Understanding the Photo Diode

The question describes a specific type of semiconductor diode that exhibits certain characteristics: it is a reverse-biased silicon or germanium pn junction, and its reverse current increases when exposed to light.

Let's examine the properties of the diodes mentioned in the options:

• Tunnel diode: This is a heavily doped p-n junction diode that exhibits negative resistance characteristics due to the quantum mechanical tunneling effect. It is typically operated in forward bias, although the tunneling effect occurs near zero bias. It is not primarily used for light detection in this manner.
• Varactor diode: Also known as a varicap diode, this is a p-n junction diode whose capacitance varies significantly with the applied reverse voltage. It is used in tuning circuits and voltage-controlled oscillators. It is not designed for light detection.
• Shockley diode: This is a four-layer (PNPN) semiconductor device. It is a type of thyristor (though often considered the simplest thyristor). It acts as a switch and has two stable states (ON and OFF). It is not a light-sensitive diode in the way described.
• Photo diode: This is a p-n junction diode designed to operate in reverse bias. When light (photons) strikes the junction, it generates electron-hole pairs. These carriers are then swept across the junction by the electric field created by the reverse bias, causing an increase in the reverse leakage current. The amount of reverse current is proportional to the intensity of the light. Photo diodes are commonly made from silicon or germanium.

Comparing the characteristics described in the question with the properties of the diodes:

Based on this comparison, the device that matches the description of a reverse-biased silicon or germanium pn junction in which reverse current increases when exposed to light is the Photo diode.

The increase in reverse current in a photo diode due to light is explained by the photoelectric effect within the semiconductor. Incident photons with sufficient energy generate electron-hole pairs near the junction. Under reverse bias, these carriers are accelerated across the junction, contributing to the flow of reverse current, often called photocurrent.