A very fast operation is achieved with a type of diode called Esaki Diode. Quantum mechanical effect will be used to produce microwave frequency. The diode was invented in 1957 by Leo Esaki, Yuriko Kurose and Takashi Suzuki. Esaki received the Noble prize for Physics in 1973. The award was presented jointly with Brian Josephson for inventing the electron tunneling effect. As per the law of physics, a charged particle should possess energy equal to the barrier so that it can cross the barrier. There exists a non-zero probability that the particle with energy less than the barrier can cross the barrier. It is called tunneling effect and the probability will increase with the decrease in barrier energy.
Esaki diode which is also known as Tunnel diode is a heavily doped p-n junction. The junction is about 10nm wide. There will be broken band gap with the heavy doping. The conduction band electron states on the n-side will be aligned to valence bond hole states on the p-side. The production of tunnel diodes was started by Sony in 1957. Other companies followed the suit in 1960. The diode will be made from germanium in most of the cases. However, it can be made with gallium arsenide and silicon materials as well. These diodes are used in the preparation of frequency converters and detectors. As these parts contain negative differential resistance in the operating range, they are used in the production of oscillators and amplifiers as well.
Metal–insulator–metal (MIM) diode is another type of tunnel diode. However, the application is limited to the research and development only. If there is an additional insulator layer, it will lead to Metal–insulator–insulator–metal (MIIM) diode. There will be precise diode control with the additional insulator layer.
Operation of esaki diode
There will be an increase in voltage under normal forward bias. The electrons will go through the narrow p-n junction and they will fill the electron states in the conduction band on the n-side. There will be further increase in voltage and it will lead to misaligned state and the current will be dropped. When the voltage increases, the current will decrease and it is called the negative resistance. As there is further increase in voltage, the diode will operate like a normal diode and electrons will travel through the p-n junction.
If the diode is used in reverse direction, the tunnel diode is called back diode or backward diode. It can act like a fast rectifier and there will be zero offset voltage. When the diode is under the reverse bias, the p-side will be aligned with the empty states on the n-side and electrons will go through the pn junction barrier in reverse direction.
Applications of tunnel diode
Tunnel diodes are used for low voltage and high-frequency switching applications. The tunneling will take place at the speed of light and there will be low cost and low noise generation. The diode can be fabricated very easily and there will be low power dissipation. Tunnel diodes are used in high-frequency microwave oscillator as well.