Diodes

There are very many types of diodes, and each is designed to perform specific functions. Diodes are presented functionally, according to the following structure:
1. Rectifier diodes
2. Other types of diodes and their function

The best documentation you can find about any individual electronic component is in its ata Sheet.

RECTIFIER DIODES

The diode is a PN (Positive-Negative) semiconductor junction. That means it is made of two semiconductor parts joined together: one is enhanced with impurities that allow positive currents to pass through, the other one is the reverse.

The following schematics are presented:
1. biasing dides
2. AC rectification
3. overvoltage protection
4. voltage clamping

A PN junction is biased (polarized) directly or in reverse, as follows:

Fig 1: Direct biased diode Voltage measured in point A The current and voltage passing through may be as high as to destroy the diode; they need to be limited by a resistor (R).
Fig 2: Reverse biased diode Voltage measured in point A The current and voltage passing through may be as high as to destroy the diode; they need to be limited by a resistor (R).

AC TO DC DIODE RECTIFICATION

Diodes are used to rectify AC current in order to obtain DC current. Following are few of the most common schematics used.

Schematic	Wave
Fig 3: Half-Wave - IA = IT / π IT = current supplied by transformer
Fig 4: Simple Full-Wave - IA = 2*IT / π IT = current supplied by transformer
Fig 5: Bridge Full-Wave - IA = 2*IT / π IT = current supplied by transformer

All diodes have some small internal resistance, and that develops a small voltage drop accordingly, as follows:
1. for germanium diodes, the voltage drop is 0.2..0.3 V; the average is 0.25 V
2. for silicon diodes, the voltage drop is 0.6..0.7 V; the average is 0.7 V

OVERVOLTAGE DIODE PROTECTION

Another important role diodes play is overvoltage protection. Please compare Figs 6 and 7. You should notice there is no difference in the graphs of the two protection schematics employed.

However, the schematic in Fig 7 is cheaper, it handles way greater currents, and heat dissipation is drastically reduced when compared to employing a Zener diode. Fig 7 is recommended.

Schematic	Graph
Fig 6: Overvoltage protection using a Zener The Zener works reverse biased. On high currents it heats a lot and it may easily break down.
Fig 7: Overvoltage protection using 2 ordinary rectifiers (type 1N4007) This is named "Totem Pole" configuration. Both diodes work directly biased and heat dissipation is minimum. This circuit is way more reliable.

VOLTAGE CLAMPING

Another important function diodes have is voltage clamping. That may be achieved in many types of circuits, and I present two of the most common.

Schematic	Graph
Fig 8: Positive voltage clamping in point A Diode D works directly biased and it clamps positive voltages.
Fig 9: Negative voltage clamping in point A Diode D works directly biased and it clamps negative voltages.

OTHER TYPES OF DIODES AND THEIR FUNCTIONS

Diodes come in many types, and they have various functions. Some diodes are:

• Led
• Shockley
• SCR
• Diac
• Triac
• Zener
• Tunnel
• Varactor
• Laser: used in very many application, from laser pointers to DVD reading/writing.
• Infrared: used for example in TV remote controls.
• microwave: well, we all use one of those.
• PIN diode: is a three layer diode working as a variable resistor when forward biased, and as a variable capacitance in reverse bias schematics.
• Back diode: works better in reverse bias than in direct one. Because it is very stable over temperature variations it is used in frequency control.
• Step recovery diode: is a Shockley diode realized out of an ordinary PN junction.

Fig 10: Few common schematic symbols

	Led diodes: can have one, two, or three colours. They are used as visual indicators
	Shockley diode: is a unidirectional thyristor working in its voltage breakover region to trigger other thyristors or diodes. Shockley diodes are N type junctions, and they develop about 0.2 V voltage drop. They are just semiconductor and metal (aluminum, silver, gold, platinum) and their switching speed in roughly 10 ns.
	SCR: Silicon Controlled Rectifier is a diode of the thyristor family. ;SCRs handle the highest amounts of current and voltages, and they are constructed as four layer PNPN junctions
	Diac: is similar in functionality to the Shockley thyristor, except it is bi-directional. It is also used to trigger other thyristors into conduction.
	Triac: acts a switch having a gate to control the switching state
	Zener: it is a voltage (overvoltage) clamping diode, working only reverse-biased. Plays the role of voltage stabilizer. (Not good!)
	Varactor: variable capacity diode, also named Varicap. It is used in RF and frequency control circuits. It is a DC controlled variable capacitor.
	Tunnel diode: very fast action (conducting) diode, it is sometimes named Esaki diode. This is a negative resistance diode used in very high frequency oscillators. In fact, they are the fastest switching diodes, and their speed is limited only by the shunting capacitance of the connecting circuitry.