Quantum well devices for millimetre wave applications

Abstract

Quantum well devices exhibit strong non-linearity with very short time responses which make them attractive for non-linear applications at very high frequencies. In this paper, we review several topics concerning the design, the fabrication and the frequency evaluation of these devices. We discuss notably new proposals in order to improve their frequency and power capabilities in the context of recent progress in strained layer epitaxy and finite superlattices. Concerning fabrication, the more recent developments for integrating the device in a whisker-less technology are reported. On this basis a few selected recent achievements for demonstrating the ability of these devices to operate at very high frequency are considered. Lastly, we focus on recent advances related to the applications for the devices used as oscillators, harmonic multipliers, detectors and mixers. IntroductionThe remarkable advancement of the last decade in the techniques of crystal growth has stimulated active research in many laboratories on heterostructure devices where electronic transport is perpendicular to heterolayers. In particular, a variety of experiments have been conducted with heterostructures which consist of two barriers in series separated by a potential well. When the thickness of the layers of this double barrier heterostructure (DBH) are typically smaller than 5 nm, significant quantum effects, such as size-quantization and tunnelling can be expected giving rise to the so-called resonant tunnelling effect [1].Basically, these quantum well devices are useful for very high frequency applications at millimetre and submillimetre wave lengths because they are able to respond very quickly and sensitively to a voltage control. In addition, they have potential advantages such as stronger non-linearity and/or special symmetry, with respect to conventional devices, which make them attractive for non-linear functions. Lastly, one can expect a reduction of noise term which should be benefit for applications. In this context, quantum well devices have received a widespread interest aimed at establishing a better understanding of their electrical properties and hence to use them in practical applications.In this paper, we review various aspects concerning the modelling, the fabrication and the frequency evaluation of these new devices. We will see that a DBH acts as an energy filter which exhibits peaks in electron transmission and consequently in current-voltage characteristics at certain values of voltage. Therefore, the devices exhibit negative differential resistance effect so that they can be used as oscillators especially. Generally speaking, the important figures of merit for quantum well devices are the peak current density (Jp) and the peak to valley current ratio (PVCR). In a first section we will see how these parameters can be optimized by a proper choice of structural parameters and material systems. Another challenging topic concerns the processing techniques for the fabrication of very small area devices reported in sectio...