Editing Diode (section)

====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.