Tunnel Diode: Definition, Characteristics & Applications

A tunnel diode, also known as an Esaki diode, is a type of semiconductor diode that exhibits “negative resistance” due to the quantum mechanical phenomenon called tunneling. Tunnel diodes have a pn junction that is heavily doped and approximately 10 nm wide. This extreme doping results in a broken bandgap, where conduction band electron states on the N-side align with valence band hole states on the P-side.

Due to transit time and other effects, transistors are limited in their application in high-frequency ranges. However, many devices utilize semiconductors’ negative conductance properties for such applications. One such device is the tunnel diode, also known as the Esaki diode, named after L. Esaki for his work on this effect.

The dopant concentration in the p and n regions is extremely high, ranging from 1024 to 1025 m-3. The pn junction is abrupt, resulting in a narrow depletion layer width. When a forward bias is applied, the current-voltage characteristics of a tunnel diode exhibit a negative slope region.

The term “tunnel diode” refers to a phenomenon within the diode due to quantum mechanical tunneling. The diode is heavily doped, so at a temperature of absolute zero, the Fermi levels lie within the bias of the semiconductors.

Characteristics of Tunnel Diode

When a reverse bias is applied to a p-n junction, the Fermi level on the p-side becomes higher than that of the n-side. This leads to the tunneling of electrons from the balance band of the p-side to the conduction band of the n-side. As the reverse bias strength increases, 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. This causes the tunneling of electrons from the n-side to the p-side. The amount of current generated by this tunneling is much larger than the normal junction current. As the forward bias increases, the tunnel current also increases to a certain limit.


When the band edge of the n-side is aligned with the Fermi level in the p-side, the tunnel current reaches its maximum. The tunnel current decreases as the forward bias increases, and the desired negative conduction region is achieved. It further increases the forward bias, resulting in a normal pn junction current exponentially proportional to the applied voltage. These features can describe the V-I characteristics of the tunnel diode. The negative resistance is commonly utilized to generate oscillation.

Tunnel Diode Symbol

The symbol for a tunnel diode is depicted below.


Tunnel Diode Applications

A tunnel diode is a semiconductor diode capable of operating at very high speeds, particularly in the microwave frequency. It derives its unique characteristics from a quantum mechanical effect known as tunneling. Thanks to its negative slope characteristics, it is ideal for use in fast oscillators and receivers. However, its applications are somewhat limited due to its inability to be used in large integrated circuits.

When a voltage is applied, a current begins to flow through it. As the voltage increases, the current also increases. However, once the voltage reaches a certain point, the current suddenly increases again, and the tunnel diode behaves like a normal diode. This unique behavior makes it useful for various special applications, which we’ll explore below.

Oscillator Circuits:

Tunnel diodes are often utilized as high-frequency oscillators due to their rapid transition between high electrical conductivity states. These diodes can generate oscillations as high as 5 GHz; they can even produce oscillations up to 100 GHz in digital circuits.

Used in Microwave Circuits:

Ordinary diode transistors are not suitable for efficient performance in microwave operations. To overcome this limitation, tunnel diodes are utilized in microwave generators and amplifiers. These diodes were previously popularly used in microwave waves and satellite communication equipment. However, in recent times, their usage has declined considerably due to the availability of transistors that operate in this frequency range.

Resistant to Nuclear Radiation:

Tunnel diodes have remarkable resistance to magnetic fields, high temperatures, and radioactivity. This makes them an ideal component for modern military equipment. Additionally, they are used in nuclear magnetic resource machines. However, the most significant application of tunnel diodes is in satellite communication equipment.

  1. Varactor Diode
  2. Zener Diode
  3. Forward bias and Reverse Bias of PN Junction Diode
  4. Diffusion Capacitance of Diode
  5. Depletion Width of Diode

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