The resonant-tunneling diode is one of the structurally simplest active (gain-producing) semiconductor devices. Because it operates by quantum-mechanical tunneling, it has also been considered to be a prototypical nanoelectronic device. Interest in this device was dates to 1983, and for more than ten years the RTD defied accurate theoretical description. A major theoretical effort initiated in 1994 and organized by Texas Instruments resulted a thorough understanding of the operation of the device. The results indicate that the current is dominated by direct tunneling; inelastic processes are a very minor effect. However, the conventional approximations used to describe tunneling must be discarded. One must employ accurate semiconductor band-structure models and a realistic self-consistent potential. The symmetry which permits a separation of spatial variables is only approximately obeyed, and the assumption of such symmetry leads to spectacularly large errors in the results. In general, the thrust of these results is simply that one has to correctly describe the detailed physics.

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