What is Tunnel diode?


Tunnel diode
Tunnel diode
 


Tunnel diode is a heavily doped PN Junction diode which shows negative resistance. It works on the basis of tunneling effect. It means the electrons passes through the wall or the barrier and reappearing at the other side. It is also called Esaki diode, named after the inventor Leo Esaki who invented in August 1957 and received Nobel prize in 1973 in Physics for discovering electron tunneling effect used in this diodes.

Symbol of Tunnel diode:
 

Symbol of Tunnel diode
Symbol of Tunnel diode


Tunnel diode is a two terminal device anode and cathode.

Construction of Tunnel diode:


Construction of Tunnel diode
Construction of Tunnel diode


 
Gallium arsenide, Gallium antimonide and Germanium are used in the manufacturing of tunnel diode. The doped density is 1000 times higher than the normal diode. A very small tin dot of 50micrometer is soldered or alloyed to a heavily doped pellet which is of n type. The pellet is then soldered to the kovar pedestal. The tin dot is connected to cathode via mesh screen to reduce inductance.  Kovar forms the anode and the cathode. The diode is covered by a ceramic body.

What is  Tunnel Effect:



Normal PN Junction diode
Normal PN Junction diode


 
When the normal PN Junction is forward biased the depletion region decreases and the barrier height also decreases. The electrons in the N type layer cannot penetrate through the depletion region because of the opposition of the inbuilt voltage. When the applied external voltage is increased and when it is higher than the potential of the barrier voltage it passes over the barrier and enters the P side.
 
Tunneling effect
Tunneling effect


Unlike the normal PN Junction diode in Tunnel diode there is a high difference in the energy levels of P type and N type. The energy level of conduction band and valence band of N type is lower than the P type. So the conduction band of N type overlaps with the valence band of the P type. The depletion is very small in tunnel diode so the electrons from N type conduction band can directly pass through the barrier to the P type valence band.

Working of Tunnel diode:


Step 1:when no voltage is applied (Unbiased tunnel diode)

 
Step 1:when no voltage is applied (Unbiased tunnel diode)


No tunnel current
No tunnel current




The diode is said to be in unbiased condition when no external voltage is applied. Since the tunnel diode is heavily doped and the conduction band of N type is overlapped with the valence band of P type, the holes from the P type passes into N type and the electrons from the N type passes into the P type. But the net current is zero since the equal number of charges flow through the opposite side.

Step 2 :  When small voltage is applied
 

when small voltage is applied
when small voltage is applied


 

Small current flows
Small current flows



When small voltage is applied small amount of electrons from the conduction band of N type passes through the barrier into the valence band of P type. Thus small tunnel current is produced.

Step 3: When applied voltage is increased


 

When applied voltage is increased
When applied voltage is increased


 
When the applied voltage is increased more number of free holes and electrons are generated and the conduction band of N type and valence band of P type becomes equal so maximum tunnel current flows. 

Step 4: when applied voltage is further increased