Subject: Advance Communication System

Difficulty: Medium

Marks: 4 marks

Operation:

• Tunnel diode is a thin junction diode which under low forward bias conditions exhibits negative resistance useful for oscillation or amplification.
• The junction capacitance of the tunnel diode is highly dependent on the bias voltage and temperature.
• A very small tin dot about 50μm in diameter is soldered or alloyed to a heavily doped pellet of n- type Ge, GaSb or GaAs.
• The pellet is then soldered to a kovar pedestal, used for heat dissipation, which forms the anode contact.
• The cathode contact is also kovar being connected to the tin dot via a mesh screen used to reduce inductance.
• The diode has a ceramic body and hermetically sealing lid on top.
• In tunnel diode semiconductor material are very heavily doped, as much as 1000 times more than in ordinary diodes.
• This heavy doping result in a junction which has a depletion layer that is so thin (0.01μm) as to prevent tunneling to occur.
• In addition, the thinness of the junction allows microwave operation of the diode because it considerably shortens the time taken by the carriers to cross the junction.
• A current-voltage characteristics for a typical Germanium tunnel diode is shown in figure.
• Forward current rises sharply as voltage is applied. At point A, peak voltage occurs.
• As forward bias is increased past this point, the forward current drops and continues to drop until point B is reached, this is the valley voltage.
• At point B current starts to increase once again and does so very rapidly as bias is increases further.
• Diode exhibits dynamic negative resistance between A and B therefore, useful for oscillator applications.

Application:

1)Tunnel diode maybe used as a mixers.

2)Being high speed device ,tunnel diodes also used for high speed switching and logic operations as flip flop gates

3)They are used s oscillators upto 100 Ghz.