Overview and background

Recent progress in crystal growth and microfabrication technologies have allowed us to explore a new field of semiconductor device research. The quantum-mechanical wave-nature of electrons is expected to appear in mesoscopic semiconductor structures with sizes below 100 nm. Instead of conventional devices, such as field effect transistors and bipolar transistors, a variety of novel device concepts have been proposed based on the quantum mechanical features of electrons. The resonant tunnelling diode (RTD), which utilises the electron-wave resonance in multi-barrier heterostructures, emerged as a pioneering device in this field in the mid-1970s. The idea of resonant tunnelling (RT) in finite semiconductor superlattices was first proposed by Tsu and Esaki in 1973 shortly after molecular beam epitaxy (MBE) appeared in the research field of compound semiconductor crystal growth. A unique electron tunnelling phenomenon was predicted for an AlGaAs/GaAs/AlGaAs double-barrier heterostructure, based on electron-wave resonance, analogous to the Fabry–Perot interferometer in optics. In the particle picture, each electron is constrained inside the GaAs quantum well for a certain dwell time before escaping to the collector region. The bias dependence of the tunnelling current through the double-barrier structure shows negative differential conductance (NDC) as a result of RT. Experimental results reported in the early days showed only weak features in current–voltage (I–V) characteristics at low temperatures and did no more than confirm the theoretical prediction of resonant tunnelling.