Photodiodes: Light Meets Semiconductors

2024-01-05 | By Antonio Velasco

Let's continue talking about diodes with a little dive into optoelectronics--basically, electronics dealing with light! For this blog, I'll be talking about the LED's doppelganger: the Photodiode.

Okay so, what is it?

Put simply, a photodiode does the opposite of what an LED does: convert light into an electrical current. This is done through the photoelectric effect, where photons of light strike the semiconductor material, exciting electrons and creating electron-hole pairs. These electron-hole pairs can then move freely within the material, generating an electric current and thus the detection of light. This can also be done with a photoresistor, something I mentioned in another article, but instead of having a resistance based on light, this time it is current.

The science behind it relies mostly on the material used for the diode: semiconductors. Silicon, germanium, and gallium arsenide (sometimes also referred to as GaAs) are common materials that are utilized and doped to get certain properties. The most commonly doped material is silicon. By doping silicon with specific impurities, engineers tailor the wavelength sensitivity and other characteristics of the photodiode. For germanium, it is often doped and made to be sensitive to infrared light, allowing it to be perfect for any remote controls or IR sensors (something I also previously wrote about!) Gallium arsenide photodiodes excel in high-frequency applications, including fiber-optic communications, thanks to their rapid response times. You'll hear a lot about GaAs in semiconductor manufacturing.

The Mathematics of Photodiodes

Now, let's shed some light (haha, get it?) on the math behind photodiodes. Let's take a look at some graphs and equations for this!

Responsivity quantifies how efficiently a photodiode converts incident light into an electrical current. It's expressed in units of A/W (amperes per watt) and can be calculated as:

Rλ = Ip / P

Where:

- R is the responsivity.

- Ip is the photocurrent generated by the photodiode.

- P is the incident optical power at a given wavelength.

Photodiodes: Light Meets Semiconductors

Shown is a graph depicting the responsivity (in A/W) of a photodiode with respect to its wavelength. You'll notice that this is particularly responsive with wavelengths between 400 to roughly 1000nm--the specific wavelengths for visible light!

Quantum efficiency measures the percentage of incident photons that generate electron-hole pairs in the photodiode. Put simply, it is just looking at the overall sensitivity of the diode. It's calculated as:

QE = (number of collected electrons) / (number of incident photons)

Furthermore, another important factor in photodiodes is the bandgap, or essentially the range of wavelengths to which it is sensitive. In the previous graph, we saw it most sensitive from 400nm to 1000nm, depicting a photodiode that is perfect for visible light. These are typically made of Silicon, whereas as mentioned, IR light detectors (from 800nm to 1700nm) tend to be made of germanium. GaAs also has a similar bandgap to germanium, except it is better suited for high-frequency applications due to its extremely high response speed--perfect for fiber-optic!

Wrap-Up

Photodiodes are suitable for a number of projects--whether it's a daylight sensor or trying to make your own remote! They're used in a lot of automatic lights these days, especially the ones in your garden that might turn on in the dark and turn off during the day. Photodiodes are simple to use and extremely useful!

For other articles on diodes, see below!

Demystifying the Diode!

Diodes...in Transistors?

Zener Diodes

Shining Some Light on LEDs!