"Diodes with a breakdown level equal to zero are called tunnel diodes, or Shockley
diodes. The tunnel diode is a heavily doped diode that is used in high-frequency communication
circuits for such applications as amplifiers, oscillators, modulators, and demodulators. The unique
operating curve of the tunnel diode is a result of the heavy doping used in the manufacturing of the
diode. The tunnel diode is doped about one thousand times as heavily as a standard pn-junction diode.
This type of a diode exhibits a negative resistance."
Read this topic