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Applet Tutorial
(The pn junction diode under applied bias)
(you may skip this if you are already familiar with pn junction theory)

By using the checkboxes at top, familiarize with the four
basic current components:

electron leakage : this is part of the reverse saturation current.

electron injection : once injected (into the pregion, red), electrons
move by means of diffusion (due to its concentration gradient in pregion).
So, it is also called the diffusion current.

electron recombination: Whenever the thermalequilibrium condition of a
physical system is distributed, processes exist to restore the system to
equilibrium. In pn junction, the bandto band recombination where an electronhole
recombines will occur. This transition of an electron from the conduction
band to the valence band is made possible by emission of a photo or by
transfer of the energy to another free electron or hole. These electrons
form the electron recombination current.

hole leakage : this is the hole part of the reverse satusation current.

hole injection : after injection (into the neutral nregion, blue), they
move (away from the junction) via diffusion.

hole recombination: this is the hole part of the recombination current.

The sample schematics (top redwhiteblue rectangle) and the band diagram
are for a given applied bias that is set using the scrollbar. The bias
may be changed using the little scrollbar in the upperleft corner. The
band diagram shows both majority carriers (electron=blue rectangle; hole=red
rectangle) and minority carriers. The heights of the colored rectangles
in the conduction(valence) bands in n(p) side are proportional to the log
of the majority carrier density. The heights of the colored rectangles
in the conduction(valence) bands in p(n) side changes with the change of
the position into the sample. When x=xp(xn), the height of the rectangles
is equal to the height of log of the majority carrier density above
the conduction(valence) band in n(p) side. Then the carrier density decreases
as it goes deep into the sample, at last it appoaches to the log of the
minority carrier density under zero bias and remains as a constant. The
length which minority carrier density appoaches to a constant is equal
to the length that the recombination current can occur. The change
of the carrier density in this process is according to a decrease exponential
function with a parameter named diffusion length. Diffusion length is the
average distance a minority carrier travels before it recombines with a
majority carrier, it is typically a few microns to a few millimters.

You should see the minority carrier drift (i.e., leakage) at all applied
bias. Injection current increases under forward bias (positive voltage),
and the recombination current is observed only under forward bias.

At zero applied bias, the number of electrons moving from the pside to
the nside is exactly equal to the number moving in the opposite direction
(ie, from the nside to the pside). This is true for the holes as well.
Thus the total current across the junction is zero at zero bias (as it
should be).

When you apply a positive bias to the pside with respect to the nside
(forward bias), then more electrons in the nside lie above the Ec of the
pside (ie, the band edge of the other side of the junction) and these
electrons can cross the junction without being objected by the potential
barrier. These electrons constitute the electron part of the injection
current. (Of course, even at zero bias electrons flow from the nside to
pside. But these are cancelled by the electrons flowing in the opposite
direction.) Those electrons in the nside which lie below Ec of the opposite
side will not be able to cross the junction, and do not contribute to any
current.

When you apply the reverse bias (ie, pside negatively biased wrt the nside),
the electron flow from the nside to the pside decreases and eventually
disappears from certain reverse bias value. Observe that, beyond this reverse
bias, the current is constant of the applied reverse bias.

The same is true for the injection hole current under the forward or reverse
bias.

Based on this applet, you can derive the currentvoltage relationship for
a pn junction diode.