Physical operations of photosensitive pn junctions
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Physical operations of photosensitive pn junctions

Posted Date: 2024-02-02

silicon pn junction

An interesting thing happens when a piece of n-type silicon comes into contact with a piece of p-type silicon: diffusion current flows from the p side to the n side, forming a depletion region, and drift current flows from the n side to the p side.

Diffusion current

Holes are the majority carriers on the p-side, while free electrons are the majority carriers on the n-side. These carriers undergo diffusion, which is the tendency of particles to move from a higher concentration to a lower concentration. Holes diffuse across the junction, from p to n, and electrons also diffuse across the junction, from n to p. These carrier movements are a form of electric current; we call it diffusion current.

Diffusion current is described as flowing from the p-side to the n-side because regular current flows in the same direction as positive charge carriers, even though the positive charge carriers are not actually present in the circuit.

In this case, the holes are moving, so we actually have positive charge carriers, so a conventional electrical current is scientifically more coherent than a circuit that doesn't contain diodes or transistors.

depletion zone

We have free electrons on the n side and holes on the p side. As free electrons diffuse across the junction, they meet holes on the other side. The electron "falls" into the hole, so to speak, and recombines near the junction.

This results in an overall negative charge region near the p-side junction because recombination eliminates the holes that previously balanced the bound negative charge in the p-type semiconductor. The same thing happens on the other side, in an n-type semiconductor, except the bound charge there is positive.

We call this the depletion region because the total positive and negative charge on either side of the junction is partly caused by the depletion of the majority charge carriers, which in turn is a result of diffusion currents and recombination.

Drift current

Doping is not the source of mobile charge carriers in semiconductors. Thermal energy causes the random generation of electron-hole pairs, resulting in the existence of minority carriers, that is, electrons on the p side and holes on the n side.

If holes on the n-side or free electrons on the p-side enter the depletion region, the electric field in the depletion region will strengthen the movement to the other side of the junction. This is the drift current: minority carriers crossing the junction under the influence of an electric field. It flows from the n side to the p side.

So does current continue to flow through the diode even though it is completely disconnected from power and other components? of course not. The pn junction naturally maintains a balance between diffusion and drift currents. They flow in opposite directions with the same magnitude, so the net current is zero.

photosensitive pn junction

When a junction is exposed to light, we have an additional source of mobile charge carriers, the energy transferred by the incident photons. If photons create electron-hole pairs in or near the depletion region, the electric field in the depletion region can push mobile charge carriers across the junction.

This is what we call photocurrent: an electric current produced by the movement of light-induced carriers.

Photocurrent is reverse current. Like a drift current, it flows from the n-side to the p-side, notice how it crosses the junction under the influence of the depletion region electric field, just like a drift current. We will return to drift current later in this introduction when we discuss dark current.

Depletion region in photodiode

As mentioned above, light-generated electron-hole pairs will only generate photocurrent when they are in or near the depletion region. This shows that we can make photodiodes more sensitive by increasing the width of the depletion region: the wider the depletion region, the same intensity of incident light will produce more photocurrent because more photogenerated charge carriers can reach to push them The electric field passing through the junction.

There is another way the depletion region affects photodiode operation. The depletion region functions like a capacitor within a diode, and in a photodiode, this capacitance limits the device's ability to respond to rapid changes in illumination.

Therefore, the depletion region is related to two important considerations in the design of photodiode-based systems.


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