A diode is an electronic component that only conducts electric current in one direction. It has zero resistance in one direction and infinite direction in the other direction. It consists of a vacuum tube with two electrodes; a cathode and a plate. The most common type of diode is the semiconductor diode, which is made of a crystalline piece of semiconductor material with p-n junction in between the two electrical terminals; the cathode and the plate.
When the cathode is heated electrons start to flow in only one direction, from the cathode to the plate hence referred to as diode’s forward direction or forward biased. This occurs only when a certain cut-in voltage or threshold voltage is met. The unidirectional behavior is referred to as rectification and is applied in converting alternating current to direct current.
The semiconductor diode’s characteristics depends on the semi-conductor materials used and the doping materials introduced during manufacture. These leads to different purpose diodes that perform different functions. Example: Zener diodes are used to regulate voltage; avalanche diodes are used to protect circuits from high voltage surges; varactor diodes are applied in tuning radio and TV receivers; Gunn diodes, tunnel diodes and IMPATT diodes are used to generate radio-frequency oscillations; and Light Emitting Diodes are used to produce light.
Semiconductor and vacuum diodes can be used as short-noise generators. The vacuum-tube diodes are applied in high power applications, audiophile applications and in musical instruments.
Vacuum tube (thermionic) diodes consists of a sealed evacuated glass that contains two electrodes; cathode and plate. The cathode is either directly heated or indirectly heated. If directly heated, the cathode is made of tungsten wire and is heated by current passing through it from an external voltage source. If the cathode is indirectly heated, it is heated by infrared radiation from a nearby heater formed of Nichrome wire and supplied with current from an external voltage source.
The heated cathode releases electrons into the vacuum through a process called thermionic emission. The plate being not heated, it absorbs the emitted electrons. Alternating voltage to be rectified is applied between the cathode and the plate. Current starts to flow only when the plate voltage is positive with respect to the cathode.
P-n junction diode
A p-n junction diode is made of a crystal of semiconductor of silicon, germanium or gallium arsenide. Impurities are added to create a positive (p) side that contains an excess of holes and the negative (n) side that contains an excess of electrons. This allows current to flow only in one direction, from the n side to the p side, creating a third region between the two where no charge carriers are present. This region is called the depletion region.
The boundary or interface between the n-type and p-type regions is called the p-n junction and it is where the diode action takes place. If the voltage at the p-side (the anode) is comparatively higher than at the n-side (the cathode), electrons flow through the depletion region from the n side to the p side. The junction does not allow flow of electrons in the opposite direction when the voltage is applied in the reverse direction resulting into a breakdown of the diode.
Types of semiconductor diode
1. Avalanche diodes
Avalanche diodes conduct in the reverse direction when the reverse bias voltage exceeds the breakdown voltage. They are similar to Zener diodes but break down by a different mechanism; the avalanche effect.
2. Zener diodes
Just like the avalanche diodes, they also conduct in reverse direction, but the breakdown or Zener breakdown, occurs at a precisely defined voltage. This makes the diode to be used as a precision voltage reference.
3. Schottky diodes
Schottky diodes have a lower forward voltage drop than p-n junction. They are mostly used in voltage clamping applications and prevention of transistor saturation.
4. Laser diodes
They are mostly used in optical storage devices and in high speed optical communication.
5. Light Emitting Diodes (LEDs)
LEDs are used as low-efficiency photodiodes in signal applications. They can also be used to form opto-isolator when combined with a photodiode or phototransistor.
Photodiodes are used in solar cells, optical communications, and in photometry as photodetectors.
7. Thermal diodes
They are used to monitor temperature because of their varying forward voltage with temperature.
8. PIN diodes
A PIN diode has a central un-doped/intrinsic layer forming a p-type/intrinsic/n-type structure. They are used are used as photodetectors, ionizing radiation detectors and as radio frequency switches.
9. Constant current diodes
They allow current to rise to a certain value and then level off at a certain value.
10.Transient voltage suppression (TVS) diodes
They are avalanche diodes designed to protect other semiconductor devices from high-voltage transients. This is because they have a p-n junction that has a larger cross-sectional area than the normal diode, allowing them to conduct large currents to ground without damaging.
Application of diodes
1.Ionizing radiation detectors.
Semiconductor diodes are sensitive to more energetic radiation, hence they are used in detecting different ionizing radiation by measuring the charge conducted.
2. Logic gates.
Diodes are used in constructing AND and OR logic gates.
3. Radio demodulation.
This was the first application of the diode; demodulating amplitude modulated (AM) signal in radio broadcasts. Since the AM signal consists of alternating positive and negative peaks of a radio carrier wave, whose amplitude or envelope is proportional to the original audio signal, diodes are used in rectifying the AM radio frequency signal, leaving only the positive peaks of the carrier wave.
4. Power conversion
Diodes are used in constructing rectifiers that are used to convert alternating current to direct current.
5. Over-voltage protection.
Diodes are used in protecting sensitive electronic devices from damaging high voltages.
6. Temperature measurements
Diodes are used as temperature measuring devices since
the forward voltage drop across the diode depends on temperature.