Current through a PN Junction Diode

Q1: How can I describe relative amounts of charge carriers in semiconductors?
A1: Recall that the p-type semiconductor is characterized by a high concentration of holes while the n-type semiconductor is characterized by a high concentration of free electrons.
Hence, holes and electrons are the majority carriers in the p-type and n-type semiconductor (respectively).
In turn, electrons and holes are the minority carriers in the p-type and n-type semiconductor (respectively). Theses minority carriers are generated by thermal excitation.

Q2: Why introduce majority and minority carriers?
A2: You can use majority and minority carriers to examine describe the 2 types of current in a junction diode.
1. Drift Current: Current that flows due to an applied electric field
2. Diffusion Current: Current that flows due to charge carriers moving from a high to low concentration
Recall that holes and electrons diffuse across the junction, which sets up an internal electric field responsible for the junction voltage. The diffusion of these majority carriers also sets up a diffusion current:
$I_{diff}$
Recall that this junction voltage prevents further diffusion of:
1. Holes from the p-type to n-type semiconductor
2. Free electrons from the n-type to p-type semiconductor
In other words, the junction voltage prevents further diffusion of both majority charge carriers.

However, recall that the minority carriers are still present in the diode. When they reach to the depletion layer, the electric field associated with the junction voltage pushes:
1. Free electrons from the p-type semiconductor to the n-type semiconductor
2. Holes from the n-type semiconductor to the p-type semiconductor.
This movement of minority carriers establishes a drift current that flows to from the n-type to p-type semiconductor.
$I_S$
We call this drift current the reverse saturation current of the diode.

Q3: How are are the diffusion current and the reverse saturation current related?
A3: Suppose that no external electric field is applied to the diode. The diode has a diffusion and reverse saturation current, but the diode must remain neutral overall, so:
$I_{diff}=-I_S$

Q4: How can I distinguish between the diffusion current and the reverse saturation current? Besides looking at whether the carriers they specify are majority/minority carriers?
A4: As mentioned at the end of A1, minority carriers are thermally generated. Because of this, the magnitude of the reverse saturation current increases with temperature. Roughly speaking, the reverse saturation current doubles for every 7°C rise in temperature.

On the other hand, the diffusion current of majority carriers depends on the externally applied voltage reaching a certain threshold, after which point the diffusion current scales exponentially.

Q5: How does diffusion current and reverse saturation current relate to forward and reverse biasing?

A5:

Let $i_d$ be the current through the diode

In forward biasing, the externally applied voltage has overcome a certain threshold. So the diffusion current dominates.

$i_d\approx I_{diff}$

In reverse biasing, the externally applied voltage is below that threshold. So the reverse saturation current dominates.

$i_d\approx -I_{S}$