D. Chattopadhyay

Electron transport in direct-gap III-V ternary alloys

Electron drift, Hall mobility and thermoelectric power in InPxAs1-x, GaxIn1-xAs and InAsxSb1-x have been calculated by an iterative solution of the Boltzmann equation. A Kane-type band structure corrected for the lattice disorder is assumed. All the relevant scattering mechanisms are included together with band non-parabolicity, wavefunction admixture, degeneracy of the distribution function and screening of the scattering probabilities due to free carriers. Only the ionised impurity concentration (Ni) and the alloy scattering potential ( Delta Ea) have been treated as adjustable parameters to be determined from a fit with the experimental data. Variations of the other different material parameters across the alloy system have been taken from different sources, or reasonable assumptions are made regarding such variations. A large number of experimental results on both mobility and thermoelectric power are considered and good agreement is obtained with Delta Ea larger than that given by the difference in electron affinity between the constituent compounds.

Hot-Phonon Effects on Millimeter and Submillimeter Wave Response of Two-Dimensional Hot Electrons in GaAs Quantum Wells

Small-signal ac transport of degenerate two-dimensional hot electrons in GaAs quantum wells is studied for lattice temperatures of 77 and 300 K including deformation potential, acoustic, polar optical and ionized impurity scatterings. The consideration of nonequilibrium optical phonons or hot phonons is found to enhance considerably the 3 dB cut-off frequency (f 3dB ), where the ac mobility μ ac falls to 0.707 of its low-frequency value. f 3dB is shown here to decrease with increasing channel width, and to initially decrease and then rise slowly with increasing carrier concentration.

Negative electron diffusivity in high electric fields

Abstract The prediction that the electron diffusivity may be negative in high electric fields under some conditions has been examined starting from the Boltzmann equation and assuming a Maxwellian distribution function. It is found that the diffusion constant is positive for predominant acoustic phonon, polar optical phonon or impurity atom scattering. But the constant may be negative when effects of nonparabolicity are important and energy relaxation is limited by non-polar optical phonon scattering but the momentum relaxation is dominated by impurity atom scattering. Calculations with the parameter values of InSb, silicon and germanium show that only in materials like germanium at low temperatures of about 27 K the diffusion constant may be negative for impurity concentrations of 10 17 –10 18 cm −3 .

Hot electron diffusivity in silicon

Hot electron diffusion coefficients in silicon at room temperature are theoretically studied by incorporating the band non-parabolicity and the effect of the diffusion current on the distribution function. Two models of intervalley scattering are considered: in one model, coupling with high-temperature intervalley phonons only is assumed; in the other, low-temperature intervalley phonons are also included. Both the models give practically identical results and the calculated values are found to agree closely with the experiment.

Diffusion of Charge Carriers in High Electric Fields

The diffusion constant of electrons in high electric fields is studied by solving the Boltzmann equation and incorporating the effect of the carrier density gradient on the distribution function. The relaxation-time description of the scattering mechanisms and a constant isotropic effective mass for the carriers are assumed. For charge diffusion in gases, the method yields values of the longitudinal to transverse diffusivity ratio (DL/DT) in excellent agreement with those obtained from other methods. The method has also been applied to study the hot-electron diffusion in semiconductors, namely, germanium and silicon. It is found that the quantity (DL/DT) is less than unity and differs markedly from the values calculated on the basis of modified Einstein’s relations. Satisfactory agreement with recent experiments in the case of silicon has been obtained.

Warm electrons in polar semiconductors

A method based on an interactive solution of the Boltzmann equation is presented for the calculation of the warm electron coefficient beta in polar semiconductors. The effect of electron-electron collisions is incorporated by using a Maxwellian energy distribution function for the carriers. With the aid of the method the warm-electron coefficients in InAs, InSb, InP and Hg0.8Cd0.2Te are computed taking account of all the relevant scattering mechanisms. The effects of band nonparabolicity, p-function admixture and electron screening are included in the calculations. The sensitivity of beta on the choice of the scattering parameters is studied and the available experimental data are analysed in the light of the calculated values. Agreement between theory and experiment is found to be satisfactory in the case of InAs only.

Drift and Hall mobilities of electrons in InSb at 30 and 77 K

The drift and Hall mobilities of electrons are calculated for various degrees of impurity content incorporating the effects of deformation-potential-acoustic, piezoelectric, polar-optical and ionized-impurity scattering and the non-parabolicity of the energy band. An overall agreement with the experimental results is obtained if the acoustic deformation-potential constant is taken to be 7.2 eV.

Effect of ionised impurity scattering on hot-electron diffusion in silicon

The electric field variation of the diffusivity in n-type silicon at 300K is studied theoretically for different impurity densities (Ni) in the range 1014 to 1018 cm-3. Increasing Ni makes the parallel diffusion constant weakly field-dependent for high values (1017-1018 cm-3) of Ni.

Warm electron coefficient in polar semiconductors in the presence of strong electron-electron collisions

An iterative calculation of the warm electron coefficient is presented considering polar mode scattering in a parabolic band. Electron-electron collisions are assumed strong enough to establish a Maxwellian distribution function. The results vary considerably from those of the displaced Maxwellian calculation which has been used earlier for the interpretation of experimental results.

Screening effects on polar mode scattering at high electric fields

The effects of screening of polar optical mode of scattering at high electric fields are studied by performing Monte Carlo calculations on InSb at 77 K. The average carrier energy is found to be markedly affected by the inclusion of screening.