Some properties of the moving high-field domain in Gunn effect devices

Some experimentally observed characteristics of the high-field domain in long samples (0.1 to 0.25 cm) of GaAs are described. Accurate determinations of domain velocity and amplitude in a number of devices have been made using a surface potential probe. Although the probe resolution was insufficient to observe the domain widening as it supports more voltage, as is predicted by the Ridley model, it has been shown that domain fields in excess of 75 kV/cm exist in long samples. In presenting the results it has been convenient to recognise two classes of behaviour: (1) Samples showing only small departures from the Ridley model and good waveform coherence; these have been termed ´well behaved;´ and (2) samples in which additional perturbing processes modify the Gunn effect. Of the two such processes studied, namely, that giving rise to the emission of recombination radiation from the moving domain, and, secondly, electron traping within the domain, the former is by far the most important since, depending on sample length and carrier concentration, it will always occur at sufficiently high applied fields. From considerations of the space-charge distribution necessary to support the high domain fields a model has been developed, the chief feature of which is a triangular domain field distribution. The peak value of the domain field is dependent on the domain voltage drop and the material carrier concentration. The model explains most of the experimental observations and, in particulars the factors which determine whether a sample is well behaved or emits recombination radiation. In addition, the model gives a semiquantitative picture of the primary current-field characteristics of the transferred electron mechanism.