So increased voltage results in a drop in current. As the voltage continues to increase the junction begins to function as a normal diode as electrons travel by conduction through the p-n junction and no longer tunnel through it. Therefore the most important region for operation in a tunnel diode is the negative resistance area.
Used in reverse direction Esaki diodes are called back diodes and act as very fast rectifiers with a zero offset with extreme linearity for power signals and have an accurate square law characteristic in the reverse direction. As back diodes, with reverse bias at high reverse voltage, electrons actually flow in the opposite direction, as electron states on each side of the p-n junction become increasingly aligned, allowing electrons to tunnel through the p-n junction going the opposite direction -- this is called the Zener effect which also occurs in zener diodes.
Above is a rough approximation of the VI curve for a tunnel diode, showing the negative differential resistance region.
Normally, in a standard semiconductor, conduction operates with the p-n junction forward biased and current flow is blocked when the junction is reverse biased. Up until there is a "reverse breakdown voltage" where the device is destroyed by voltage, the semiconductor functions. The tunnel diode has sufficient dopant (phosphorous) concentration to create a zero voltage breakdown, but the voltage begins to conduct in the opposite direction. In the forward bias, while voltage is moving forward, there is actually the phenomenon of "quantum mechanical tunneling" occurring, where, in one region there is an increase in forward voltage accompanied by decrease in forward current. This region is called the negative resistance region and can be utilized in a solid state version of the dynatron oscillator (Tunnel 11).
In the 1950s the tunnel diode promised to be an oscillator and the kind of high frequency threshold or trigger device that had not been seen before, since it could operate at higher frequencies than the tetrode, even into the microwave bands. But more convential semiconductors have since exceeded its performance, utilizing conventional oscillator techniques. However, tunnel diodes are also resistant to nuclear radiation, compared to other diodes, which would make them better suited to higher radiation environments, such as in space applications.
While the Esaki diode has not been widely used, its application is nearing widespread use in computers. The diode circuits have been developed and crash programs have been implemented by major computer makers in order to apply it to communications equipment. Yet there still is resistance to the use of these phenomenal little circuits, where currents may be reversed. Some designers and engineers prefer transistors. Proven circuits and solid-state technology that is already acceptable seem to be sufficient, they say. But interest in the diode is so great that they are being forced to try to apply this not-so-new technology to obtain ultra-high speed and ultra-low power consumption in the computer industry. The diode phenomenon dominated 1960 Physics Conferences.
Interest in the diode is so great that it dominated the solid-state conference. More than 3100 engineers and physicists from the U.S., Japan, and Europe were drawn to Philadelphia for the conference. Ten of the 43 papers and two heavily attended informal discussions were devoted to the Esaki diode (Scrupski Mar. 2000).
The Esaki diodes have been used in some limited applications in computers, but have not been used as widely as popular interest predicted. Tunnel diodes are a long way from wide-spread use in switching applications. The promise of its potential use continues and the General Electric Transistor Manual, Fifth Edition, has two chapters on the tunnel diode. Written by GE applications engineers, this book was most popular of all manufacturer-issued transistor handbooks (Scrupski Oct. 2000).
The ability of particles of electrons to tunnel has been recorded and even controlled. This phenomenon, however, appears to have limited use and power. The design of the Esaki diode has shown us that it can be measured and utilized, but to what purpose has yet to be found. It may be used in future atomic reactors and space programs, as it is resistant to radiation, but those uses may even be exceeded by other devices. However, this knowledge is well recorded and known, so that future scientists and physics engineers may find it a common tool one day, when the need arises.