Esakis Quantum Tunneling Diode

The tunneling effect is basic in quantum mechanics, a fundamental consequence of the probabilistic wave function as a measure of the location of a particle. Unlike a tennis ball, a tiny electron may penetrate a barrier. This effect was first exploited in semiconductor technology by Leo Esaki, who discovered that the current-voltage (I/V) curves of semiconductor p-n junction rectifier diodes (when the barrier was made very thin, by increasing the dopant concentrations) became anomalous, and in fact double-valued. The forward bias I vs. V plot, normally a rising exponential exp(eV/kT), was preceded by a distinct "current hump" starting at zero bias and extending to V=50mV or so. Between the region of the "hump" and the onset of the conventional exponential current rise there was a region of negative slope, dI/dV<0!

The planar junction between an N-type region and a P-type region in a semiconductor such as Si contains a "depletion region" separating conductive regions filled with free electrons on the N-side and free holes on the P-side. It is a useful non-trivial exercise in semiconductor physics to show that the width W of the depletion region is

Here ee0 is the dielectric constant, e the electron charge, VB is the energy shift in the bands across the junction, and JVD and NA, respectively, are the concentrations of donor and acceptor impurities.

The change in electrical behavior (the negative resistance range) resulting from the electron tunneling (in the thin depletion region limit) made possible an entirely new effect, an oscillation, at an extremely high frequency! (As often happens with the continuing advance of technology, this pioneering device has been largely supplanted as an oscillator by the Gunn diode, which is easier to manufacture.)

The Esaki tunnel diode is perhaps the first example in which the appearance of quantum physics at the limit of a small size led to a new device. In our terminology the depletion layer tunneling barrier is two-dimensional, with only one small dimension, the depletion layer thickness W. The Esaki diode falls into our classification as an element of nanotechnology, since the controlled small barrier W is only a few nanometers in thickness.

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