What is Tunnel Diode?

A tunnel diode or Esaki diode is a type of semiconductor diode that is capable of very fast operation, well into the microwave frequency region, by using the quantum mechanical effect called "Tunneling". 

A tunnel diode is a high conductivity two terminal P-N Junction diode doped heavily about 1000 times higher than a conventional junction diode. Tunnel diodes are useful in many circuit applications in microwave amplification, microwave oscillation and binary memory.  

The tunnel diode exhibits a special characteristic know as negative resistance. This feature makes it useful in oscillator and microwave amplifier applications. Tunnel diodes are constructed with germanium or gallium arsenide by doping the p and n regions much more heavily than in a covenional rectifier diode. 

Symbol

This heavy doping allows conduction for all reverse voltages so that there is no breakdown effect as with the conventional rectifier diode.

Working of Tunnel Diode

When a small forward bias voltage is applied across a tunnel diode, it begins to conduct current. As the voltage is raised, the current increases and attains a peak value known as peak current. If the current is increased a little more, the current actually begins to decreases until it reaches a low point called the valley current. If the voltage is increased further yet, the current begins to increase again, the time without decreasing into another “valley”. The region on the graph where the current is decreasing while applied voltage is increasing is known as the region of the negative resistance.

It has negative resistance in the shaded voltage region, between v₁ and v₂.

Mechanics behind working

According to classical mechanics theory, a particle must have an energy at least equal to the the height of a potential-energy barrier if it has to move from one side of the barrier to the other. In other words, energy has to be supplied from some external source so that the electrons on N side of junction climb over the junction barrier to reach the P-side. 

However if the barrier is thin such as in tunnel diode ,the Schrodinger equation(Quantum Mechanics) indicates that there is a large probability that an electron will penetrate through the barrier. This will happen without any loss of energy on the part of electron. This quantum mechanical behavior is referred to as tunneling and the high-impurity P-N junction devices are called tunnel-diodes. The tunneling phenomenon is a majority carrier effect.

Why tunneling? 

It is that the reduced depletion layer can form result in carriers “punching through” the junction with the velocity of light even when they do not possess enough energy to overcome the potential barrier. The result is that large forward current is produced at relatively low forward voltage (less than 100mv) such a mechanism of conduction in which charge carriers (possessing very little energy) punch through a barrier directly instead of climbing over it is called tunnelling. That’s why such diodes are known as tunnel diodes. Because of heavy doping the tunnel diode can conduct in reverse as well as in formed direction but it is usually used in forward biased mode. 

Reverse Bias 

In the tunnel diode, the dopant concentration in the p and n layers are increased to the point where the reverse breakdown voltage becomes zero and the diode conducts in the reverse direction.  

Applications of Tunnel Diode 

• The tunnel diode showed great promise as an oscillator and high-frequency threshold (trigger) device since it would operate at frequencies far greater than the tetrode would, well into the microwave bands. 

• Applications for tunnel diodes included local oscillators for UHF television tuners, trigger circuits in oscilloscopes, high speed counter circuits, and very fast-rise time pulse generator circuits. 

• The tunnel diode can also be used as low-noise microwave amplifier.

• Tunnel diodes are also relatively resistant to nuclear radiation, as compared to other diodes. This makes them well suited to higher radiation environments, such as those found in space applications 

Courtesy:  http://www.expertsmind.com