Jawaharlal Nehru Technological University Hyderabad B.Tech II ANALOG AND DIGITAL ELECTRONICS Previous Question Paper (September - 2021 - R18)

A photodiode is a p-n junction device designed to operate in reverse bias, where it converts incident light energy into electrical current. When photons with energy greater than the semiconductor bandgap strike the depletion region, they generate electron-hole pairs, increasing the reverse leakage current proportional to the light intensity.

Key characteristics include:

• Sensitivity: High sensitivity to light, allowing it to detect low-intensity signals.
• Linearity: The output current is directly proportional to the incident light intensity over a wide range.
• Response Time: Very fast response time, making it suitable for high-frequency applications.
• Dark Current: A small current flows even in the absence of light due to thermal generation.

Applications include optical communication systems, light intensity measurement, smoke detectors, and medical imaging equipment.

The reverse saturation current (I₀) of a diode is highly sensitive to temperature variations. It arises due to the thermal generation of minority charge carriers within the semiconductor material. As temperature increases, the number of thermally generated electron-hole pairs increases exponentially.

The relationship is governed by the following approximation:

This formula indicates that for silicon diodes, the reverse saturation current approximately doubles for every 10°C rise in temperature. The physical reasoning is that higher thermal energy provides sufficient kinetic energy for electrons to break covalent bonds, crossing the forbidden energy gap from the valence band to the conduction band. Consequently, the concentration of minority carriers increases, leading to a significant rise in the reverse leakage current. This temperature dependence is a critical factor in circuit design, as it can lead to thermal instability or thermal runaway in power semiconductor devices if not properly managed with heat sinks or temperature compensation circuits.