Question

Difference between zener and tunnel diode

Answer 1

I have no clue where sandyboy got his information, but he is not correct. A zener diode is used for voltage regulation. It is used to establish and hold a certain voltage value, dependent on supply voltage, and the series limiting resistor that establishes the minimum conduction current for the diode. The zener is used for voltage reference, as well as to limit certain voltages where 1 or 2 diodes would not provide the needed voltage level for a circuit. 
The Tunnel diode can be used as an amplifier, or usually as an oscillator, but it can also be used as a switch.

Answer 2

    Zener diode is a heavily doped p-n junction diode. It is biased in the reverse direction. It can conduct current in the forward as well as in reverse direction. Near its breakdown voltage the electric field at the junction make it possible for minority carriers from the junction to start flowing. So heavy currents can flow but maintaining the same voltage across the junction diode. Its main function is to regulate voltage across a load resistance. It is used in many power electronics and other electronic circuits.  Zener is not used at very very high frequencies.

    Tunnel diode is a p-n junction diode. It is operable at very high frequency. It is used in oscillators (clocks) which are in megaHertz range or above. The tunnel diode can be used in the negative bias region also.

   Tunnel diode in its positive bias, has a positive resistance in its V-I characteristics. After a certain voltage, the increase in voltage results in decrease in the current. This is due to reduction in the tunneling. This effect is used in making the high frequency oscillator circuits. They are used in TV tuners, Oscilloscopes in labs etc.
 
See the two diagrams for V-I characteristics.