Instabilities of Current in III–V Semiconductors

Abstract

A description is given of a newly discovered phenomenon which is observed when an electric field of a few thousand V cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> is applied to a homogeneous sample of n-type GaAs or InP. Above a well-defined threshold field, a time-dependent decrease in current is observed, which is largely independent of external circuit conditions. In long specimens this decrease is aperiodic, resembling random noise with a bandwidth of ∼10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> c/sec. In short specimens, coherent oscillations are observed whose period is equal to the transit time of electrons between the ohmic electrodes of the structure. Frequencies over the range of 0.5–6.5 Gc/sec have been generated in this way, using experimental techniques which are described. Measurements of the efficiency of dc-to-rf conversion (from 1 to 2%), and of peak power outputs (up to 0.5 W), suggest that the new effect may have useful applications. Some diagnostic experiments are described and the results are discussed in terms of various possible mechanisms. Although the quantitative agreement with theory is poor it is concluded that the current instability may possibly be due to the amplification of lattice optical modes.