Junction Diode
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The Junction Diode
 In electronics, a diode is a component that restricts the direction of movement of charge carriers. Essentially, it allows an electric current to flow in one direction, but blocks it in the opposite direction. Thus, the diode can be thought of as an electronic version of a check valve. Circuits that require current flow in only one direction will typically include one or more diodes in the circuit design.
Short history
 Thermionic and solid state diodes developed in parallel. The principle of operation of thermionic diodes was discovered by Frederick Guthrie in 1873. The principle of operation of crystal diodes was discovered in 1874 by the German scientist, Karl Ferdinand Braun.
 Thermionic diode principles were rediscovered by Thomas Edison on February 13, 1880 and he took out a patent in 1883, but developed the idea no further. Braun patented the crystal rectifier in 1899. The first radio receiver using a crystal diode was built around 1900 by Greenleaf Whittier Pickard. The first thermionic diode was patented in Britain by John Ambrose Fleming (scientific adviser to the Marconi Company and former Edison employee on November 16, 1904. Pickard received a patent for a silicon crystal detector on November 20, 1906.
 At the time of their invention such devices were known as rectifiers. In 1919 William Henry Eccles coined the term diode from Greek roots; di means 'two', and ode (from odos) means 'path'.
 A semiconductor diode's current-voltage, or I-V, characteristic curve is ascribed to the behavior of the so-called depletion layer or depletion zone which exists at the p-n junction between the differing semiconductors. When a p-n junction is first created, conduction band (mobile) electrons from the N-doped region diffuse into the P-doped region where there is a large population of holes (places for electrons in which no electron is present) with which the electrons "recombine". When a mobile electron recombines with a hole, the hole vanishes and the electron is no longer mobile. Thus, two charge carriers have vanished. The region around the p-n junction becomes depleted of charge carriers and thus behaves as an insulator.
 However, the depletion width cannot grow without limit. For each electron-hole pair that recombines, a positively-charged dopant ion is left behind in the N-doped region, and a negatively charged dopant ion is left behind in the P-doped region. As recombination proceeds and more ions are created, an increasing electric field develops through the depletion zone which acts to slow and then finally stop recombination. At this point, there is a 'built-in' potential across the depletion zone.
 If an external voltage is placed across the diode with the same polarity as the built-in potential, the depletion zone continues to act as an insulator preventing a significant electric current. This is the reverse bias phenomenon. However, if the polarity of the external voltage opposes the built-in potential, recombination can once again proceed resulting in substantial electric current through the p-n junction. For silicon diodes, the built-in potential is approximately 0.6 V. Thus, if an external current is passed through the diode, about 0.6 V will be developed across the diode such that the P-doped region is positive with respect to the N-doped region and the diode is said to be 'turned on' as it has a forward bias.
 In a normal silicon diode at rated currents, the voltage drop across a conducting diode is approximately 0.6 to 0.7 volts. The value is different for other diode types - Schottky diodes can be as low as 0.2 V and light-emitting diodes (LEDs) can be 1.4 V or more (Blue LEDs can be up to 4.0 V). Referring to the I-V characteristics image, in the reverse bias region for a normal P-N rectifier diode, the current through the device is very low (in the µA range) for all reverse voltages up to a point called the peak-inverse-voltage (PIV). Beyond this point a process called reverse breakdown occurs which causes the device to be damaged along with a large increase in current. For special purpose diodes like the avalanche or zener diodes, the concept of PIV is not applicable since they have a deliberate breakdown beyond a known reverse current such that the reverse voltage is "clamped" to a known value (called the zener voltage or breakdown voltage). These devices however have a maximum limit to the current and power in the zener or avalanche region.
Some types of semiconductor diode
 Laser diodes
– When an LED-like structure is contained in a resonant cavity formed by polishing the parallel end faces, a laser can be formed. Laser diodes are commonly used in optical storage devices and for high speed optical communication.
Diodes (The PN Junction)
 If a piece of intrinsic silicon is doped so that half is n-type and the other half is p-type, a pn junction forms between the two regions as indicated in on the first slide
 The p region has many holes (majority carriers) from the impurity atoms and only a few thermally generated free electrons (minority carriers).
 The n region has many free electrons (majority carriers) from the impurity atoms and only a few thermally generated holes (minority carriers).
 Diodes (The PN Junction)
 Diodes (The Depletion Region)
 Diodes (The Barrier Potential )
 Any time there is a positive charge and a negative charge near each other, there is a force acting on the charges.
 In the depletion region there are many positive charges and many negative charges on opposite sides of the pn junction.
 The forces between the opposite charges form an electric field.
 This electric field is a barrier to the free electrons in the n region, and energy must be expended to move an electron through the electric field.
 That is, external energy must be applied to get the electrons to move across the barrier of the electric field in the depletion region.
Diodes (Forward Bias )
 Since unlike charges attract, the positive side of the bias-voltage source attracts the electrons from the N-Region into the P-Region.
 The holes in the P-Region provides a medium for electrons to move through the P-Region. The electrons move from hole to hole on to the left.
 As they move they leave holes behind. The holes ellectively not actually move towards the pn junction.
 This is called hole current.
Effect of Forward Bias on the Depletion Region
 As more electrons flow into the depletion region, the number of positive ions is reduced.
 As more holes effectively flow into the depletion region on the other side of the pn junction, the number of negative ions is reduced.
 This reduction in positive and negative ions during forward bias causes the depletion region to narrow.
Effect of the Barrier Potential During Forward Bias
 When forward bias is applied, the free electrons are provided with enough energy from the bias-voltage source to overcome the barrier potential and effectively move and cross the depletion region.
 The energy that the electrons require in order to pass through the depletion region is equal to the barrier potential. In other words, the electrons give up an amount of energy equivalent to the barrier potential when they cross the depletion region.
 This energy loss results in a voltage drop across the pn junction (0.7 for silicon and 0.3 for germanium).
 An ideal diode does not have a barrier potential.
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