Tunnel diode, construction and working

What is 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

Tunnel diode is a two terminal device anode and cathode.

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

 

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

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)

 

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

 

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 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

When applied voltage is increased further

 

When the applied voltage is further increased misalignment of the valence band of P type and conduction band of N type takes place. So the flow of the tunnel current decreases

 

Step 5: When applied voltage is largely increased

When it is further increased it behaves like normal PN junction. At that point the tunnel current is zero.

when applied voltage is largely increased

 

V-I Characteristics of Tunnel diode:

V-I Characteristics of Tunnel diode

 

When the voltage is applied , it starts conducting immediately because of its heavy doping. When the voltage is increased further it shows the negative resistance and the current decreases. When the applied voltage is increased more it conducts like the normal diode.

Advantages:

• It is a low power device
• It is a low noise device
• It is a low cost device
• High speed
• Fast response
• It is easy to operate

Disadvantages

• No isolation between input and output circuit since it is a two terminal device.
• The output power is very low, it is of only few milliwatts.

Application of Tunnel diode:

• It is used in FM receivers because of its low power requirement
• It is used in high frequency components since it shows a fast response.
• It is used as logic memory storage device.
• It is used in high speed switches.
• It is used in microwave circuits.