Diode
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A semiconductor device meant to allow current flow one way (in the "forward biased" direction) but not the other (the "reverse biased" direction). A Light emitting diode is an example of a diode.
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Usual characteristics of a diode
Diodes conduct well in one direction (in the forward bias) but not the other (the reverse bias). Hence they are like one-way valves for electric current.
Diodes have a voltage drop, that is usually in the range of 0.3V to several volts depending on the diode. The voltage drop is usually marked Vf in diode datasheets and called forward voltage. Germanium diodes usually have a lower voltage drop than the more common silicon diodes.
A diode used in converting AC to DC is sometimes called a rectifier.
Diode Types
Mercury Arc Rectifiers
These are also known as Cooper-Hewitt or Hewittic rectifiers, and later versions are called steel tank rectifiers. They were used before the silicon diode was invented. They did essentially the same thing as a silicon diode, but were much larger (a 250 amp 3 phase mercury rectifier would about 3 meters long and almost 50cm in diameter, compared to a silicon diode that would be less that 10cm square) and more delicate. They work by striking an arc between a pool of mercury and an electrode made of graphite. This would cause some of the mercury to vaporise, which formed a diode. As time progressed a steel tank was used instead of a glass envelope and the Steel Tank rectifier was born. The concept was the same, but the steel envelope allowed larger rectifiers to be used.
Vacuum Tube Diode
This is where the diode got its name, it is essentially an evacuated glass envelope that has two metal elements in it. They are called the anode and the cathode. The cathode is heated, and is usually called the filament. Because it is hot it emits electrons easily, which are absorbed by the second element, the anode. Thus electricity can only flow one direction. Because there were 2 electrodes, it was called a diode, di- being a prefix for 2 an -ode being a suffix for electrode.
P-N Junction Diodes
The most common type of diode and what is often meant by "diode".
Silicon, and to a lesser extent Germanium, is the most common material used in P-N junction diodes. The typical forward voltage for silicon is 0.65 V and 0.3 V for Germanium. The forward voltage is dependent on temperature. Silicon rectifiers may be gold or platinum doped to decrease the recombination time and increase switching speed. These "ultrafast" rectifiers commonly switch faster than 25 nS while rated for more than 1 kV PIV (Peak Inverse Voltage, the reverse voltage the diode can block)
Zener
A zener diode is a special kind of diode, that behaves like a normal diode when forward biased. But the special thing about zener diodes is that they can conduct in reverse biased direction too, when the voltage over the zener is bigger than it's zener voltage Vz. This means a zener diode can be used as a Voltage regulator.
Diodes rated below about 4.3V are zener diodes and those above about 4.3V are avalanche diodes. Although they're usually all called zener diodes. The mechanisms are different and their temperature coefficients are different. Their voltages move in opposite directions when heated. Combinations of zener and avalanche diodes can result in very stable voltage references, The best stability from a single diode is found between 4.3V and 6.8V.
Other odd voltage regulating diodes are the stabistor which is several P-N diodes in series in the same package. The backwards diode which is a low voltage regulating diode (usually employed to bias a tunnel diode.)
Schottky
The schottky diode is a metal to semiconductor barrier device. Because there is no P-N junction and no electron recombination, their recovery time or switching speed is very fast. A schottky diode's forward voltage can be below 0.2V at low current. Small schottky devices are used as detectors (radio frequency rectifiers) up into the microwave frequency range. Large schottky devices are often used as low-voltage high-current rectifiers in switch-mode power supplies. Schottky power rectifiers have lots of capacitance and much more leakage than P-N junction diodes, the leakage increases with temperature. Newer devices made with gallium arsenide operate at high voltage with low leakage. They are as near to a perfect rectifier as we can presently have except for their high cost.
Current Regulating Diodes
These are really J-FETS with their gate and source terminals tied together which are then packaged like a diode.
PIN Diodes
Used for RF switching and Attenuation the doping of the inexpensive 1N4005 rectifier makes it a usable PIN diode.
Hyper-Abrupt Junction or VARI-CAP Diodes
Many P-N diodes will serve as variable capacitance diodes when reverse biased. Some diodes are doped to optimize the range of capacitance versus reverse voltage that's achievable. They have replaced the variable capacitor in many low power applications. The tuning capacitor in radio receivers is one example.
Step-Recovery and Snap diodes
These P-N diodes are doped to accentuate a parameter that is considered undesirable in rectifiers. They are mainly used to generate harmonic energy in RF frequency multiplier circuits.
Uses for Diodes
Diodes are generally used to force current to go in one direction, although they can also be used to create a controlled voltage drop, regulate current, regulate voltage, and even provide light.
Rectifying
The simplified definition of rectify is 'to make an AC current into a DC one'.
This can be done any number of ways, most of which involve diodes, although mechanical methods can also be used.
There is one thing to take into account when rectifying voltage: the difference between RMS (Root Mean Square) voltage and peak voltage. Most of the time AC voltage is measured in RMS voltage, because it allows you to use AC voltage and DC voltage in most formulas interchangeably. However the RMS voltage is not the actual AC peak voltage, the peak to peak voltage is 
(See Constants) times higher! (See Sine Wave) This becomes evident when rectifying an AC voltage, as the DC output will be either 
or 
the RMS input voltage, depending on what type (half or full wave respectively) of rectification is used.
For example, if you half wave rectified 120v AC RMS, you would get 85vDC, and if you full wave rectified it you would get 170vDC.
Half Wave Rectifier
The easiest way to rectify an AC source would be to use a single diode in series with it, so current could only flow in one direction. This would essentially cut off all of the negative cycles from the source, so you get pulsed DC. Because pulsed DC is not very useful, a capacitor is put on the output. This helps smooth the voltage, giving a DC output that, without any load on the capacitor, has a constant voltage.
This method of rectifying has three problems. First, only half of the power that the transformer can give out is used, the other half of the time the transformer has no current draw. Second, you only get half of the voltage out that you put in when under load, again because you cut off all of the negative cycles of the AC source. (Although, off load the voltage will reach the same as for full-wave rectification.) Finally, when you begin to draw current the output stops being a fixed voltage, and acquires a ripple to it. The loaded output of this circuit looks like this:
Full Wave Rectifier
Also called a bridge rectifier and available as discrete components.
To solve some of the problems with the half wave rectifier, a full wave rectifier is used. A full wave rectifier uses 4 diodes and will reverse the polarity of the incoming AC wave for half of the incoming cycles, so all of the available power that a transformer can give is used. It also gives out the full voltage that it receives. It also reduces the size of filter capacitor needed to get the desired amount of ripple. The loaded wave form of a filtered full wave rectifier looks like this:
The ripple would be calculated as:
where:
I= current (amps, A)
F= frequency (hertz, Hz) this is either 50 or 60 depending on where you live
V= desired ripple voltage
Input/Output Polarity Protection
Yet another use for diodes is polarity protection.
Series Polarity Protection
The first way to protect the device from reverse polarity is to put a diode in series with the input. The would only allow current to flow the correct way into the device, preventing any damage from occurring to it someone plugs it in backwards. The disadvantage to this is that the diode will always drop the incoming voltage a small amount, and will waste some power.
Parallel Polarity Protection
The second way to polarity protect something would be to put a diode in parallel with the device. When the device is powered backwards the diode absorbs most of the incoming energy, preventing the device from being damaged. An advantage to this is that the diode wastes no power when the circuit is operating properly, as it is reverse biased so it draws almost no current. The problem is that if the device were connected to a high current source (car battery for example) the diode would burn out, and the device may then be subjected to the reversed voltage. To solve this a fuse is put in series with the device, so the fuse blows before the diode does; and all that is needed to correct the problem is to correct the polarity and replace the fuse.
