We investigate the effect of a uniform intense terahertz radiation on hot-electron transport in semiconductors driven by a dc or slowly varying electric field of arbitrary strength. Using a vector potential for the high-frequency field and a scalar potential for the dc or slowly varying field, we are able to separate the center-of-mass motion from relative motion of electrons and to distinguish the slowly varying part from the rapidly oscillating part of the center-of-mass velocity. Considering the fact that relevant transport quantities are measured over a time interval much longer than the period of the terahertz radiation field, we obtain a set of momentum and energy balance equations, without invoking a perturbational treatment of the electron-photon interaction. These equations, which include all the multiphoton processes, are applied to the examination of hot-carrier transport in a GaAs-based quantum well subjected to a weak or a strong dc bias and irradiated by a terahertz radiation of various frequency and strength in both the parallel and vertical configurations. Up to as many as unu 550 absorption and emission multiphoton channels are included in the numerical calculation. The present approach turns out to be a very convenient and efficient tool to study the effect of an intense high-frequency radiation on dc or slowly varying carrier transport in semiconductors. Its applicable frequency range and its connection with previously developed balance-equation treatment are discussed.

The rate of hot-electron energy loss to the lattice in polar semiconductors is studied within a two-temperature model. We call attention to the role of the internal thermalization of the LO-phonon system in implementing this model and in establishing the concept of electron energy loss to phonons. With the help of the Feynman diagrammatic technique for the Keldysh closed-time-path Green's function, we have derived a formula for the electron-energy-loss rate, taking account of contributions from all orders of the electron-phonon interaction and including the hot-phonon effect. This formula, which carries the internal thermalization time of the LO-phonon system and the relaxation time of the whole lattice as parameters, reduces exactly to the conventional Kogan formula in the limit of weak electron-phonon coupling. In the zero-hot-phonon-effect Iimit (if it is allowed) this formula is shown to be equivalent to that given by Dharma-wardana [Phys. Rev. Lett. 66, 197(1991)]. Comparison beween our formula and that of Das Sarma and Korenman [Phys. Rev. Lett. 67, 2916(1991)] is also given. An interesting prediction of the present formula is the low-temperature enhancement of the electron-energy-loss rate over the conventional Kogan result. A numerical calculation for a two-dimensional GaAs system shows agreement between this theory and the experimental trend.

Bloch electron conductivity perpendicular to the layers of a superlattice (period d) is evaluated using an extension of the balance-equation qpproach [X.L. Lei and C.S. Ting. Phys. Rev. B 32, 1112(1985)] to narrow-band transport. The perpendicular park drift velocity Tp and The critical field Ec, at which the drift velocity peaks, are analyzed as functions of miniband width. Our theoretical predication that Ecd increases with decreasing miniband width agrees well with the data of Sibille et al. [Phys. Rev. Lett. 64, 52(1990)], even for the samples of narrowest miniband width in their expriment.

The balance-equation approach for hot-electron transport previously developed is extended to systems com posed of two groups of carriers, each of different effective mass. This is the simplest model for a real band structure of a multivalley semiconductor. The separation of the center-of-mass (c.m.) motion from the relative motion of the electrons is incomplete due to the possibility of exchange of particle number between the two systems and thus is taken into account in the Liouville equation for the density matrix. General expressions for the rates of change of the c.m. momenta, electron system energies, and partcle numbers are obtained. These equations in their classical forms are used for a model calculation for the high-field steady-state transport in GaAs and the calculated results show reasonably good agreement with experiments.

The balance-equation approach developed by Lei and Ting for steady-state hot-electron transport is extended here to include nonequilibrium phonon occupation under the assumption that the effect of phonon-phonon interaction can be represented by an effective phonon relaxation time Tp, which may be mode dependent. Both single heterojunctions and multiplayer quantum-well superlattices, as well as three-dimensional (3D) bulk systems, are discussed. A 3D phonon model and a quasi-2D phonon model are employed in describing the various interaction electron-phonon systems. The expressions for the frictional forces and energy transfer rates obtained in steady state are structurally similar to those without hot-phonon effects and the balance equations with finite phonon relaxation can be solved with the same computational effort as in the case of Tp=0. We have examined hot phonon effects on the energy-transfer rate, the electron temperature, and the linear and nonlinear mobilities. It is given lattice temperature. However, It significantly increases the velocities. For an n-type GaAs heterosystem the normalized electron-energy-loss rate at Tp=3.5ps is seen to be a factor of 5-10 smaller than that at Tp=0. The functional dependence of the energy-loss rate on carrier temperature shows considerable difference form the prediction of a carrier temperature-type theory, and is in reasonably good agreement with experiments.

Hot-electron transport in a GaAs-AlGaAs heterojunchon is studied by the use of a non-Boltzmann balace-equation approach recently developed. The occupation of the lowest and next lowes subbands and coulomb interactions between inrasubband and intersubband elelctrons are taken into account. We include the scattering by remote charge impurities, acoustic phonos (via deformation potential and piezoeletric coupling), and polar opteical phonons in the force and energy balance equations to give the carrier mobility as a function of dirft velocity and/or eletric field. Theoretical results are in reasonably good agreement with experiments.

A non-Boltzmznn theory of steady-state transport for two-dimensional systems in a strong electric field is developed, which includes a force-and an energy balance equation. The electron termperature, impurity-, and phonon-limited mobilities are determined solely from these balance equations. The theory is applied to the calculation of ohmic and nonlinear transport in GaAsGaAlAs heterojunctions at low temperatures. Temperature-dependent ohmic mobilities are calculated and compared with experiments. Nonlinear effects in electronic transport at low temperatures are discussed and some numerical results are presented. We also compare the present balance equations with those in the carrier temperature mode).

A Green's function approach to nonlinear electronic transport in a static electric field is developed microscopically for the system composed of interacting electrons with impurities and phonons. The essemtoal idea is to separate the center-of-mass motion of electrons. An electron temperature is introduced as a measurement of the internal energy of the relative electrons electrons and phonons in the initial state, we obtain the density matrix for the electron-lattice sysytem to the first order of mass of interaction but nuder arbitrarily strong electric field. The frictional force experienced by the center of mass of the Green's-function technique, and the force-and energy-balance equations for steady state are obtained. These equations are qpplied to the calculations of the ratio of electron temperature to the lattice temperature and the electron redsistivity as functions of drift velocity imputity, acoustic-phonon, and optcal-phonon scatterings. The dynamic nature of Coulomb screening by charge carriers is studied numerically. One of the interesting predictions is the possible cooling of electtons at low temperatures in samples with low impurity comcentration.

The nonlinear transport for interacting electrons with phonons and impurities is studied in the presence of a strong electric field E. By separating the center-of-mass motion from the relative motion of electrons we are able to obtain force-and energy-balance equations in steady states, form which the electron temperature and current density can be determined self-consistently as a function of E.

Microwave-radiation induced giant magnetoresistance oscillations recently discovered in highmobility two-dimensional electron systems are analyzed theoretically. Multiphoton-assisted impurity scatterings are shown to be the primary origin of the oscillation. Based on a theory which considers the interaction of electrons with electromagnetic fields and the effect of the cyclotron resonance in Faraday geometry, we are able not only to reproduce the correct period, phase, and the negative resistivity of the main oscillation, but also to predict the secondary peaks and additional maxima and minima observed in the experiments. These peak-valley structures are identified to relate, respectively, to single-, double-, and triple-photon processes.