Jawaharlal Nehru Technological University Hyderabad B.Tech II Year ELECTRONIC DEVICES AND CIRCUITS Previous Question Paper (October - 2020 - R13)

A PN junction diode operates based on the behavior of the depletion region under external voltage. In forward bias, the positive terminal of the battery is connected to the P-region and the negative terminal to the N-region. This reduces the potential barrier, allowing majority carriers (holes in P, electrons in N) to cross the junction, resulting in a low-resistance path and significant current flow. In reverse bias, the positive terminal is connected to the N-region and the negative to the P-region. This increases the potential barrier and widens the depletion region, preventing majority carrier flow and resulting in a high-resistance state with only a negligible reverse saturation current due to minority carriers.

The diode current equation, known as the Shockley diode equation, is derived from the diffusion of minority carriers across the depletion region. The concentration of minority carriers at the edges of the depletion region is given by the Boltzmann relation:

where V is the applied voltage and V_T is the thermal voltage (kT/q). The excess minority carrier concentration is:

The diffusion current density is proportional to the gradient of these minority carriers. By integrating the diffusion current contributions from both the P and N sides and applying boundary conditions, the total diode current I is expressed as:

Here, I_0 is the reverse saturation current, V is the applied voltage, V_T is the thermal voltage, and η is the ideality factor (1 for Germanium, 2 for Silicon).