No abstract

As a consequence of the Gunn effect travelling space charge wave (SCW) amplification in n‐GaAs is possible. Within the frame of the drifted Maxwellian approach the frequency dependence of wave propagation is investigated. The applicability of this approach is thoroughly discussed. The most remarkable properties of the SCW are high gain and early onset of relaxation effects although diffusion, surprisingly, increases the limiting frequency at field strengths just above threshold. Farther above threshold the limiting frequency is much lower than for the real part of the conductivity. Gain and limiting frequency increase with electron density. A stability criterion results which for low electron densities is consistent with the usual n × L‐product of 1012 cm−2 while larger n × L‐products should be admittable at higher concentration.

No abstract

Within the frame of the drifted Maxwellian approach, the Gunn effect in GaAs at 300 °K can be sufficiently described by only three balance equations describing carrier, momentum, and energy exchange between central and upper valleys if an ohmic damping is taken into account, representing the satellite valleys. The approximation holds for fields up to 20kV/cm and frequencies up to 200 GHz. The limiting frequency increases with the rate of particle transfer between nonequivalent valleys and is reduced by the upper valley ohmic damping and by the relative amount of energy scattered back from the upper to the central valley.

In polar semiconductors the electron temperature T can become smaller than E This "cooling effect" has been studied by many authors the lattice temperature T (1, 2) neglecting the effect of impurity scattering. L'Paranjape and de Alba (1) expanded the distribution function to second order in the drift momentum p They obtained monotonically decreasing T with increasing 0' E electric field E which indicates the insuffiency of this approximation. However, if one expands the momentum and energy balance equation up to second order in p the results for the J-E-characteristics a r e in good agreement with those obtained without expansion (2). 0 Throughout this work the following assumptions a r e made: isotropic material, parabolic band structure, combined scattering by polar optical modes and ionized impurities, drifted Maxwell-Boltzmann distribution ( P -P J 3/2 [ -' n f(p) = ( 2 r m* kBTE) 2m" kgTE Conservation of momentum and energy yields two equations to determine the J-E-characteristic. To evaluate the collision integral the matrix elements of Frohlich and Paranjape (3) and Brooks (4) were used.Fig. 1 shows the dependence Tv'e. E for various impurity concentrations % E for n-type InSb at 77 OK. An increase of r+, results in a larger ratio of the randomized 3 2 energy (-k T ) to the drift energy (p /(2m*)). At a certain concentration n = 2 B E 0 D = I$ the cooling effect does not appear anymore. To determine I$ we set TE = T, , .This value I$ depends strongly on the lattice temperature (Fig. 2).1) This work was sponsored by the Ludwig-Boltzmann-Gesellschaft.