A tunnel diode is a highly conductively two terminal P-N junction doped heavily about 1000 layer width is a high conductively two terminal conventional diode. Because of heavy doping depletion layer width is reduced to an externally small value of the order of 10-5mm reverse breakdown voltage is also reduced to a very small value of the order resulting in appearance to the diode to be broken for any reverse voltage, and a negative resistance section is produced in volt ampere CHARACTISTICS of the diode. 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.
CONSTRUCTION OF TUNNEL DIODES: Tunnel diodes are usually fabricated from germanium, gallium arsenide or gallium antimonite. Silicon is not used in the construction of the tunnel diodes because the ratio of peak value of forward current to the value of valley current is maximum in the case of gallium arsenide (approximately 15) and comparatively smaller for germanium (roughly 8) but very in case of silicon (about 3). Tunnel diodes are ordinary manufactured by allowing from gallium arsenide. The source materials are highly doped semiconductor crystal with an impurity concentration of the order of 1025 per cubic meter or more. It is noted that the tunnel diode being a low power device, can be easily damaged by heat and static electrically.
WORKING OF THE TUNNEL DIODES: Tunnel diodes exploit a strange quantum phenomenon called resonant tunnelling to provide interesting forward bias charactistics. 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.