Definition:
A heavily doped two-terminal semiconductor device through which electric current flows because of tunneling ( or tunnelling) of electrons is known as Tunnel Diode. It possesses the negative resistance characteristic in which current decreases even when the applied voltage is increased.
Tunneling basically denotes the penetration of electrons through the depletion region. As highly doped diode has a thin depletion region. It works on the principle of Quantum mechanical tunneling. As tunnel diodes show fast switching characteristics thus it finds extensive applications in the field of the microwave.
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| Symbol of Tunnel diode |
Working principle of Tunnel Diode
As we have already discussed that the doping concentration in case of tunnel diode is very high. Due to this, the reduced width of the depletion region causes the penetration of charge carriers across the junction even when the carriers do not have enough energy to jump across it.
Let us understand the working of a tunnel diode by considering some conditions.
1. In a zero biased condition, the highly doped diode is not provided with any input voltage. So, in no biased condition, an overlapping is noticed between the conduction band of n region and valence band of p region
This overlapping causes the two bands to appear at the almost same energy level. As the depletion width is very small thus, without any applied potential, due to temperature variation, electrons from n side tunnels to p side.
Similarly, holes from p side tunnels to n side.
As the tunneling is equal for both the carriers thus no current is noticed in case of zero biased condition.
2. Now, when a certain forward voltage is applied to the diode, no any current will flow due to this applied potential. As this forward voltage is less than the barrier potential of the depletion region.
But, still, a small current flows through the device. This is so, because, the electrons in the conduction band of n region will penetrate over thin depletion region and reaches valence band of p region.
The velocity of punching of these electrons is somewhat similar to the velocity of light. Hence, this punching generates a forward current through the device.
3. If the forward potential applied to the device is increased further. Then the conduction band and valence band shows some overlapping. This causes a small current to flow through the device. However, here with the increased potential current through the device now starts decreasing.
After this, the applied potential is increased further. But now this applied voltage will cause the current through the device to reach nearly zero. As the conduction and valence are no longer forming an overlap connection. But, the applied potential will now overcome the barrier potential of the depletion region. So, current as in case of the normal diode will flow through the tunnel diode also.
This is how a tunnel diode works.
Applications of Tunnel Diode
It is widely used in microwave and high-frequency applications because of its fast response. It can be also used as amplifiers and in oscillator circuits.
Key terms related to Tunnel Diode
Tunneling current: It is the current flowing through the device due to the punching of electrons across the junction. The thin depletion width allows some free electrons to tunnel the junction rather than jumping over it. This penetration of electrons generates a current known as Tunneling current.
Negative resistance characteristics: The negative resistance characteristic is the special ability of tunnel diode in which current represents fall even when the voltage provided to it rises.
Valley current: When tunneling reaches its minimum value with the increase in forward potential. Then the lowest value of tunneling current is the valley point of the device.
So, we can conclude that the device possesses conduction initially due to quantum mechanics. After reaching some specified large forward potential the tunnel diode begins to behave like a normal diode.
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