The classical diode, also known as the pn diode, is a low-cost diode that is used for general-purpose applications. These diodes have a forward diode voltage around 0.7 volts and a forward current rating in the range of 1 milliamperes to 20 milliamperes. They are rarely used in high-power or high-frequency applications.
Photodiodes produce a current that is proportional to the wavelength of light that strikes its surface. Photodiodes are designed for specific light wavelength ranges. One photodiode may be designed to respond to red light and another to infrared light, light that cannot be seen by the human eye.
The light emitting diode, often called an LED, emits light, measured in lumens, when a voltage is applied across its two leads. The lumens that an LED produces are proportional to the amount of current that flows through it. High-brightness LED have maximum current ratings of about 1,000 milliamperes and forward voltage ratings of 3 and 4 volts. Low-current LEDs, found on your computer and electronic gadgets, have much lower current ratings and lower forward voltage ratings than high-brightness LEDs. Typical forward current ratings can be as low as 2 milliamperes and forward voltage ratings as low as 1.7 volts.
The rectifier diode, often called a power diode, is most often used in power supplies. There, they are used to convert alternating current (AC) to direct current (DC). Because power supplies often require high amounts of currents, the maximum current rating for a rectifier diode is often much higher than a pn diode. You can readily find rectifier diodes that have current ratings specified above 10 amperes.
Schottky diodes have low forward voltage specifications. They can conduct current when the voltage across the diode is as low as 0.4 volts. The Schottky diode also has a faster switching speed specification than a typical diode would. Because of these better specifications, the Schottky diode is often used in high-efficiency switching power supply applications and in high-frequency communications applications.