Signal diodes are the ‘smallest’ diodes. They are designed for small voltages and currents, but they can operate at high speed. Indeed, assume that the applied voltage changes polarity very fast, in other words ‘at a high frequency’, then the depletion layer must be built up and removed very fast. This displacement of holes and electrons needs time, but in signal diodes this can happen quickly. Breakdown voltages of signal diodes are typically some tens of a Volt, and should not be exceeded. Also maximum power ratings are rather low.
Rectifier diodes or power diodes are designed for high voltages and high currents. They are used in applications dealing with high power or energy. A typical application is a rectifier that converts an ac voltage into a dc voltage. Rectifiers will be discussed later in this chapter. Breakdown voltages are typically a few hundreds of a Volt, much larger than for a signal diode. Breakdown must again be prohibited, since it is destructive. Power diodes are also larger and bulkier than signal diodes, since they have to deal with larger currents. That also makes them slower than signal diodes. This is a typical trade-off in electronics: small components are fast, but can not deal with large currents, while large components are slow, but they can deal with large currents.
Zener diodes on the other hand can endure breakdown. Moreover, they are designed to operate in the breakdown region. Breakdown voltages are mostly rather small, some Volts. Typical circuits are voltage regulators and clampers (see further).
Light Emitting Diodes
Light Emitting Diodes or LEDs are diodes that emit light with an energy proportional to the current through the diode. They are typically used as indicators, e.g. to display whether a certain device is ‘on’ or ‘off’. They are also used as emitters, like e.g. the infrared sender in a remote control. LEDs are more and more used as true light source, e.g. at the back of your cell phone, or as decorative lighting in a ceiling.
Photodiodes and solar cells
Photodiodes and solar cells do the opposite: they convert light into electrical energy. The most important difference between both is the design and the application that follows from it. Photodiodes are designed to receive signals carrying information, e.g. at one end of a glass fiber. Therefore they are very sensitive, i.e. they should be able to convert weak light signals in measurable currents. They have to react very fast as well, so that information can be transmitted very fast. Solar cells on the other hand are designed for high efficiency. They have to harvest as much energy as possible from the incident sunlight. They don’t have to be fast at all, since the sunlight changes very slowly.