Photodiodes - (working & why it's reverse biased) | Semiconductors | Physics | Khan Academy

Summary

Photodiodes are semiconductor devices that convert light into electricity, operating in reverse bias to detect light intensity. When light hits the diode, it generates electron-hole pairs that create current, with more light producing more current. Reverse bias ensures a large depletion region, making the photodiode sensitive to light without interference from other currents. Photodiodes are used in applications such as automatic street lighting and note counters, where they detect light changes and trigger responses. Their ability to measure light intensity makes them essential in various electronic systems.

Takeaways

• 😀 Photodiodes are devices that convert light into electricity, functioning as the opposite of LEDs which convert electricity into light.
• 😀 Photodiodes are used to detect the brightness of light, with applications in various fields like solar panels, but they are not the same as solar cells.
• 😀 To operate as a photodiode, a PN junction must be reverse biased, meaning the P side is connected to the negative terminal and the N side to the positive terminal.
• 😀 In reverse bias, the depletion region of the PN junction widens, preventing electrons and holes from recombining, which is key to the photodiode's function.
• 😀 When light strikes the photodiode, photons with sufficient energy can cause electrons to jump into the conduction band, creating electron-hole pairs.
• 😀 If the electron-hole pair is formed within the depletion region, the electron is attracted toward the positive charge and the hole towards the negative, creating a current.
• 😀 Brighter light increases the number of photons, which leads to more electron-hole pairs and thus a higher current, making the brightness detectable through current measurement.
• 😀 Reverse bias is preferred because it ensures that the electron-hole pairs are formed in the depletion region, maximizing the current generated by light.
• 😀 Forward bias narrows the depletion region, reducing the effectiveness of photon absorption, and allowing other currents to interfere with light-based current generation.
• 😀 The current generated by a photodiode is mainly dependent on the amount of light, while voltage has little impact once a certain threshold is reached.
• 😀 Photodiodes can be used in applications like automatic street lights (activated by low current when it's dark) and note counters in banks (detecting changes in current as notes pass through a laser beam).

Q & A

What is a photodiode?

A photodiode is a type of diode that converts light into electricity. It is the opposite of an LED, which converts electricity into light.

What is the primary use of photodiodes?

Photodiodes are primarily used to detect the brightness of light by converting light into electrical current.

What is the key difference between photodiodes and solar cells?

While photodiodes and solar cells both convert light into electricity, photodiodes are primarily used for detecting light intensity, while solar cells are specifically designed to generate electrical power from sunlight.

How does reverse biasing work in a photodiode?

In reverse biasing, the P-side of the photodiode is connected to the negative terminal and the N-side to the positive terminal, which causes the depletion region to widen, allowing the photodiode to detect light more effectively.

Why is reverse biasing preferred in photodiodes instead of forward biasing?

Reverse biasing is preferred because it widens the depletion region, allowing more photons to generate electron-hole pairs that contribute to the current. In forward bias, the depletion region narrows, reducing photon absorption and disrupting the detection of light.

What happens when light hits a photodiode?

When light hits a photodiode, photons with enough energy excite electrons, creating electron-hole pairs. These pairs are separated by the reverse bias, resulting in an electric current.

How does the intensity of light affect the current in a photodiode?

The intensity of light directly affects the current in a photodiode. Brighter light results in more photons, which leads to more electron-hole pairs being formed and, therefore, a higher current.

What are the voltage-current (VI) characteristics of a photodiode?

The voltage-current characteristics of a photodiode show a negative current under reverse bias. The current generated is largely independent of voltage and primarily depends on the amount of light hitting the photodiode.

What is dark current in photodiodes?

Dark current is the tiny amount of current generated by a photodiode even in the absence of light, caused by thermal energy that excites electrons and creates electron-hole pairs.

What are some practical applications of photodiodes?

Photodiodes are used in automatic street lighting systems, optical note counters, light sensors, and various other applications where light intensity needs to be detected and measured.