A diode is an electronic component that conducts electric current in one direction, very much like a check valve on a water pipe.
Characteristics of diodes
Diodes are generally rated for forward current, reverse voltage, turn-off time or frequency, reverse leakage current, and forward voltage drop.
forward current is how many amps they can carry while conducting electricity.
reverse voltage is how many volts they can withstand when reverse biased before they "break down" and self-destruct.
turn-off time is how long (usually in nanoseconds, for example) it takes the diode to stop conducting electricity when voltage is reversed.
reverse-leakage current is how much current (usually in the micro amps) flows backwards across the diode when it is reverse biased.
Forward voltage drop is how many volts of loss there is across the diode while it is conducting electricity.
Types of diodes
Rectifier diodes were originally made using selenium, then germanium, and now silicon. They range from tiny as a grain of sand for surface mount to very large modules weighing pounds. This type is used for converting alternating electricity into pulsed-direct-current electricity, or for preventing current from flowing backwards in any circuit. For example, when a solar panel charges a battery during the daytime, it can then drain the battery at night - but a diode in series will prevent the current from flowing back through the solar panel at night.
The forward voltage drop of a germanium diode is around 0.2v at low currents, and the forward voltage drop is around 0.7v for silicon based diodes, however schottky diodes can have as low as 0.4 volts of forward drop, which makes them more efficient because less energy is lost. Schottky diodes also have zero turn-off time which makes them more efficient for high frequency applications. They may tend to have higher leakage, however.
A zener diode is a specialized silicon diode. It has about a 0.7v forward voltage drop, but it has an exceptionally low reverse breakdown voltage, ranging anywhere from a few volts to hundreds of volts. More importantly, the diode is constructed in such a way that when too much voltage happens across the diode in the reverse state, the diode just begins conducting electricity, but does not damage itself as would a regular diode. For example, if a 6.3v zener diode were connected backwards across 6 volts, no current would flow. But if it were connected across 7 volts, way too much current would flow. More specifically, if you had a 10v power source, and you connected a 6v zener diode in series with a lightbulb, the zener would drop 6 volts and leave only 4 volts for the lightbulb.
The zener diode is used for reducing voltage and for regulating voltage by the shunt-regulation method. It is also used for protecting the input connectors on electronic circuits because it can divert static electricity safely to ground while still allowing an audio or data connection.
Light emitting diodes, also called LEDs, are made with many different specialized semiconductor material so that they give off light when current flows forward through them. Depending on the different types of chemical elements used in making the diode, different colors of light can be produced.
LEDs at the basic level create only a single color of light. Thus, to create a "white LED" it actually takes more complicated construction. The two ways to create a white LED are to build an LED module that actually has 3 primary color LED chips inside it (red, green, and blue.) This kind of LED can produce many colors by mixing various quantities of each of the 3 primary colors. The most common kind of white LED is that which is used in LED flashlights: It is actually an ultraviolet LED chip covered with phosphor - just like the inside of a fluorescent tube light: The UV light hits the phosphor which in turn gives off yellow-white light which mixes with the blue tint of the UV led to produce a nearly white color.
Tunneling diodes, also called Esaki diodes, have a very unique characteristic in that they exhibit a negative resistance at a low voltage. This means that at a very low voltage, they conduct an electrical current, but as you increase the voltage, the current reduces. If you continue to increase the voltage, the current then begins to increase again and it works like a normal diode more or less. But the negative resistance characteristic is very important to microwave circuitry because it is possible to form a very simple circuit using a tunneling diode to produce extremely high frequencies.
A trigger diode is used in dimmer and AC control circuits. When connected to a voltage less than the trigger voltage, the diode is an open circuit. However, when the voltage across the diode reaches a certain voltage, it "triggers" and turns on, and stays on until the current is removed from through it.
A vericap diode, also called a tuning diode, is a special diode who's capacitance changes in response to the reverse voltage across it. This kind of diode can be inserted into a radio tuning circuit, and the radio frequency of the tuned circuit can be adjusted by varying the voltage on the diode. Early portable digital tuning AM/FM radios used this method.
A photodiode is a diode that is optimized to detect light and/or (especially) near infrared. It is built with perhaps an enlarged junction area, and is built in a case which is clear or has a clear window to allow light to reach the surface of the die.
Photo diodes work exactly like a very tiny solar cell: They produce a small electrical current when light shines on them.
They are used in many different technologies including fiber optic data reception, daylight sensors in automatic lights, IR Beam intrusion alarms, and even in CMOS and CCD image sensors where each pixel is actually a small photodiode.
Vacuum tube diode
A vacuum tube diode is constructed with a glass envelope sort of like a light bulb, and it has a filament like a light bulb which heats up the cathode which in turns emits electrons. It also has a metal conductor inside called a plate, and when the cathode is more negative than the plate, electrons flow from the cathode through the vacuum to the plate where they then continue on through the rest of the circuit. If, however, the plate is more negative than the cathode, then the electrons are repelled by the plate and stay near the cathode and no current flows, generally speaking.
High voltage vacuum tube diodes can produce x-rays because the high voltage can accelerate the electrons toward the plate with such velocity that they emit x-ray particles when the electrons are slowed down by the plate (Bremsstrahlung).
Vacuum tube diodes were used of course before solid state diodes were invented, but continued to be used in high voltage applications even into the early days of solid state because it was still cheaper to build electron tubes then high voltage solid state diodes.
Modern x-ray tubes operate on the very same principle as a vacuum tube diode, however they usually have mechanisms to keep the plate from getting too hot and melting.