Tunnel Diode

A diode is an electronic device having two electrodes; one of which is positive (called the anode) and the other is negative (called the cathode). The node at which these two electrodes meet is called the P-N Junction. Because of having only a single P-N junction, the diode serves as a one-way conductor that allows current to flow only in one direction.

Current flows through a diode only when the diode is in the forward-bias or ON state. This happens when a positive voltage is applied to the anode and the negative voltage is applied to the cathode. During this state, the internal resistance of the diode is very small, that is why current can easily pass through it. On the other hand, when a voltage is applied across the diode such that the anode is connected to the negative voltage and the cathode is receiving the positive voltage, the diode is said to be in reverse-bias OFF state. While in the reverse-bias state, the diode has a very high internal resistance making it impossible for current to pass through it.

The process of allowing current to flow only in one direction is called rectification. One of the most common applications of the diode is rectification. It is often used to convert alternating current to direct current. However, there are different special-purpose diodes which perform more than rectification.

Special-Purpose Diodes

Capacitive or Varactor Diode – Also called varicap, this diode, when operated in reverse-bias, serves as capacitance. The capacitance value is in picofarad. This capacitance can be controlled by varying the reversed input voltage.

Zener Diode – The zener diode is a voltage reference diode. They are used to maintain a fixed amount of voltage. Zener diodes are designed to have a specific reverse “breakdown” voltage. The breakdown voltage is the voltage that the diode can keep constant while operating in reverse-bias. This type of diode is widely used for voltage regulation in dc power supplies.

Tunnel Diode – Also called Esaki Diodes. The important feature of this type of diode is its negative resistance while operating on a certain range of forward voltage. With this characteristic, the diode allows an increase in current when the voltage is decreased. Because of this, the tunnel diode can be used as an amplifier.

Light-Emitting Diode (LED) - The LED is a special-purpose diode that can produce light as current passes through it. Because of this characteristic, the LED is widely used as a basic display component, especially that it requires less power to operate compare to other display units. The typical voltage value needed is 1.6 V to produce 20mA of current. LED’s come in different colors like red, green, blue, yellow, and orange. There are also multi-colored LED’s and blinking LED’s.

Photodiode – This type of diode is made up of photosensitive materials such as sulfide and cadmium, where the resistance decreases with more light. The resistance value without light is called the dark resistance and is typically in the range of megaohms. Being sensitive to light, the photodiode is usually used for many light-control devices.