Photodiodes are semiconductors that are used to convert light into electrical energy. Also known as photodetectors, photo sensors, or light detectors, they can perform this function thanks to their specific design and structure.
One of the most fundamental and important parts of a photodiode is the p-n junction. This is a space between two layers of semiconductor material, the p-type layer has a high number of holes while the n-type layer is packed with electrons. Though there are several types of photodiodes, this is a consistent element of the devices’ design and is usually called the depletion layer.
When no light is detected by the device, there is very little current generated within the device (often it’s close to zero). This is known as the photodiode’s ‘dark current’ and will reduce as the sensitivity of the device increases.
When light energy is detected by the device (usually above a certain set level called the bandgap) this causes new holes and electrons to be created, generating an electrical current in the p-n junction.
In photodiodes that are reverse-biased, the holes move towards the anode and the electrons to the cathode, creating a current in the depletion region. As the brightness of light increases, so does the current in the device.
Depending on the specific requirements or design of the photodiode, the device will operate in a photovoltaic or photoconductive mode:
- Photovoltaic mode is where no external voltage is applied to the device meaning it's not biased. So when light falls on the device, electrons move towards the cathode holes to the anode, generating a current in the depletion zone
- Photoconductive mode is when a reverse-biased power source is applied to the device, increasing the depletion area, dark current and response speed of the photodiode while reducing its junction capacitance