Working Of a Semiconductor Diode

A semiconductor diode is an electronic component made up joining a block of P-type of semiconductor and an N-type of semiconductor in a single crystalline form. Most of these diodes are made of silicon, although germanium diodes are also available. The semiconductor diodes are also called as a PN junction diode. These diodes are used for a variety of applications such as rectification, voltage regulation, detection, light emission, solar cells, photo detection, etc.

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When a potential is given to the terminals of a diode, the current can flow only in one direction. The current flows through the diode only when the positive of the battery is connected to the P side of the diode and the negative of the battery is connected to the N side of the diode. This is called as the forward bias condition. The diode also has a junction potential of 0.6 volts where  the conduction or flow of current takes place through the  diode. Only when the voltage across the terminals crosses 0.6 volts in  the forward bias mode the conduction of the diode takes place. In the forward bias condition, the positive holes are repelled by the positive anode and the free electrons are repelled by the cathode the negative terminal of the battery. The electrons and holes move across the junction and they combine together. More electrons are siphoned off by the  positive anode and move through the terminals and through the battery. The holes thus created moves across to the N side through the PN junction. Thus a steady current flows through the diode.

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In the reverse bias condition, the positive of the battery is connected to the N side of the diode and the negative  of the battery is connected to the P side of the diode.  Here the diode is at an off or non conducting position. The electrons are attracted to the positive pole of the battery. The holes in the P side of the diode are attracted to the negative of the battery.  The  electrons and holes leave the junction creating an area completely devoid of electrons or holes. This area which is devoid of either electrons or holes is called as a depletion zone. There is no conduction of the diode in the reverse biased diode.

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When an AC voltage is applied to the terminals of the diode, the diode conducts during the alternate halve cycles of the AC waveform and cuts off the voltage during the reverse cycles. The resultant waveform is the broken DC pulses during alternate half cycles of the  AC waveform. A full waveform can be obtained using a transformer and diode combination. The DC pulses can be smoothed out by various waveform filters to obtain a smooth DC voltage.

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When the voltage is increased in a reverse biased diode, the depletion zone becomes wider.  At a critical point, the depletion field becomes large enough so that the carrier, either the electron or hole gain sufficient momentum to break electrons from the atoms with which it collides. These stripped electrons gains sufficient speed until it collides with other atoms striking still more electrons. The operation is accelerated by heat and heated generated carriers in the depletion zone. This process is called as the avalanche. At the avalanche area, the diode begins to conduct in the reverse mode. Usually the breakdown voltage is much higher than the the voltage used in the circuit. There are diodes which operate in the avalanche region and these are called as the zener diodes, which are voltage regulator diodes.