A diode is a two terminal device using a PN junction. Diodes may be made from Silicon, Germanium, Selenium, or Gallium Arsenide. Most diodes will be made of Silicon. Normally a forward voltage drop of 0.7 volts will be seen with Silicon Diodes, and a forward voltage drop of 0.3 volts will be seen with Germanium Diodes. Diode types:

Rectifier Diode: Heavy duty [high current] used to convert AC to DC

Tunnel Diode: Has a negative resistance region. The current through the diode increases up to a certain voltage, than with increasing voltage the current decreases until a voltage point is reached and the current begins to increase again

Zener Diode: Designed to conduct in the reverse [bias] direction: with a precise breakdown voltage [Vz]

Varactor Diode: 'Varicap' or 'VVC', Voltage [dependent] variable capacitor [varies in picofarads]

Schottky Diode: A Schottky Diode is designed with a rapid turn on and off once the threshold voltage is reached

Light Emitting Diode: 'LED', Gives off light when energized [forward biased]

Photodiode: Operated in reverse-bias, reverse current increases 'almost' linearly with increase in incident light

Varistor: Resistance [flow of current] changes as a function of applied voltage voltage. A varistor may also be called a VDR [Voltage Dependent Resistor]. Varistors will have a negative voltage coefficient [high resistance at low voltages and a low resistance at high voltages].

Metal Oxide Varistor: [MOV] See listing above. MOV devices are used in parallel with the load.

Transient Voltage Suppressor: 'TVS', Designed to absorb a transient over-voltage. The device may be designed as Uni-Polar device protecting in one direction or a Bi-Polar device protecting the circuit in both directions. TVS manufacturers are listed on the Transient Voltage Suppressor page [Nonlinear Resistor Manufacturers]. TVS devices are used in parallel with the load.

Glossary of Diode Terms

Definitions by application, include:
Clamping Diode: A diode used to limit the peak voltage on a line to a pre-determined maximum voltage.
Welding Diode: A diode with a very large forward current rating.