A photodiode is a semiconductor device that converts light into an electric current. In the presence of light, the diode generates charge carriers that flow into a current from one terminal to the other. It can also be referred to as a photodetector or photosensor.
The special diode operates by the principle of the photoelectric effect. Light is a form of energy and travels from the sun to the earth in the form of packets of energy called photons. The principle explains the relationship between light and electric current. We know that a diode is a semiconductor with two oppositely doped parts i.e. positive (p) and negative (n). The positive doped has holes as majority charge carriers while the negative type has free electrons as majority charge carriers. It’s worth remembering there are oppositely charged minority carries on each side. Between the p-type and n-type is a junction or a depletion layer that is free of charge carriers.
There are three main modes of operation as will be explained below.
Photoconductive mode- the diode is reverse biased, that is the p-type is connected to the cathode and n-type to anode terminals of the battery or dc power. On reverse bias, the depletion layer is increased and inhibits the flow of majority charge carriers. However, minority charge carriers are made to flow through the diode generating leakage saturation current. Temperature also energizes electrons to become free from parent atoms generating more charge carriers, which is referred to as dark current. The light energy falling on the depletion region will energize valence electrons to leave the parent atom and become free charge carriers. When an electron is dislodged, an empty space is left behind and is referred to as a hole. The light, therefore, generates charge carriers in the depletion layer. The inbuilt electric field in the depletion layer and the applied electric field of the batteries drift the charge carriers to flow through the junction hence current flows.
Photovoltaic mode- the diode is not biased (no batteries connected). The diode generates current when connected to a closed-circuit or a voltage build-up in an open circuit. The light photons generate charge carriers in the depletion layer that are drifted by inbuilt electric field thus current flows. When covalent electrons are energized and become free, they leave behind a hole and the atom becomes a positive ion. When a free electrons join the covalent band of an atom, it becomes a negative ion. The immobile positive and negative ions are responsible for the inbuilt electric field that causes current to flow in a closed circuit of voltage build-up in an open circuit.
Avalanche mode- the diode is heavenly reverse biased. There exists leakage saturation current that is caused by minority charge carriers. It’s important to remember, minority carriers are forward biased and are drifted by the electric field. As they move they knock off covalent electrons, energize them and they become free charge carriers. The breakdown of covalent electrons to free electrons increases exponentially causing an avalanche growth of charge carriers and the diode becomes conductive.
There are four main types depending on construction. These are;
- PN photodiode- two doped regions, positive and negative
- PIN photodiode- has an additional intrinsic layer increasing its sensitivity.
- Avalanche diode- heavily reverse-biased operation
- Scotty photodiode
- Optocoupler- offers electrical circuit isolation for the safety of sensitive equipment.
- Photodetectors- detects lights and triggers an actuator accordingly
- Compact disc player
- Infrared remote control receiver – used in television and air conditioners
- Measurement of light for example light levels in-camera.
- Street light switch sensor
- Mechanical obstruction detector-used in counters and burglar alarm system
- The medical field in CT scan
- Optical communication fiber
As we have seen, the photodiode is a very key electronic component in electrical world. The advantages and applications are a big score. With quick response time, linear response, small sized, durable and its low cost it can be integrated in electronic circuitry and do it’s magic. Some of its limitations are the undesirable dark current and need for amplification of low levelled signal. As the world goes green so are the energy sources, harnessing of solar energy has been possible thanks to photodiode. Did you know solar panel building blocks are the photodiodes? The lights on the streets are automated, talk about light sensing courtesy of a photodiode. The future will utilize this diode all the more. Be updated and move with technology.