JNTU B.Tech II ANALOG AND DIGITAL ELECTRONICS Previous Question Paper (August/September - 2022 - R18)

When a p-n junction diode is reverse biased, two primary breakdown mechanisms can occur as the voltage increases:

1. Zener Breakdown: This occurs in heavily doped diodes with narrow depletion regions. Due to the high doping concentration, the electric field across the depletion layer becomes extremely strong even at low reverse voltages. This intense field exerts a force on the valence electrons, pulling them directly out of their covalent bonds (field emission), leading to a sudden increase in current. This process is temperature-dependent, where the breakdown voltage decreases as temperature increases.
2. Avalanche Breakdown: This occurs in lightly doped diodes with wider depletion regions. As reverse voltage increases, thermally generated minority carriers gain sufficient kinetic energy from the electric field to collide with crystal atoms, knocking out additional valence electrons (impact ionization). These new carriers gain energy and cause further collisions, creating a cumulative multiplication effect known as an avalanche. This process is temperature-dependent, where the breakdown voltage increases as temperature increases.

A p-n junction diode operates differently based on the applied bias polarity:

1. Forward Bias: The positive terminal of the battery is connected to the p-type and the negative to the n-type. This reduces the potential barrier, allowing majority carriers (holes in p, electrons in n) to cross the junction. Current flows easily, increasing exponentially with voltage.
2. Reverse Bias: The positive terminal is connected to the n-type and the negative to the p-type. This increases the potential barrier and widens the depletion region. Only a negligible reverse saturation current flows due to minority carriers.

The V-I characteristics show an exponential rise in current in the forward bias region (first quadrant) and a near-zero current in the reverse bias region (third quadrant) until the breakdown voltage is reached, where current increases sharply.