What is a Tunnel Diode?

A tunnel diode (also known as an Esaki diode) is a type of semiconductor diode that has effectively “negative resistance” due to the quantum mechanical effect called tunneling. Tunnel diodes have a heavily doped PN junction that is about 10 nm wide. The heavy doping results in a broken bandgap, where conduction band electron states on the N-side are more or less aligned with valence band hole states on the P-side.

The application of transistors in a very high-frequency range is hampered due to the transit time and other effects. Many devices use the negative conductance property of semiconductors for these high-frequency applications. A tunnel diode is one of the most commonly used negative conductance devices. It is also known as Esaki diode after L. Esaki for his work on this effect.

The concentration of dopants in both p and n regions is very high, at around 1024 – 1025 m-3. The PN junction is also abrupt. For these reasons, the depletion layer width is very small. In the current-voltage characteristics of the tunnel diode, we can find a negative slope region when a forward bias is applied.

The name “tunnel diode” is due to the quantum mechanical tunneling being responsible for the phenomenon that occurs within the diode. The doping is very high so at absolute zero temperature, the Fermi levels lie within the bias of the semiconductors.

Characteristics of Tunnel Diode

When a reverse bias is applied the Fermi level of the p-side becomes higher than the Fermi level of the n-side. Hence, the tunneling of electrons from the balance band of the p-side to the conduction band of the n-side takes place. With the interments of the reverse bias, the tunnel current also increases.

When a forward bias is applied the Fermi level of the n-side becomes higher than the Fermi level of the p-side, thus the tunneling of electrons from the n-side top-side takes place. The amount of the tunnel current is very large than the normal junction current. When the forward bias is increased, the tunnel current is increased up to a certain limit.

When the band edge of the n-side is the same as the Fermi level on the p-side, the tunnel current is maximum with the further increment in the forward bias the tunnel current decreases and we get the desired negative conduction region.

Tunnel Diode Applications

A tunnel diode is a type of sc diode which is capable of very fast and in microwave frequency range. It was the quantum mechanical effect which is known as tunneling. It is ideal for fast oscillators and receivers for its negative slope characteristics. But it cannot be used in large integrated circuits – that’s why its applications are limited.

When the voltage is first applied current stars flow through it. The current increases with the increase of voltage. Once the voltage rises high enough suddenly the current again starts increasing and tunnel diode stars behave like a normal diode. Because of this unusual behavior, it can be used in several special applications started below.

Oscillator Circuits:
Tunnel diodes can be used as high-frequency oscillators as the transition between the high electrical conductivity is very rapid. They can be used to create oscillation as high as 5Gz. Even they are capable of creativity oscillation up to 100 GHz in appropriate digital circuits.

Used in Microwave Circuits:
Normal diode transistors do not perform well in microwave operation. So, for microwave generators and amplifiers tunnel diodes are used. In microwave waves and satellite communication equipment, they were used widely, but lately, their usage is decreasing rapidly, as transistors that operate in this frequency range are becoming available.

Resistant to Nuclear Radiation:
Tunnel diodes are resistant to the effects of magnetic fields, high temperature, and radioactivity. That’s why these can be used in modern military equipment. These are used in nuclear magnetic resource machines also. But the most important field of its use is satellite communication equipment.