Editing Quantum tunnelling (section)

==== Tunnel diode ====
{{Main|Tunnel diode}}

[[File:Rtd seq v3.gif|thumb|upright=1.6|right|A working mechanism of a [[resonant tunnelling diode]] device, based on the phenomenon of quantum tunnelling through the potential barriers]]

[[Diode]]s are electrical [[semiconductor device]]s that allow [[electric current]] flow in one direction more than the other. The device depends on a [[depletion layer]] between [[N-type semiconductor|N-type]] and [[P-type semiconductor]]s to serve its purpose. When these are heavily doped the depletion layer can be thin enough for tunnelling. When a small forward bias is applied, the current due to tunnelling is significant. This has a maximum at the point where the [[voltage bias]] is such that the energy level of the p and n [[conduction band]]s are the same. As the voltage bias is increased, the two conduction bands no longer line up and the diode acts typically.<ref name="Krane">{{cite book|last=Krane|first=Kenneth|url=https://archive.org/details/modernphysics00kran/page/423|title=Modern Physics|publisher=John Wiley and Sons|year=1983|isbn=978-0-471-07963-7|location=New York|page=[https://archive.org/details/modernphysics00kran/page/423 423]}}</ref>

Because the tunnelling current drops off rapidly, tunnel diodes can be created that have a range of voltages for which current decreases as voltage increases. This peculiar property is used in some applications, such as high speed devices where the characteristic tunnelling probability changes as rapidly as the bias voltage.<ref name="Krane" />

The [[resonant tunnelling diode]] makes use of quantum tunnelling in a very different manner to achieve a similar result. This diode has a resonant voltage for which a current favors a particular voltage, achieved by placing two thin layers with a high energy conductance band near each other. This creates a quantum [[potential well]] that has a discrete lowest [[energy level]]. When this energy level is higher than that of the electrons, no tunnelling occurs and the diode is in reverse bias. Once the two voltage energies align, the electrons flow like an open wire. As the voltage further increases, tunnelling becomes improbable and the diode acts like a normal diode again before a second energy level becomes noticeable.<ref name="Knight">{{cite book|last=Knight|first=R. D.|title=Physics for Scientists and Engineers: With Modern Physics|publisher=Pearson Education|year=2004|isbn=978-0-321-22369-2|page=1311}}</ref>