Editing Diode (section)

===Current–voltage characteristic===
A semiconductor diode's behavior in a circuit is given by its [[current–voltage characteristic]]. The shape of the curve is determined by the transport of charge carriers through the so-called ''[[depletion region|depletion layer]]'' or ''[[depletion region]]'' that exists at the [[p–n junction]] between differing semiconductors. When a p–n junction is first created, conduction-band (mobile) electrons from the N-[[dopant|doped]] region diffuse into the P-[[dopant|doped]] region where there is a large population of holes (vacant places for electrons) with which the electrons "recombine". When a mobile electron recombines with a hole, both hole and electron vanish, leaving behind an immobile positively charged donor (dopant) on the N side and negatively charged acceptor (dopant) on the P side. The region around the p–n junction becomes depleted of [[charge carrier]]s and thus behaves as an [[insulator (electricity)|insulator]].

However, the width of the depletion region (called the [[depletion width]]) cannot grow without limit. For each [[electron–hole pair]] recombination made, a positively charged [[dopant]] ion is left behind in the N-doped region, and a negatively charged dopant ion is created in the P-doped region. As recombination proceeds and more ions are created, an increasing electric field develops through the depletion zone that acts to slow and then finally stop recombination. At this point, there is a "built-in" potential across the depletion zone.

[[File:PN band.gif|thumb|upright=2.8<!--max normally 1.8 but WP:IAR-->|none|A [[p–n junction]] diode in low forward bias mode. The [[depletion width]] decreases as voltage increases. Both p and n junctions are doped at a 1e15/cm3 [[doping (semiconductor)|doping]] level, leading to built-in potential of ~0.59V. Observe the different [[quasi Fermi level]]s for conduction band and valence band in n and p regions (red curves).]]

====Reverse bias====
{{See also|p–n diode#Reverse bias}}
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 any significant electric current flow (unless [[electron–hole pair]]s are actively being created in the junction by, for instance, light; see [[photodiode]]).

====Forward bias====
{{See also|p–n diode#Forward bias}}
However, if the polarity of the external voltage opposes the built-in potential, recombination can once again proceed, resulting in a substantial electric current through the p–n junction (i.e. substantial numbers of electrons and holes recombine at the junction) that increases exponentially with voltage.

====Operating regions====
[[File:Diode current wiki.png|thumb|upright=1.4|[[Current–voltage characteristic]] of a p–n junction diode showing three regions: '''breakdown''', '''reverse''' biased, '''forward''' biased. The exponential's "knee" is at ''V''<sub>d</sub>. The leveling off region which occurs at larger forward currents is not shown.]]

A diode's [[current–voltage characteristic]] can be approximated by four operating regions. From lower to higher bias voltages, these are:

* '''Breakdown''': At very large reverse bias, beyond the [[peak inverse voltage]] (PIV), a process called reverse [[avalanche breakdown|breakdown]] occurs that causes a large increase in current (i.e., a large number of electrons and holes are created at, and move away from the p–n junction) that usually damages the device permanently. The [[avalanche diode]] is deliberately designed for use in that manner. In the [[Zener diode]], the concept of PIV is not applicable. A Zener diode contains a heavily doped p–n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material, such that the reverse voltage is "clamped" to a known value (called the ''Zener voltage''), and avalanche does not occur. Both devices, however, do have a limit to the maximum current and power they can withstand in the clamped reverse-voltage region. Also, following the end of forwarding conduction in any diode, there is reverse current for a short time. The device does not attain its full blocking capability until the reverse current ceases.
* '''Reverse biased''': For a bias between breakdown and 0 V, the reverse current is very small and asymptotically approaches -''I''<sub>s</sub>. For a normal P–N rectifier diode, the reverse current through the device is in the micro-ampere (μA) range. However, this is temperature dependent, and at sufficiently high temperatures, a substantial amount of reverse current can be observed (mA or more). There is also a tiny surface leakage current caused by electrons simply going around the diode as though it were an imperfect insulator.[[File:DiodeGenCharacteristics1.jpg|right|thumb|500x500px|[[Semi-log]] I–V (logarithmic current vs. linear voltage) graph of various diodes.]]
* '''Forward biased''': The current–voltage curve is [[Exponential function|exponential]], approximating the [[Shockley diode equation]]. When plotted using a linear current scale, a smooth "[[Knee of a curve|knee]]" appears, but no clear threshold voltage is visible on a semi-log graph.
* '''Leveling off''': At larger forward currents the current–voltage curve starts to be dominated by the ohmic resistance of the bulk semiconductor. The curve is no longer exponential, it is asymptotic to a straight line whose slope is the bulk resistance. This region is particularly important for power diodes and can be modeled by a ''Shockley ideal diode'' in series with a fixed resistor.