Diodes are electronic components functions as a one-way valve it means it allow current to flow in one direction. These diodes are manufactured by the semiconductor materials germanium, silicon and selenium. Operation of diode can be classified in two ways, if it allows the current then it is forward biased otherwise it is reverse biased.

　　For silicon diodes the forward voltage is 0.7v and for germanium it is 0.3v. In silicon diode the dark band indicates the cathode terminal and the other terminal is anode. Generally diodes are used as reverse polarity protector and transient protector. There are many types of diodes and some of these are listed as follows.

　　1. Zener Diode

　　It is a passive element works under the principle of zener breakdown. First produced by Clarence zener in 1934.It is similar to normal diode in forward direction, it also allows current in reverse direction when the applied voltage reaches the breakdown voltage. It is designed to prevent the other semiconductor devices from momentary voltage pulses. It acts as voltage regulator.

　　2. Schottky Diode

　　In this type of diode the junction is formed by contacting the semiconductor material with metal. Due to this the forward voltage drop is decreased to min. The semiconductor material is N-type silicon which acts as an anode and the metal acts as a cathode whose materials are chromium, platinum, tungsten etc.

　　Due to the metal junction these diodes have high current conducting capability thus the switching time reduces. So, Schottky has greater use in switching applications. Mainly because of the metal- semiconductor junction the voltage drop is low which in turn increase the diode performance and reduces power loss. So, these are used in high frequency rectifier applications.

　　3. Transient Voltage Suppression Diode

　　In semiconductor devices due to the sudden change in the state voltage transients will occur. They will damage the device output response. To overcome this problem voltage suppression diode diodes are used. The operation of voltage suppression diode is similar to Zener diode operation.

　　The operation of these diodes is normal as p-n junction diodes but at the time of transient voltage its operation changes. In normal condition the impedance of the diode is high. When any transient voltage occurs in the circuit the diode enters in to the avalanche breakdown region in which the low impedance is provided.

　　It is spontaneously very fast because the avalanche breakdown duration ranges in Pico seconds. Transient voltage suppression diode will clamp the voltage to the fixed levels, mostly its clamping voltage is in minimum range. These are having applications in the telecommunication fields, medical, microprocessors and signal processing. It responds to over voltages faster than varistors or gas discharge tubes.

　　4. Light Emitting Diode (LED)

　　These diodes convert the electrical energy in to light energy. First production started in 1968. It undergoes electroluminescence process in which holes and electrons are recombined to produce energy in the form of light in forward bias condition.

　　Earlier they used in inductor lamps but now in recent applications they are using in environmental and task handling. Mostly used in applications like aviation lighting, traffic signals, camera flashes.

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