13.1.3 Photodiode

Construction and Operation:

• A photodiode is constructed by creating a junction between p-type and n-type materials.
• To allow light to interact with the junction, photodiodes have a transparent window or lens.
• Photons absorbed by the photodiode's material release charge carriers (electrons and holes), causing a current to flow.
  • The amount of current generated is directly proportional to light intensity: the greater the light intensity, the more charge carriers are released, and the higher the current.

Modes of Operation:

1. Photoconductive Mode (Reverse-Biased)

• In this mode, the photodiode is operated with a reverse bias, meaning the p-type side is connected to the positive terminal of a power source.
• Characteristics:
• Linear Relationship: The current generated is directly proportional to the light intensity, except when no light is present (in which case a small "dark current" flows).
• Reduced Response Time: The response time (time to react to changes in light) decreases as the reverse bias increases, making this mode ideal for rapid light detection.
• Applications: Used in optical systems, such as smoke detectors and communication systems, due to the fast and proportional response to light changes.

2. Photovoltaic Mode (Forward-Biased)

• In this mode, the photodiode is operated in forward bias, similar to a solar cell, where it generates a voltage without an external power source.
• Characteristics:
• Generates a small current that depends on light intensity.
• Typically has a dark current of about 500 pA when no light is present.
• Applications: Often used in solar cells for energy generation from light.

Responsivity and Spectral Response:

• Responsivity (Sensitivity) of a photodiode refers to the ratio of the current generated to the power of incident light.
  • Formula: $\text{Responsivity} = \frac{\text{Current Generated}}{\text{Power Incident on Diode}}$
• Spectral Response: Photodiodes respond differently to different wavelengths (colours) of light.
  • A diode's spectral response curve shows its sensitivity to various wavelengths.
  • Different photodiodes can be manufactured to respond better to UV, visible, or infrared light depending on the intended application.

Applications in Optical Systems:

• Smoke Detectors:
  • Photodiodes detect pulsing infrared light. If smoke scatters this light into the photodiode, a current is generated and amplified to trigger an alarm.
• Light Detectors in Fibre Optic Communication:
  • Photodiodes capture light transmitted through optical fibres. The light signal produces a current, which can be amplified and used to transmit information reliably.

Scintillators and Atomic Particle Detection:

• Photodiodes can be coupled with a scintillator (a material that emits light when hit by particles or high-energy photons) to detect atomic particles.
• Process:
  1. Particles collide with the scintillator, emitting light photons.
  2. The photodiode absorbs these photons, creating a current.
  3. The number of light pulses can be counted to detect particles, and the current levels can help measure particle energy.