A Diode is an electronic device that can allow the electric current to flow in one direction only. The diode performs the work of allowing the current to pass only in the forward direction and not in the reverse. This unidirectional property of the diode is used in rectification of alternating current and in modulation-demodulation of radio signals. However, although the application of the unidirectional property of the diodes is common; they posses other properties too, according to their type that are applied, in a wide range of applications.
The combination of a thermionic electron emitter and a collector, to which the emitted electrons can travel, can be called as a Diode. A modern valve consists of an indirectly heated ‘cathode’ that serves as an electron emitter and a ‘plate’ that acts as a collector. Both are sealed in a highly evacuated glass envelope. The cathode is an oxide-coated nickel cylinder, having an insulated heater filament enclosed within it. The plate is also a hollow cylinder made of nickel, iron or molybdenum and it surrounds the cathode cylinder. The cathode emits electron on being heated, the collector collects them and thus the current flows in the circuit.
A p-n junction diode is a semiconductor crystal having acceptor impurities (p-type) in one region and donor impurities (n-type) in the other region. The boundary between the two regions is called the p-n junction. Near this junction a depletion region is formed due to slight movement of the holes (from p region) and electrons (from n region) towards each other. This forms a potential barrier that needs to be overcome for current to flow. When the positive pole of the battery is connected to the p region and negative to the n region, it is called as forward bias. The potential barrier is overcome in this case and the diode conducts. If the poles are connected in the opposite way, it is the reverse bias. This case leads to an increase in the potential barrier and the diode does not conduct.
The reverse bias of a diode causes the holes and electron to be pushed away from the junction. Hence, usually zero or a minimal current results. A small current that occurs is due to minority charge carries in the p and n regions and it slightly increases with temperature. If the reverse bias voltage is made very high, the covalent bonds near the junction break down and a large number of electron-hole pairs are liberated. The reverse current then increases abruptly to a relatively large value. This is known as Avalanche breakdown of the diode and may damage the junction diode by the excessive heat it generates.
Diodes are varied in there making and function. The most common are LEDs, Zener diode, point-junction diode, varactor diode, tunnel diodes, photo diode etc. the LEDs are light emitter diodes and they emit light of a definite frequency when forward biased. The Photodiode on the other hand allow the current to flow when illuminated, which changed with the intensity of light. Zener diode is used in reverse bias case only as the reverse current is almost stable until avalanche breakdown in it.