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.