Question

The main cause of avalanche breakdown is:
A) ionization by collision.
B) high doping.
C) recombination of electrons and holes.
D) low doping

Answer 1

Hint: The avalanche breakdown occurs when a high reverse voltage is applied across the diode. As the applied reverse voltage is increased, the electrical field across the junction increases. This electric field exerts a force on the electrons at the junction and frees them from covalent bonds (ionization). These free electrons start moving with high velocity across the junction and hit the other atoms, thus creating more free electrons.

Complete solution:
p-Type Semiconductor: The p-Type Semiconductor is formed when an element of trivalent impurity is added to a pure semiconductor (Silicon) as a result many holes are created in it. Many holes are provided in the semiconductor material by the addition of trivalent impurities like Gallium and Indium.
n-Type Semiconductor: It is made by adding a pentavalent impurity to a pure semiconductor such as silicon or germanium. The impurities used could also be phosphorus, arsenic, antimony, bismuth, or other elements. They are called donor impurities. The impurity is named as the donor because it gives a free electron to a semiconductor.
PN Junction Diode: A PN-junction diode is formed when a p-type semiconductor and an n-type semiconductor are fused creating a potential barrier voltage across the diode junction.
Biasing of PN junction diode and this is based on the voltage applied:
1. Zero bias: When There is no external voltage applied to the p-n junction diode.
2. Forward bias: When we connect the positive terminal of the voltage potential is connected to the p-type while the negative terminal is connected to the n-type.
3. Reverse bias: When we connect the negative terminal of the voltage potential is connected to the p-type and the positive is connected to the n-type.
How does current flows in diode:
The flow of electrons towards the p-side from the n-side of the junction takes place when there is an increase in the voltage. Similarly, the flow of holes towards the n-side from the p-side of the junction takes place along with the increase in the voltage. This results in the concentration difference between both sides of the terminals. Because of the formation of the concentration difference, there will be a flow of charge carriers from higher concentration regions to lower concentration regions. The movement of charge carriers inside the PN junction is the reason for the current flow in the circuit.
Refer the image

Avalanche breakdown:
Avalanche breakdown occurs in a pn junction diode in which average doping is there and has a thick junction (means its depletion layer width is high). Avalanche breakdown usually occurs when a high reverse voltage is applied across the diode. So as we increase the applied reverse voltage, the electric field across the junction will keep increasing.
This generated electric field exerts a force on the electrons at the junction and it frees them from covalent bonds. These free electrons will gain acceleration and will start moving across the junction with high velocity. This results in a collision with other neighboring atoms. These collisions in high velocity will generate the next free electrons. These electrons will start moving and electron-hole pair recombination occurs across the junction. This results in a net current that rapidly increases.
We learned that avalanche breakdown occurs at a voltage$(V_a)$ that is higher than the Zener breakdown voltage$(V_z)$. The reason behind this is simple. We know, avalanche phenomena occur in a moderately doped diode and junction width$(d)$ is high. A Zener breakdown occurs in a diode with heavy doping and thin junction (here $d$ is small). The electric field that occurs due to applied reverse voltage can be calculated as $E={\displaystyle \frac{E}{d}}$.
In avalanche breakdown, the depletion layer`s width is more and hence much more reverse voltage needs to be applied to develop the same electric field strength (necessary enough to break electrons free)
Depletion region - As a result, the bulk charge carriers (free electrons for the N-type semiconductor, and holes for the P-type semiconductor) are depleted in the region around the junction, so this region is named the depletion region or depletion zone.

Final answer (A), Avalanche breakdown is caused by impact ionization or ionization due to collision of electron-hole pairs.

Note: 1. A very little current flows under reverse bias conditions and depletion region increases. The electric field within the depletion region of a diode is often very high.
2. Electrons/holes that enter the depletion region undergo a tremendous acceleration. As these accelerated carriers collide with the atoms, they can knock electrons from their bonds, creating additional electron/hole pairs and thus additional current.
3. As these secondary carriers are swept into the depletion region, they too are accelerated, and the process repeats itself.