Photodiodes

Photodiodes perform an essential process in many circuits, detecting light and converting it into electrical energy. They can be found in a wide array of common electronic devices, such as cameras, surveying equipment, burglar alarms, and more. They may also be referred to as photodetectors or photosensors, depending on the context.

We carry a number of different photodiodes for practically every purpose you could need one for, including from leading manufacturers such as Excelitas photodiodes and Optek photodiodes. Whatever the kind of photodiode circuit you're constructing, you'll find what you need with RS.

What is a photodiode?

Photodiodes are a type of semiconductor which use a P-N junction to convert light (photons) into an electrical current.

They can be referred to as light sensors or detectors, with the principles of the photoelectric effect governing their inner workings. When the junction of this two-terminal semiconductor photodiode is illuminated, then the electric current will start to flow through it.

How does a photodiode work?

The P-N junction of a photodiode is connected in reverse bias, resulting in a wide depletion region within the junction. The P-type has holes as the majority carrier, while the N-type has electrons as the majority carrier. Without illumination, there is still a small amount of current in microampere known as dark current.

When a photon (light) which possesses energy greater than the bandgap energy hits the diode, the covalent bond breaks and new electron-hole pairs are generated which establishes photocurrent. An increase in surface area of a photodiode will slow this process down and reduce the response time of the component.

The photocurrent that is generated is directly proportional to the absorbed light intensity, as it is that process of breaking bonds which generated the current. The intensity of photon absorption varies depending on the energy of photons, so the lower the energy of photons, the deeper the absorption is. This is known as the inner photoelectric effect.

The sum of both the photocurrent and dark current is the total current which will pass through the photodiode. The sensitivity of the photodiode sensor can be increased by minimizing the dark current.

You can also get PIN photodiodes, which are very similar to P-N junction diodes but with one major difference: an intrinsic layer is placed between the P-type and N-type layers. The purpose of the intrinsic layer is that it is highly resistive, increasing the electric field strength in the photodiode. There are many benefits to having this added intrinsic layer due to the increase in the depletion region.

Types of photodiode operation

Photovoltaic mode

Referred to as "zero bias mode" as well, this makes use of the photovoltaic effect for its operation. The photocurrent output is restricted, while the voltage is created by the illuminated diode. This mode has a small dynamic range and the voltage generated has non-linear dependence.

Photoconductive mode

Photodiodes in photoconductive mode are commonly reverse biased, meaning that the cathode becomes positive in relation to the anode. The width of the depletion layer is increased by using reverse voltage, reducing the response time and capacitance of the junction. One benefit of this mode is its rapid response, although it will exhibit electronic noise as a drawback.

Avalanche diode mode

Avalanche photodiodes use a high reverse bias condition, multiplying avalanche breakdown at each electron-hole pair photo generated. This creates internal gain within the photodiode itself, progressively increasing how responsive the device is.

What are photodiodes used for?

The properties of semiconductor photodiodes make them incredibly versatile and suitable for a wide number of applications. As photodiodes are fundamentally light sensors, they can be used to measure light intensity in certain areas and provide crucial information about an environment. Avalanche photodiodes are used in smoke detectors to sense light-scattering within a space too.

Barcode scanners, character recognition systems and obstacle detection systems will employ photodiodes as part of understanding patterns and the world around them by how light interacts with the photodiode sensor.

Find your photodiodes and more with RS

Our extensive range of photodiodes cover practically every configuration you could require for a project and its various photodiode circuit designs. Whatever combination of emitters and detectors you need, RS will have what you need. We've been a trusted distributor of quality electrical components and electromechanical products across the Americas for over 90 years.

Start building your order today, or you can complete our part quote form which we can then discuss with you in order to work out the best deal possible. Alternatively, for any other queries you may have or advice you should require, simply contact one of our regional sales offices who can provide you with the support you need when dealing with photodiodes or otherwise.