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 .