PNPN diode- Forward Blocking & conducting mode
Forward Blocking State:
- When the device is biased in the forward-blocking state in figure, the applied voltage v appears primarily across the reverse-biased junction j2.
- Although j1 and j3 are forward biased, the current is small.
- The reason for this becomes clear if we consider the supply of electrons available to ni and holes to p2.
- Focusing attention first upon j1 let us assume that a hole is injected from p1 into n1
- If the hole recombines with an electron in n1 (or in the j1 transition region), that electron must be resupplied to the n1 region to maintain space charge neutrality.
- The supply of electrons in this case is severely restricted, however, by the fact that n1 is terminated in j2, a reverse-biased junction.
In a normal p-n diode, the n region is terminated in an ohmic contact, so that the supply of electrons required to match recombination (and injection
into p) is unlimited.
- In this case, however, the electron supply is restricted essentially to those electrons generated thermally within a diffusion length of j2.
- As a result, the current passing through the j1 junction is approximately the same as the reverse saturation current of j2.
- A similar argument holds for the current through j3, holes required for injection into n2 and to feed recombination in p2 must originate in the saturation current of the center junction j2.
- The applied voltage v divides appropriately among the three junctions to accommodate this small current throughout the device.
Forward Conducting State:
- The charge transport mechanism changes dramatically when transistor action begins. As α1 α2 approaches unity by one of the mechanisms described.
- many holes injected at j1 survive to be swept across j2 into p2. This helps to feed the recombination in p2 and to support the injection of holes into n2.
- Similarly, the transistor action of electrons injected at j3 and collected at j2 supplies electrons for n1 Obviously, the current through the device can be much larger once this mechanism begins.
- The transfer of injected carriers across j2 is regenerative, in that a greater supply of electrons to n1 allows greater injection of holes at j1 while maintaining space charge neutrality.
- this greater injection of holes further feeds p2 by transistor action, and the process continues to repeat itself.