The process of destroying a depletion area at the p-n junction and allowing a significant reverse current is known as "depletion region breakdown." The depletion region can be destroyed in two ways: by applying a high enough voltage across the junction (or creating an electron-hole pair near the junction) or by putting sufficient current through the device.
In practice, these methods for destruction of the depletion region are not very different from those already discussed for forward bias conditions. In fact, the only real difference between forward-biased diodes and backward-biased diodes is that no current flows into the diode during destruction of the depletion region. Otherwise, the methods for destroying the depletion region are exactly the same.
Applying a High Voltage to the Diode
If a high enough voltage is applied across a diode, even if it is not forward biased, electrons will be emitted from the end of the n region closest to the cathode and will then fall back into that end of the n region. Thus, the voltage required to produce this effect is called the "breakdown voltage" of the diode.
A depletion area is a region at the p-n junction where the flow of charge carriers (free electrons and holes) is decreased over time until there are no charge carriers left. The depletion region varies in size depending on several factors such as the doping level of the material, but generally speaking it can be said that for low doping levels (such as those used for transistor fabrication) the depletion region is large compared to the wavelength of visible light so that it can be considered an uniform layer across all parts of the junction. As the doping level increases, the depletion region narrows down until at some point it becomes comparable in width to the wavelength of visible light or even narrower. At this point, we enter the ultra-deep n-type doped region or ultra-shallow p-type doped region.
The depletion region forms when the majority charge carriers are depleted from the semiconductor. That is, there are not enough free charge carriers to cover the space charge within the depletion region. If more charge carriers were available, they would move towards the surface of the n-type or p-type material away from the junction, reducing the potential difference across the junction and therefore preventing current flow.
The diffusion of charges causes the depletion area. Because of the concentration differential, holes diffuse across the junction from the p-side to the n-side, whereas electrons diffuse from the n-side to the p-side. Near the junction, the holes and electrons that are spreading towards one other unite. As they do, they move farther away from their respective dopants, and therefore from each other. This separation of charge creates an electric field, which is responsible for depleting the junction.
The depletion layer forms when there is a difference in doping between two regions of an element. This could be the case with doped silicon or germanium. The type of dopant determines how far the layer will spread. If you add electrons to silicon, they will spread out farther than if you add holes. Similarly, if you add holes to silicon, they will spread out farther than if you add electrons. This is because electrons are more mobile than holes. A certain amount of energy is needed to remove an electron from silicon, so fewer such electrons remain near the surface compared to holes. Thus, you get a depletion layer with electrons added.
You also get a depletion layer with holes added if you use undoped silicon or germanium. The absence of any ions makes it harder for charges to move around inside the material and therefore less likely that they will unite with opposites charges.
When the reverse bias is set too high, the current through the PN junction increases quickly, and the voltage at which this occurs is referred to as the breakdown voltage. The crystal structure fails at this breakdown voltage. This results in a big stream flowing. Zener breakdown is the process through which this mechanism of breakdown occurs. The electric field across the PN junction causes an avalanche of electrons to be injected into the n region of the diode, causing the circuit to conduct even though no signal is applied.
Breakdown occurs when the energy gained from the electron-hole pairs created by ionization exceeds the potential energy difference between the two regions (n and p). At some point, the electric field becomes strong enough to cause an electron-hole pair to be created within the silicon band gap. Since both particles are now free to move, they do so until their kinetic energy is reduced to zero. Without limiting factors such as surface states or impurities, all of the electrons will flow into the n region and all of the holes will flow into the p region. Thus, a current flows through the PN junction regardless of whether it is forward or reverse biased.
The amount of current that can flow through a PN junction without damage occurring to the device depends on several factors, including the type of material used to fabricate the junction.
Depletion Area Development: At the time of PN junction formation, free electrons around the junction diffuse across the junction into the P region and interact with holes. The depletion zone now serves as a barrier. Potential Obstacles The depletion region's electric field functions as a barrier. Any particle that arrives at this area will be accelerated by the field until it reaches some critical energy value, after which point it no longer interacts with the device.
This process leaves behind an accumulation of charge in the N region. Because the charges are negative, they create a net positive charge in the N region. This is called the "built-in" or "surface" charge. It should be noted that although these charges are built into the device during fabrication, they do not remain there forever. Over time, both the P and N regions of the diode will tend to re-balance these surface charges. However, if the diode is exposed to high enough temperatures, it will breakdown before these charges can be restored.
The barrier created by the depletion zone prevents current from flowing through the PN junction unless there is a voltage difference between the ends of the diode. When light strikes the semiconductor, electrons are excited into the conduction band. These free electrons then diffuse through the material until they reach the PN junction where they cancel out any remaining holes.