Tunnel Diode I Leo Tutorials I

This diode was first introduced by Dr. Leo Esaki in 1958. So it is called Esaki diode.

  Construction :  It is a high-conductivity two terminal p-n junction diode having doping density about 1000 times higher as compared to heavy doping produces following three unusual effects :

  1. Firstly, it reduces the width of depletion layer to an extremely small value (about 0.00001 mm) 
  2. Secondly it reduces the reverse breakdown voltage to very small value with the result that diode appears to be broken down for any reverse voltage.
  3. Thirdly it produces a negative resistance resistance on V/I characteristic of diode. It is called tunnel diode because due to extremely thin depletion layer, electrons are able to tunnel through the potential barrier at relatively low forward bias voltage ( less than 0.05 V ). Such diodes are usually fabricated from germanium, gallium arsenide ( GaAs ) and gallium antimonide(GaSb).
                        The commonly used schematic symbols for the diode. It should be handled with caution because being a low power device, it can be easily damage by heat and static energy.

    V/I Characteristic :  Forward bias produces immediate conduction i.e., as soon as forward bias is applied, significant current is produced. The current quickly rises to its peak value Ip when the applied forward voltage is increased further diode current start decreasing till it achieves its maximum value called valley current Iv corresponding to valley voltage Vv ( point B ). For voltage greater than Vv current start increasing as in any ordinary junction diode. between the peakpoint A and the point B, current decreases with increases in applied voltage. In other words tunnel diode posses negative resistance (-Rn) in the region. In fact this constitute the most useful property of the diode.Instead of absorbing power, a negative resistance produce power. By off setting losses in L and C components of the tank circuit , such a negative resistance permit oscillation. Hence a tunnel diode is used as very high frequency oscillator.

            Another point worth nothing is that this resistance increases as we go from point A to B because as voltage is increased current keeps decreasing which means that diode negative resistance keep increasing.

   Equivalent Circuit : The equivalent circuit of a tunnel diode . The capacitance C is the junction diffusion capacitance ( 1 to 10 pF ) and ( -Rn) is the negative resistance. The inductor Ls is due to mainly to the terminal leads ( 0.1 to 4nH ). These factors limit the frequency at which the diode may be used. they are also important in determining the switching speed limit.

 Application : Tunnel diode is commonly used for following purposes :

  1. As on ultrahigh speed switch due to tunneling mechanism which essentially takes place at the speed of light. It has a switching time of the order of nanosecond or even picosecond.
  2. As logic memory storage device due to trippled valued feature of its curve for current.
  3. As microwave oscillator at a frequency of about 10 GHz due to its extremely small capacitance and inductance and negative resistance.
  4. In relaxation oscillator circuits due to its negative resistance.    

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