Diode

A diode can be thought of as the electronic version of a one-way valve. By restricting the direction of movement of charge carriers, it allows an electric current to flow in one direction, but essentially blocks it in the opposite direction.

Physical explanation of semiconductor diode operation

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 charges 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. 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'.

Related Topics:
Voltage - Depletion Layer - P-n junction - Charge carrier - Insulator - Depletion width

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A diode's I-V, characteristic can be approximated by two regions of operation. Below a certain difference in potential between the two leads, the Depletion Layer has significant width, and the diode can be thought of as an open (non-conductive) circuit. As the potential difference is increased, at some stage the diode will become conductive and allow charges to flow, at which point it can be thought of as a connection with zero (or at least very low) resistance. More precisely, the transfer function is logarithmic, but so sharp that it looks like a corner (see also signal processing).

Related Topics:
Transfer function - Logarithm - Signal processing

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The Shockley ideal diode equation (named after William Bradford Shockley) can be used to approximate the p-n diode's I-V characteristic.

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:I=I_mathrm{S} left( {e^{qV_mathrm{D} over nkT}-1} ight),,

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where I is the diode current, IS is a scale factor called the saturation current, q is the charge on an electron (the elementary charge), k is Boltzmann's constant, T is the absolute temperature of the p-n junction and VD is the voltage across the diode. The term kT/q is the thermal voltage, sometimes written VT, and is approximately 26 mV at room temperature. n (sometimes omitted) is the emission coefficient, which varies from about 1 to 2 depending on the fabrication process and semiconductor material.

Related Topics:
Electron - Boltzmann's constant

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It is possible to use a shorter notation. Putting

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:rac{k T}{q} = V_mathrm{T}

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and n=1 the relationship of the diode becomes:

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:I=I_mathrm{S} left( {e^{V_mathrm{D} over V_mathrm{T}}-1} ight),

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where V_mathrm{T} = 26 mV (at room temperature) is a known constant.

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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 depending on the current.

Related Topics:
Volt - Schottky diode - Light-emitting diode

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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 upto 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 zener voltage). The devices however have a maximum limit to the current and power in the zener or avalanche region.

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