A diode is a two-terminal device, having two active electrodes, between which it allows the transfer of current in one direction only. Diodes are known for their unidirectional current property. Basically, diodes are used for the purpose of rectifying waveforms, and can be used within power supplies or within radio detectors. They can also be used in circuits where 'one way' effect of diode is required. Most diodes are made from semiconductors such as silicon, however, germanium is also used sometimes. Diodes transmit electric currents in one direction, however, the manner in which they do so can vary. Some of the different types of diodes are: LEDs or light emitting diodes are the most popularly known diodes today. They are p-n junction diodes that permit transfer of electrons between the electrodes and produce light. However, not all LEDs emit visible light. There are those that emit infrared light, which cannot be seen by human eyes. Such LEDs are used in remote controls of television, DVD players, etc. When the diode is switched on or forward biased, the electrons recombine with the holes and release energy in the form of light. This means electronic excitation spearheads photon emission, which in turn results in light emission or electroluminescence. Aluminium gallium indium phosphide or aluminum gallium arsenide are generally the conducting materials used in LEDs. The color emitted by the LED, will depend on the combination of semiconductor material used. Slightly doped p-n junctions at times encounter avalanche breakdown, which is the sudden multiplication of voltage (voltage transients) across a diode. This sudden increase often destroys diodes. However, avalanche diodes, available with breakdown voltages of 4000V are built in such a way, that they can break down the voltage and permit passage of reverse bias voltage. Thus, these diodes are used to protect circuits (especially high voltage circuits) from transient voltage. They are often used along with Zener diodes and often confused with the same. Zener diode is a type of diode that not only allows current to flow through it in one direction like any other diode, but in reverse bias, also allows current to flow in the reverse direction, if the voltage exceeds a certain limit. This voltage limit is known as Zener voltage and is fixed for Zener diodes with breakdown voltage ranging from 1.8V to 200V. Thus, Zener diodes protect circuits from damages. Unlike, avalanche diodes, such diodes have heavily-doped p-n junctions and the doping is done differently to achieve different Zener breakdown voltage. These diodes are mostly used to control voltage in electrical circuits. Also referred to as hot-carrier diodes, Schottky diodes are diodes that feature lower forward voltage drop, as compared to the ordinary silicon p-n junction diodes. The voltage drop may be somewhere between 0.15 and 0.4 volts at low currents, as compared to the 0.6 volts for a silicon diode. The lower voltage drop helps the diode switch from conducting state to non-conducting state in shorter time. Thus, they come in handy for preventing transistor saturation and are also used in voltage-clamping applications. In order to achieve this performance, these diodes are constructed differently from normal diodes, with metal to semiconductor contact. Schottky diodes are used in RF applications and rectifier applications. This type of diode is different from the LED type, as it produces coherent light, which is nothing but radiation in which the waves are of the same frequency and in the same phase. These diodes are small in size, however, compared to their size, the output is commendable. Laser diodes can again be divided into two types: low power diodes and high power diodes. The coherent light produced by these diodes make them perfect for devices such as DVD and CD drives, laser pointers, high-definition TVs, barcode readers, etc. Laser diodes are more expensive than LEDs. However, they are cheaper than other forms of laser generators. Moreover, these laser diodes have limited life. Photodiodes are used to detect light and convert light falling on it into electric current. They feature wide, transparent p-n junctions and work on the mechanism of photoelectric effect. These diodes operate in reverse bias, wherein even small amounts of current flow, resulting from the light, can be detected with ease. Photodiodes can also be used to generate electricity, used as solar cells and even in photometry. Some photodiodes feature an undoped layer sandwiched between the p and n layers, and such diodes are called PIN photodiodes. This kind of photodiode is more popularly used today, because of its higher efficiency. Also referred to as varicap diodes, these varactor diodes are diodes with variable capacitance. They are diodes with standard p-n junctions, that are always operated in reverse-bias. In this reverse-bias mode, they exhibit variable capacitance along with variable voltage. As the reverse voltage increases, the capacitance of the varactor diode decreases. The doping amount controls the amount of capacitance, which means more the doping, more the capacitance range of the diode. These varactor diodes mainly find their use in radio-frequency circuits, wherein they are used to tune circuits, such as in communication equipment. This is why these diodes are also referred to as tuning diodes. These diodes are used to rectify alternating power inputs in power supplies. This means they convert alternating current (AC) to direct current (DC). They are extremely useful in DC devices which require continuous flow of direct current. Rectifier diodes can be further classified into two types: full wave and half wave rectifiers. The half wave rectifier allows the flow of only the positive half of the wave through it. The full wave rectifier on the other hand features a combination of two diodes that rectify not only the positive half of the wave, but also the negative half of the wave. Diodes are used widely in the electronics industry, right from electronics design to production, to repair. Besides the above mentioned types of diodes, the other diodes are point contact diode, signal diode, step recovery diode, tunnel diode and gold doped diodes. The type of diode to transfer electric current depends on the type and amount of transmission, as well as on specific applications.