Forward Bias Effects on PN Junction Diodes

[Garoll]

Hello,

Here is my question:

If we have a regular PN junction diode, which is forward biased we will observe the effect "injection", we will have more minority carriers than normal near the PN junction. I would like to ask you whether this "injection" is observed during the whole period when the diode is forward biased or it is observed just at the moment of forward biasing the diode.
I am asking this question, because i have got an issue with the frequency characteristics of the diode.
In my textbook it is said:
"When the frequency is low the minority carriers in the base are able to recombinate fully before changing the polarity of the voltage and there is current only in the positive semi-period of the signal.
When we make the frequency higher the rectifying properties of the diode are getting worse. The minority carriers are not able to fully recombinate before changing the polarity and we are observing current in negative semi-period, which is bad."
This explanation leads me to the fact that we observe "injection" only in the moment when we forward bias the diode, but not at the whole time when the diode is forward biased.

Could i ask anyone to clear out these issues, it will be of big help for me.

Regards

 

[uart]

Hi Garoll. When a diode is forward biased carriers are injected raising the minority carrier concentrations in both the P and N regions (consequently increasing the rate of recombination in each of these regions). This is a equilibrium type process. Initially the rate of injection will exceed the rate of recombination so the minority carrier conc increases. As the minority carrier conc increases it causes the rate of recombination to increase and an equilibrium is soon established where the rate of minority carrier injection is balanced by the rate of recombination in each region. The more forward current that the diode carries then the more the minority carrier conc will rise before equilibrium is established (and visa versa if the current is decreased).

So directly to your first question the answer is NO. The injection does not only occur at the start of the forward conduction, it is ongoing during foward conduction operation.

Where you're getting confused is with the description of the diode operation at high frequencies and a (usually undesirable) condition known as "reverse recovery". As explained above a forward conducting diode rapidly reaches an equilibrium whereby the minority carrier concentrations are increased in proportion to the amount of forward current flow. At low frequencies, like 50Hz or 60Hz for example, the forward current decreases slowly enough (when moving from forward to reverse bias operation) such that the excess minority carrier concentrations can remain in approx equilibrium with carrier injection at all times. This means that the excess minority carriers will decrease to zero before any reverse voltage is applied. In this condition the diode is ready to reverse block immediately that reverse voltage is applied (desirable).

Now at very high frequencies, or in particular in switched mode operation where a diode may be required to go from forward conduction to reverse blocking very rapidly, then the excess minority carriers do not have time to recombine before the reverse voltage condition is applied. In this condition (large excess minority carrier concentration) the diode will not reverse block and you actually end up having a brief period of time (called the reverse recovery time) where the diode quite freely carries current in the reverse direction. This is generally very undesirable hence fast reverse recovery is a critically important feature of diodes used in high frequency switched mode applications.

 

[Garoll]

Dear uart, thank you for your quick and kind reply, now i understood this process, thanks.