# What is a Tunnel Junction?

G.W. Poulos

A tunnel junction is a point where two different electrically conductive or magnetic materials meet, usually separated by a thin barrier, for the purpose of passing electrons from one material to the other. The defining aspect of a tunnel junction is that, mechanically speaking, the electrons are too weak to penetrate the junction barrier but do so anyway though a principle called quantum tunneling. Tunnel junctions are useful in many fast-acting electronic devices, such as flash memory chips, increasing the efficiency of photovoltaic cells, and the construction of extremely fast diodes capable of reacting at higher frequencies than would otherwise be possible.

The principle of quantum tunneling, on which the operation of all tunnel junctions is based, is established on theories of quantum mechanics. These theories state that even though, mathematically, an electron lacks the active mechanical energy to pass through the stored energy of a given barrier, the chances of any given electron breaching the barrier, though extremely small, are not zero. As the passing of an electron though an obviously superior barrier isn’t normally mathematically or mechanically possible, but exists nonetheless, scientists have surmised that the electron accomplishes this as the result of a quantum mechanics theory called wave-particle duality.

Wave-particle duality theory states that all forms of matter, electricity in the case of a tunnel junction, exist in two separate states simultaneously. First, the matter exists as a particle, such as an electron, which has a certain amount of active mechanical energy due to its mass and velocity. Second, the matter exists as a waveform, which operates and vibrates at a certain frequency.

As a result of wave-particle duality, an electron may not have the active mechanical energy to pass through a barrier; however, at a high enough frequency, it may have enough waveform energy to pass through the barrier. At a high enough frequency, the waveform energy of an electron can literally vibrate through the low-frequency barrier in an action referred to as quantum tunneling. As a result of the very high frequencies involved with quantum tunneling, the actions of the electrons involved happen extremely quickly, which allows a device that uses a tunnel junction to operate extremely quickly. This speed can then either be used to accelerate the operation of electrical equipment or to detect, identify, and react to very fast-moving forms of energy such as light waves.

In practice, tunnel junctions are used primarily in electronics. They provide the speed for reading and writing to and from flash memory, allow the manufacture of extremely fast oscillators that increase the operational speed of computers, and permit the construction of scientific instruments that can detect and operate in high-radiation environments.

The tunnel junction can also be used to interact with light energy and is involved in a number of light-related research projects. In clean energy research, it is being incorporated into high-efficiency solar cells, where its high operational frequencies allow it to capture more energy than conventional cells from the same amount of light. It is also being used in conjunction with superconductors to produce detectors similar to those used in digital cameras, with the exception that they can see ultraviolet, x-rays, and many other types of waveform energies and radiations.