Tunnel Diode


In a conventional semiconductor diode, conduction takes place while the p–n junction is forward biased and blocks current flow when the junction is reverse biased. This occurs up to a point known as the “reverse breakdown voltage” when conduction begins (often accompanied by destruction of the device). In the tunnel diode, the dopant concentration in the p and n layers are increased to the point where the reverse breakdown voltage becomes zero and the diode conducts in the reverse direction.
This reverse resistance occurs because as doping is increased, reverse voltage will decrease and a time will come when there will be reverse  breakdown voltage in forward bias condition. The application of a reverse voltage to the p-n junction will cause a transient current to flow as both electrons and holes are pulled away from the junction. When the potential formed by the widened depletion layer equals the applied voltage, the current will cease except for the small thermal current i.e. as voltage will increase , current will  decrease.


                                                     


However, when forward-biased, an odd effect occurs called “quantum mechanical tunnelling” which gives rise to a region where an increase in forward voltage is accompanied by a decrease in forward current due to change in conduction band position. Quantum tunnelling refers to the quantum mechanical phenomenon where a particle tunnels  i.e. transmitted through a barrier that it classically could not be able to cross.Barrier is the depletion region of p-n junction.


                                                              dynatron oscillator

This negative resistance region can be used  in  the dynatron oscillator .A dynatron oscillator is an electronic circuit that uses negative resistance to keep an LC tank circuit oscillating .If an ideal capacitor is connected in parallel with an ideal inductor, they form a resonant circuit that, once it begins oscillating, will oscillate forever as the energy is transferred back and forth between the capacitor and the inductor.In practice, however, the two components are not ideal. Real inductors and capacitors are equivalent to an ideal component in parallel (or in series) with a resistance; a real resonant circuit is equivalent to an ideal capacitor, inductor, and resistor connected in parallel. If a negative resistance equal in magnitude to this positive resistance can be connected in parallel with the above circuit, then the two resistances will cancel and the circuit will oscillate forever .