Forward Bias Effects on PN Junction Diodes

  • Thread starter Thread starter Garoll
  • Start date Start date
  • Tags Tags
    Diode
AI Thread Summary
When a PN junction diode is forward biased, the injection of minority carriers occurs continuously, not just at the moment of biasing. Initially, the rate of injection surpasses recombination, leading to increased minority carrier concentrations until an equilibrium is reached. At low frequencies, these carriers can recombine fully before the polarity changes, allowing the diode to block reverse voltage effectively. However, at high frequencies, the excess minority carriers do not have enough time to recombine, resulting in undesirable reverse current during the reverse recovery period. Understanding this dynamic is crucial for applications requiring fast switching in high-frequency scenarios.
Garoll
Messages
7
Reaction score
0
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
 
Engineering news on Phys.org
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.
 
Dear uart, thank you for your quick and kind reply, now i understood this process, thanks.
 
Hey guys. I have a question related to electricity and alternating current. Say an alien fictional society developed electricity, and settled on a standard like 73V AC current at 46 Hz. How would appliances be designed, and what impact would the lower frequency and voltage have on transformers, wiring, TVs, computers, LEDs, motors, and heating, assuming the laws of physics and technology are the same as on Earth?
I used to be an HVAC technician. One time I had a service call in which there was no power to the thermostat. The thermostat did not have power because the fuse in the air handler was blown. The fuse in the air handler was blown because there was a low voltage short. The rubber coating on one of the thermostat wires was chewed off by a rodent. The exposed metal in the thermostat wire was touching the metal cabinet of the air handler. This was a low voltage short. This low voltage...
While I was rolling out a shielded cable, a though came to my mind - what happens to the current flow in the cable if there came a short between the wire and the shield in both ends of the cable? For simplicity, lets assume a 1-wire copper wire wrapped in an aluminum shield. The wire and the shield has the same cross section area. There are insulating material between them, and in both ends there is a short between them. My first thought, the total resistance of the cable would be reduced...
Back
Top