Semiconductors and Diodes

Avalanche breakdown is caused by impact ionization of electron-hole pairs. A very little current flows under reverse bias conditions and depletion region increases. The electric field in the depletion region of a diode can be very high. Electron/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. As these secondary carriers are swept into the depletion region, they too are accelerated and the process repeats itself.

The avalanche breakdown in p-n junction is due to cumulative effect of conduction band electron. In reverse bias, due to applied electric field, electrons acquire enough energy to free more electron bound to the atom. The abundant number of electron-hole pairs are created for conduction, this is cumulative effect.

The reverse breakdown voltage depends on doping of the diode. Hence, in the Zener diode the heavy doping of its p-n junction is done. The depletion region formed in the diode is very thin ($<1\ m$) and the reverse bias voltage of about $5\ V$ which is less than ordinary diode.

When the reverse bias breakdown voltage is exceeded, a conventional diode is subject to high current due to avalanche breakdown. Avalanche breakdown occurs in reverse bias when the applied voltage is high enough, the free electron may move fast enough to knock other electrons free, creating more free-electron-hole pairs (i.e., more charge carriers), increasing the current. Thus, the current flow in a Zener diode is mainly due to collision generated charge carriers

We know that zener diode works on the reverse bias. When the reverse bias is equal to the break-down voltage, the voltage across the zener remains almost constant and the current increases rapidly.

When the reverse voltage across a diode is very large, the valance electrons become free due to applied high electric field and get enough acceleration to make other electrons free, thus create a lot of electron-hole pairs in a short time. As the number of charge carriers increases, current also increases.

Zener diode is a p-n junction diode working in the breakdown region. It is used as a voltage regulator/stabilizer to provide a constant voltage from a source whose voltage may fluctuate over a wide range.

 A voltage regulator circuit can be designed using a zener diode to maintain a constant DC output voltage across the load in spite of variations in the input voltage or changes in the load current. 

current increases as the resistance decreases, as zener current is inversely proportional to the series resistance

For Zener Diodes, the breakdown voltage decreases with increasing doping concentration.

The zener voltage will be a constant only when it is operating in the breakdown region

Barrier capacitance, also known as transition or depletion capacitance, is inversely proportional to the width of the depletion region. Increasing the reverse bias voltage widens the depletion layer, which in turn decreases the barrier capacitance. Therefore, option B is correct.

Zener breakdown occurs due to high electric fields (internal field emission) in heavily doped diodes, while avalanche breakdown occurs due to impact ionization. Both are mechanisms associated with reverse bias breakdown.

Before the breakdown voltage is reached, a Zener diode acts like a normal diode under reverse bias, where only minority carriers contribute to a very small leakage current. This reverse saturation current is typically on the order of microamperes, making option D correct.