination mechanisms in mid-infrared GaInAsSb quantum well lasers

    PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2007
    K. O'Brien
    Abstract Hydrostatic pressure and spontaneous emission techniques have been used to examine the important recombination mechanisms in type-I GaInAsSb/GaSb quantum well lasers. High pressure results indicate that Auger recombination dominates the threshold current of 2.11 ,m and 2.37 ,m devices and is the origin of their temperature sensitivity around room temperature. While the characteristics of the 2.37 ,m devices are much improved by the suppression of the CHSH Auger process, since its spin,orbit splitting energy is greater than its band gap, other important Auger processes such as CHHL and CHCC persist. In the larger band gap 2.11 ,m devices, an increase in threshold current with pressure is observed suggesting that CHSH Auger is present in these devices at atmospheric pressure and contributes to performance degradation at these shorter wavelengths. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Carrier recombination processes in 1.3 ,m and 1.5 ,m InGaAs(P)-based lasers at cryogenic temperatures and high pressures

    PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 14 2004
    S. J. Sweeney
    Abstract We describe measurements of the threshold current of 1.3 ,m and 1.5 ,m InGaAs(P)-based quantum-well lasers measured at cryogenic temperatures and at high pressures. At low temperatures (,100 K), we find that the threshold current of the devices increases with increasing pressure consistent with the calculated pressure variation of the radiative current. This is in sharp contrast with their pressure dependence at room temperature (RT), where the threshold current decreases with increasing pressure due to the decrease in importance of Auger recombination. These low-temperature, high-pressure results agree well with previous temperature dependence measurements on the same devices, which show a transition from radiative to non-radiative Auger recombination dominated behaviour as the laser temperature is increased from ,100 K to room temperature. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Carrier leakage suppression utilising short-period superlattices in 980 nm InGaAs/GaAs quantum well lasers

    PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 14 2004
    D. Lock
    Abstract Short-period Super Lattices (SSLs) have been proposed as a method of reducing carrier leakage in semiconductor lasers. SSLs increase the carrier confinement by increasing the effective barrier height due to the carriers being "reflected" from the SSL. Thermionically driven leakage of electrons from the quantum wells has been suggested as an important non-radiative current path in (Al)GaAs based devices which is exacerbated at high temperatures. Due to the fact that the application of high pressure causes a reduction in the zone centre ,,X minima splitting, pressure can be used to investigate the extent to which such leakage increase the threshold current. In this study, we compare devices with and without SSL layers and find direct evidence for leakage suppression in devices containing SSLs up to a temperature of 60 °C. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Overlapping representations of the neck and whiskers in the rat motor cortex revealed by mapping at different anaesthetic depths

    EUROPEAN JOURNAL OF NEUROSCIENCE, Issue 1 2008
    Shashank Tandon
    Abstract The primary motor cortex of mammals has an orderly representation of different body parts. Within the representation of each body part the organization is more complex, with groups of neurons representing movements of a muscle or a group of muscles. In rats, uncertainties continue to exist regarding organization of the primary motor cortex in the whisker and the neck region. Using intracortical microstimulation (ICMS) we show that movements evoked in the whisker and the neck region of the rat motor cortex are highly sensitive to the depth of anaesthesia. At light anaesthetic depth, whisker movements are readily evoked from a large medial region of the motor cortex. Lateral to this is a small region where movements of the neck are evoked. However, in animals under deep anaesthesia whisker movements cannot be evoked. Instead, neck movements are evoked from this region. The neck movement region thus becomes greatly expanded. An analysis of the threshold currents required to evoke movements at different anaesthetic depths reveals that the caudal portion of the whisker region has dual representation, of both the whisker and the neck movements. The results also underline the importance of carefully controlling the depth of anaesthesia during ICMS experiments. [source]