Negative Electron Mobility Research Articles

Negative electron mobility in diamond

By measuring the drift velocity of electrons in diamond as a function of applied electric field, we demonstrate that ultra-pure diamond exhibits negative differential electron mobility in the [100] direction below 140 K. Negative electron mobility is normally associated with III–V or II–VI semiconductors with an energy difference between different conduction band valleys. The observation of negative mobility in diamond, an elemental group IV semiconductor, is explained in terms of repopulation effects between different equivalent conduction band valleys using a model based on the Boltzmann equation.

Ultrahigh field multiple Gunn domains as the physical reason for superfast (picosecond range) switching of a bipolar GaAs transistor

The superfast (picosecond range) high-current switching observed recently in a GaAs junction bipolar transistor is explained by practically homogeneous carrier generation in the volume of the switching channels by a moving train of avalanching Gunn domains of large amplitude. The very fast (∼200ps) reduction in the collector voltage is determined by shrinkage of each domain, provided the negative electron mobility in ultrahigh electric fields is taken into account and current filamentation takes place. The results of one-dimensional simulations show good quantitative agreement with experimental voltage and current wave forms when the simulated switching area is equal to the summed areas of the filaments observed in the experiment.

Negative electron mobility under condition of the resonant optical excitation in gas mixtures

The negative electron mobility (NEM) effect has been analyzed in gas mixtures consisting of heavy inert gas (Ar, Kr or Xe), molecular nitrogen and lithium vapor. The mixture parameters, which are optimal for NEM observation, have been determined by using an analytical technique and simulating the Boltzmann equation for the nonequilibrium electron energy distribution function. It has been shown that NEM may be realized under condition of ionization evolution generated by selective optical excitation of resonant transition of lithium atom Li(2S)–Li(2P).

Ionization kinetics of optically excited lithium vapour underconditions of negative electron mobility

This work, based on the numerical solution of the Boltzmannequation, analyses the influence of ionization processes onthe shape of the electron energy distribution function (EEDF)in a mixture of molecular nitrogen, heavy inert gases andresonant excited lithium vapour. The convergent close-couplingmethod is used to estimate the singly differential cross sectionsfor the ionization of Li(2S) and Li(2P) atoms byelectron impact. The composition of the three-componentmixture, when the negative electron mobility (NEM)can be observed, is determined. The mixture's ionization degree rangeand the relative density of the resonant excitedlithium atoms favourable for the maximal NEM are found. Itis shown that under optimal conditions the ionization rateis determined by electron-impact ionization of the Li(2P)states. In this case the characteristic time of EEDF formation proved to beshorter than the ionization time. Based on this fact, it is suggested howto realize NEM in a quasi-stationary regime under conditionsof increasing electron density caused by the resonant radiation.

Negative Electron Mobility in Attachment-Dominated Plasmas

The temporal evolution of the electron velocity-distribution function(EVDF), the concentration, mean energy, and the drift velocity of theelectrons is studied on a kinetic basis in a weakly ionized Ar/F2mixture plasma under conditions when the electron concentration temporallydecreases as a result of the electron attachment to fluoride molecules. Usingan appropriate relaxation model, the time-dependent electron Boltzmannequation was solved in multiterm and two-term approximations of the velocitydistribution function. The multiterm results confirmed predictions on theoccurrence of negative electron mobilities in such a decaying Ar/F2plasma, which were made in a former study using the conventional two-termapproximation. The investigations particularly showed that this approximationgives almost accurate results for the EVDF and related electron swarm parametersexcept for in the very beginning of the relaxation process. It has been furthershown that for a certain range of the reduced electric field strength, thedrift velocity becomes negative in the process of temporal evolution and remainsnegative even when approaching the hydrodynamic stage of the electronswarm. In addition, the role played by the back heating from the gas byelastic collisions on the EVDF formation is studied and various comparisonswith corresponding Monte Carlo results are performed.

On the possibility of negative electron mobility in a decaying plasma

The electron energy distribution function and the electron drift velocity are studied numerically in a decaying plasma in the external electric field in the mixture Ar:F2 = 1:0.005 at atmospheric pressure. For a reduced electric field strength over the range 0.055-0.55 Td the negative electron mobility was predicted earlier by the solution of the Boltzmann equation using the two-term approximation for the velocity distribution function. The applicability of this approximation for calculations of negative mobility is discussed. The Monte Carlo simulation method is used to verify results obtained from Boltzmann equation analysis. It is shown that there is a reasonable agreement between the Boltzmann equation and Monte Carlo results.

Electron mobility in mixtures of optically excited sodium vapor, molecular nitrogen and heavy inert gases

A study is made of the processes of formation of a nonequilibrium electron energy distribution in mixtures of sodium, molecular nitrogen, and heavy inert gases. It is shown that in these mixtures, a local maximum on the electron energy distribution, can form in the energy range corresponding to the Raumsauer minimum in the elastic cross section for inert gases. The question is discussed of whether the electron mobility can become negative during the resonant optical excitation of the 3S-3P transition in sodium. The electron distribution function is calculated parametrically to determine the critical plasma parameters for which negative electron mobility can be observed in experiments, specifically, the composition of the mixture, the concentration of resonantly excited sodium atoms, the temperature corresponding to the distribution of vibrationally excited molecules, the degree of ionization and the electric field strength.

Effect of negative electron mobility in a mixture of argon, molecular nitrogen and optically excited lithium vapours

Parametric computations of the electron energy distribution function and of the electron mobility in a photoplasma of a lithium vapour mixture with argon and molecular nitrogen are presented. The possibility of the occurrence of a steady-state negative electron mobility when using a certain ratio of mix components under optical resonant excitation of the Li(2S-2P) transition is considered. The critical plasma parameters (the lithium excited atomic density, the temperature of the vibrational distribution of nitrogen molecules, the plasma ionization degree, the external electric field strength) determining this effect are revealed.

On the possibility of steady state negative mobility in externally ionised gas mixtures

The possibility of the occurrence of steady state negative electron mobility in externally ionised gas mixtures is considered. The requirements for the existence of the effect are discussed, using the example of externally ionised argon-fluorine gas mixtures. Results of numerical calculations of the drift velocity in electron-beam irradiated Ar/F2 gas mixtures are presented, which predict the occurrence of the effect in actual cases.