JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II Year ANALOG AND DIGITAL ELECTRONICS Previous Question Paper (August/September - 2022 - R18)

In a p-n junction, capacitance arises from the storage of charge. Drift capacitance, more commonly referred to as transition or depletion capacitance (C_T), occurs under reverse bias. As reverse voltage increases, the depletion region widens, acting like a parallel plate capacitor where the depletion region is the dielectric and the ionized impurity atoms form the plates. It is defined as C_T = dQ/dV, where Q is the charge of the ionized donors/acceptors.

Diffusion capacitance (C_D) occurs under forward bias. When a diode is forward-biased, minority carriers are injected across the junction, creating a stored charge profile that decays with distance. As the forward voltage changes, the amount of stored charge changes, resulting in a capacitive effect. It is defined as C_D = dQ/dV, where Q is the excess minority carrier charge. C_D is typically much larger than C_T because it is proportional to the forward current.

Zener Breakdown occurs in heavily doped p-n junctions. Due to the high doping concentration, the depletion region is extremely narrow. When a strong reverse bias is applied, the electric field intensity across the junction becomes high enough (approx. 10⁶ V/cm) to pull electrons directly out of the covalent bonds. This quantum mechanical tunneling effect creates a large number of free charge carriers, leading to a sharp increase in reverse current at a specific Zener voltage (V_Z).

Avalanche Breakdown occurs in lightly or moderately doped junctions where the depletion region is wider. As reverse bias increases, minority carriers gain significant kinetic energy from the electric field. These high-energy carriers collide with atoms in the crystal lattice, breaking covalent bonds and creating new electron-hole pairs through a process called impact ionization. These newly created carriers are then accelerated by the field to cause further collisions, resulting in a cumulative, self-sustaining multiplication effect known as the avalanche effect, leading to a sudden, rapid increase in current.