Unique Properties & Uses of Esaki Diode

We have already learnt about diodes and what are rectifier diodes and the uses of diodes in full/half wave rectification. There are another type of diodes which you may not be very familiar with since they are not much in practical use today. That certainly does not mean that they are obselete, but they are still used in various applications. In this article we will find out what is a tunnel diode and why this diode is so special?

Generally tunnel diodes are 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 their 'fast response' tunnel diodes 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.

Just like other general diodes, these to consist of a pn junction in it. 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 it, a very narrow stream of electrons forces its way across and through the pn junction. This phenomenon is called 'tunnelling' 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

If we look at the graph, it indicates how the diode behaves when a forward voltage and current is applied to it. It is clearly evident that it has a '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). Theirs no doubt that the diodes resistance is negative and is expressed as -Rd.

We will only discuss the general design and not go in details as the calculations are much complicated. Due to the negative characteristic of the 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 resistances 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 the operating principle of tunnel diodes.

A group of carefully selected tunnel diodes 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}