We have already learned about diodes, rectifier diodes, and the uses of diodes in full/half wave rectification. There is another type of diode that you may not be very familiar with since they are not often in practical use today. That certainly does not mean that they are obsolete, but they are still used in various applications. In this article we will find out what a tunnel diode is, and why this diode is so special.
Generally a tunnel diode is made up of Gallium-Indium Antimonide(GISp). It is also called the Esaki diode, named after its Japanese inventor. Long ago in the early fifties these diodes were extensively used in many semiconductor circuits. Due to its "fast response," the tunnel diode played an important role in RF, mixer, oscillator and detector circuits then. Though not in great demand today, they are still produced in limited quantities.
How Tunnel Diodes Function
Just like other general diodes, a tunnel diode consists of a pn junction. But the depletion layer in it is extremely narrow. Even the fastest of silicon diode is no match against these diodes.
When a forward voltage is applied to a tunnel diode, a very narrow stream of electrons forces its way across and through the pn junction. This phenomenon is called "tunneling" and hence the name tunnel diode. This happens because the doping level used in it is extremely high. The electronic symbol used for a tunnel diode is shown in the adjacent figure.
Unique Property of Tunnel Diodes
If we look at the graph, it indicates how a tunnel diode behaves when a forward voltage and current is applied to it. It is clearly evident that it has "negative resistance characteristics," i.e. after an initial peak, the forward current starts decreasing with an increase in the forward voltage up to a certain extent (indicated by the shaded region). There is no doubt that the diode's resistance is negative, and this is expressed as -Rd.
Its Use to Generate Electricity
We will only discuss the general design and not go in details as the calculations are very complicated. Due to the negative characteristic of an Esaki diode, a charge current is generated when heat is applied to it. It can be explained in this way: when a normal resistance R is connected to a battery, it starts discharging (here according to Ohms law I=V/R).
Therefore logically a negative resistance should start charging the battery (now -I=V/-R). Similarly if the power dissipated in a normal resistance is P=I2R watts, conversely a negative resistance should generate wattage since in this case P=-I2-R. The negative sign of current (I) indicates generation of power, and this is the operating principle of tunnel diodes.
A group of carefully selected tunnel diode in series when fitted to a large heat(external) absorbing metal, can produce enough power so as to charge a 1.2V NiCd battery. The heat applied may be solar or any other form.
Idea courtesy: J. Freshwater and C. Sanjay