This might be a stupid question but I am either having a mental block or I'm getting something. I'm learning about excess minority charge carriers.
So, when an electric field is applied across a pn diode (in such a way as to decrease the depletion layer width, i.e. forward biased), the carriers move from their "majority" sides to their opposite, "minority" sides.
We call this excess of the minority charge the excess minority charge concentration.
As the length travelled increases, more and more charges recombine with their opposites. So in short diodes, the charges go straight across to the "wire"/contact with only a few of them recombining.
Question:
But in (say very) long diodes, the excess charge carriers recombine with their opposites (almost) fully. So then, how is charge carried across the pn diode?
It feels like I'm missing something here. I don't fully understand the significance of the excess minority charge carriers? My book says that the initial gradient of the decay of minority charge carriers is the current. I don't understand this at all.
Can anyone shed any light on where I'm going wrong?
Best Answer
As the injected minority carriers recombine, they have to recombine with something. What they recombine with is some of the majority carriers. So majority carriers near the junction are being "used up" by the recombination process.
As the majority carriers are taken up by recombination, new majority carriers flow in from the area farther away from the junction. This majority carrier flow is what carries the current in the "far away" part of the device.
I found a diagram showing this here: