If a zener diode is made from heavily doped semiconductors, its depletion region becomes very narrow and the structure becomes: 

When a Zener diode is made from heavily doped semiconductors, its depletion region will be very narrow very and the structure becomes a Tunnel Diode.

Tunnel Diode:

• Symbol:

• It is a highly doped PN Junction diode, used for low voltage high-frequency switching applications.
• It works on the tunneling principle.
• It has a high charge carrier velocity.
• When compared to a normal p-n junction diode, the tunnel diode has a narrow depletion width.
• In normal forward-biased operation, it exhibits the “Negative resistance region” as shown:

• The negative differential resistance in their operation, allows them to be used as oscillators, amplifiers, and switching circuits.
• Their low capacitance allows them to function at microwave frequencies.
• Microwave frequencies range between 109 Hz (1 GHz) to 1000 GHz
• Tunnel diodes are not good rectifiers, as they have relatively high leakage current when reverse biased.