A diode is an electronic component that has two terminals and conducts electricity in one direction. The word "diode" most often refers to a semiconductor diode that is normally composed of crystalline material, but it could also refer to a vacuum tube diode. Used primarily in high-power technologies, the vacuum tube diode is much rarer than the semiconductor diode. A high power diode, then, is a semiconductor that can handle large levels of current.
Sometimes a diode is referred to as a p-n junction, which is a structure that forms the connection between the positive and negative aspects of the semiconductor. The p-type semiconductor is connected to a terminal called the anode and the n-type semiconductor is connected to a terminal called a cathode. Electric current can flow from the anode to the cathode but is normally prevented from flowing in the opposite direction. If current is allowed to flow backward, the diode is called a rectifier.
Another way to view a high power diode is to think of it as a type of valve that controls the flow of electric current. The anode side could be seen as the "upstream" side of the high power diode and the cathode could be seen as the "downstream" side. The valve allows current to flow "forward," or from the anode to the diode, but prevents the current from flowing "backward," or from the cathode to the anode.
A high power diode can come in one of several types. An avalanche diode allows current to flow in the reverse direction when the reverse voltage exceeds the forward voltage. The flow of the current forms an "avalanche" of ionization across the p-n junction, resulting in a high amount of current. These diodes are often confused with Zener diodes.
Constant current diodes control the flow of electricity such that there is a constant level of power running through the semiconductor. An Eskai or tunnel diode allows signals to be amplified, and is commonly used in harsh environments such as those with low or high temperatures, high radiation or high magnetic fields. Light-emitting diodes (LEDs) are very common today because they produce light in wavelengths from infrared to near ultraviolet. Laser diodes are LED-type diodes that are contained in a resonant cavity and are often used for lasers, optical storage devices, and for fiber optics such as high-speed communication.