Microwave and Millimeter-Wave Resonant-Tunnelling Devices

Abstract

The concept of particles interacting coherently with finite multiple-barrier structures is over 35 years old, and yet it forms the basis for an area of intense activity today, both for practical devices and for studies of the underlying physics. An example is the two barrier structure shown in Fig. 6.1. In the quantum theory textbook written by Bohm [6.1] in 1951, the double-barrier problem was solved in the WKB approximation. He showed that, at certain energies, unity transmission resonances (resonant tunnelling) occur for particles incident upon the structure. Ten years elapsed before it was recognized that this phenomenon could be useful for devices. The first suggestion for a resonant-tunnelling transistor was made by Davis and Hosack [6.2] and Ioganson [6.3] in 1963, following the suggestion by Mead [6.4] in 1960 of a nonresonant double-barrier transistor. Early in the next decade Esaki and Tsu [6.5] pointed out that superlattices should show negative resistance, and Kazarinov and Suris [6.6] showed that negative resistance could arise from a finite superlattice. In 1973 Tsu and Esaki [6.7] derived the two-terminal current-voltage (I—V)curves for finite multiple-barrier structures using a wave function matching formulation based on a method of Kane [6.8]. This technique has been remarkably successful at explaining experimental results, as will be discussed in Sect. 6.7. In 1974 Chang et al. [6.9] were the first to observe resonant tunnelling in a mono-crystalline semiconductor. They used a two-barrier structure and observed the resonances in the current by measuring the I—V curve. A decade later, interest in the field was renewed when Sollner et al. [6.10] showed that the intrinsic charge transport mechanism of a two-barrier diode could respond to voltage changes in times of the order of 0.1 ps. More recently, the negative differential resistance characteristic of resonant tunnelling has been obtained at room temperature [6.11–13]. At present, several laboratories are actively investigating resonant-tunnelling devices.