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Most of us know that an LED is a light emitting diode – but not as many people understand what exactly a diode is or why don’t all of them emit light (or do they?). Diodes are actually very similar to check valves, or one-way valves. Generally speaking, diodes allow electrical  current to flow only in one direction and block current in the other direction. The most common kind of diode in modern circuit design is the semiconductor diode, although other types are widely available. Some ways in which the diode can be used include:
- Converting AC to DC for a power supply device.
- Separating the signal from radio frequencies.
- As an on/off switch that controls current.
Diodes Explained
Certain elements are normally insulators, but we can turn them into conductors with a chemical process called doping. We call these materials semiconductors and silicon and germanium are two of the best known examples. Silicon is normally an insulator, but if you add a few atoms of the element antimony, you effectively sprinkle in some extra electrons and give it the power to conduct electricity. Silicon altered in this way is called n-type (negative-type) because extra electrons can carry negative electric charge through it.
In the same way, if you add atoms of boron, you effectively take away electrons from the silicon and leave behind "holes" where electrons should be. This type of silicon is called p-type (positive type) because the holes can move around and carry positive electric charge.
Basically, when the two are sandwiched together, a barrier forms, called a P-N junction and surrounding it is what’s called a depletion zone. An article at pbs.org explains it simply that one side of the semiconductor boundary is like mud, one like water. If you try to get electricity to move from the mud side to the water side, there's no problem. The electrons just jump across the boundary, forming a current. But try to make electricity go the other way and nothing will happen. Electrons that didn't have to work hard to travel around the water side just don't have enough energy to make it into the mud side. (In real life, there are always a few electrons that can trickle in the wrong direction, but not enough to make a big difference.)
Once voltage is applied in the correct direction (forward bias) across the diode, the P-N junction shrinks and electrons can travel from one side to the other. Voltage applied in the opposite direction (reverse bias) causes the depletion zone to expand and prevents current from traveling. But, just like lightning through air (normally an insulator), enough voltage can break through the barrier – a condition known as breakdown. Zener diodes are designed to take advantage of this by acting almost like a flood gate. The maximum reverse-bias voltage that a diode can withstand without “breaking down” is called the Peak Inverse Voltage, or PIV rating.
Zener Diode
Where the common diode is designed to never let current flow in the reverse direction, the zener is designed to break down and let current flow backward at some specific voltage. They conduct like a normal diode in the forward direction but in the reverse direction will not allow the voltage across the diode to exceed the rated zener voltage. Consequently, they make great voltage regulators or references. The current must be externally limited because when one of these “turns on” - it turns on hard and will be damaged if the current isn't held to a safe value. This is usually accomplished with a simple resistor.
Light Emitting Diodes
All diodes actually emit energy, although it is not all visible. LEDs, although they are still PN junctions, are not made from silicon (which will emit infrared light), and therefore can be made to emit visible light. What most people do not realize, however, is that the color of the light emitted by an LED has nothing to do with the color of the plastic casing. Often, the plastic case is made of the same color as the light the actual PN junction will emit, and is made to diffuse the light in order to make it easier to see from any angle. But the color of the light emitted actually depends on the material used in the PN junction (which is why blue LEDs are so expensive - they are made from rare materials). Some LEDs diffuse (spread out) the light, while others produce a tighter beam that is useful for transmitting information through light. You simply can not tell what color will be emitted by just looking at the LED until it is plugged in.
Photodiodes
All semiconductors are subject to optical charge carrier generation. This is typically an undesired effect, so most semiconductors are packaged in light blocking material. Photodiodes are intended to sense light (photodetector), so they are packaged in materials that allow light to pass Solar cells are also PN junctions (made of silicon). They are like photodiodes, except the PN junction area is made incredibly larger so that it can be used not just to detect light, but to produce electrical energy from it.
Rectifiers
Rectifier is sometimes used interchangeably with diode. A rectifier is simply a diode (or diodes) used to convert AC to DC. Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with only one diode.
Bridge rectifiers
Diodes can be connected in several ways to make a rectifier to convert AC to DC. The bridge rectifier is one of them and it is available in special packages containing the four diodes required. Bridge rectifiers are rated by their maximum current and maximum reverse voltage. They have four leads or terminals: the two DC outputs are labeled “+” and -, the two AC inputs are labeled “ ~”.
    
Transient voltage suppression diode (TVS)
Avalanche diodes designed specifically to protect other semiconductor devices from high-voltage transients are called Transient voltage suppression diode (TVS). Their p-n junctions have a much larger cross-sectional area than those of a normal diode, allowing them to conduct large currents to ground without sustaining damage. This makes them perfect for use in surge suppression equipment.
Sources:
http://en.wikipedia.org/wiki/Diode
http://www.kpsec.freeuk.com/components/diode.htm
http://electronics.howstuffworks.com/led1.htm
http://www.allaboutcircuits.com/vol_3/chpt_3/1.html
http://hyperphysics.phy-astr.gsu.edu/Hbase/Electronic/diodecon.html |
**Specifications subject to changes**
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