Etude bi-dimensionnelle des instabilites dans une lame mince d'arseniure de gallium en contact avec un autre semi-conducteur a resistance positive

Abstract

Bidimensional effects on instabilities are studied in a thin slab of GaAs beyond the threshold field E c when side-loaded by another semiconductor (1). Two cases are considered: 1. The surface charges in the semiconductor (1) are not drifted. By the variation of the conductivity σ 1, the instabilities in GaAs are modified. Two distinct modes of operation may exist, a large amplitude mode and a small amplitude mode. When the conductivity of the semi-conductor (1) is increased the last mode is the one existing and the phase velocity of the space charge wave associated with the drifting surface charges in GaAs is approaching the drift velocity of these charges. In this case it is shown that the domain-formation condition using the Kino-Robson criterion becomes: nd > 0,8·10 11( ω 1 ω ) 2 cm −2 and the value of the growth factor, obtained by numerical computations, is between 10 and 20 dB/cm. ω 1 is equal to σ 1/ϵ 1, ω the pulsation of the applied RF signal, and the ratio of mobilities, longitudinal to transverse is taken equal to 1/10. (The RF signal is applied at the transit time frequency when domain formation does not occur). 2. When the surface charges in the semiconductor (1) are drifted by application of a continuous field, an interaction between the space charge wave and the two ‘electron beams’ takes place. When the drift velocities in both GaAs and semi-conductor (1) are equal, the space charge propagates at a phase velocity equal to the drift velocities of surface carriers. This phenomenon appears even below threshold field E c , i.e. for negative or positive longitudinal mobility in GaAs. In the neighbourhood of synchronism, for E > E c , two cases have been considered, function of the conductivity of the external semiconductor. For the same conductivity that in the first case treated, a domain suppression occurs as it would be obtained by side-loading with a metal. But with a conductivity reduced by ten, the growth factor is considerable and the domain formation condition more restrictive than without side-loading by an external semiconductor: nd > 0,8·10 11 [( ω 1 ω ) 2+1] ( ω 1 ω )(k−1) 1 2 cm −2 k−1>0 with k = v 1 v 2 ( v 1 in semiconductor (1), v 2 in GaAs). As a result from numerical computations, GaAs under threshold appears to be the more adequate semiconductor for side-loading the GaAs slab at room temperature. Physical experiments are now being carried through.