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A photodiode is a type of light detector that converts light into voltage or current, based on the mode of operation of the device.
It consists of built-in lenses and optical filters and can have small or large surface areas. With an increase in their surface areas, photodiodes have a slower response time. Conventional solar cells, used for generating electric solar power, are a typical photodiode with a large surface area.
A photodiode is a semi-conductor device, with a p-n junction and an intrinsic layer between p (positive) and n (negative) layers. They are designed to work in reverse bais mode, meaning the p-side of the diode is linked to the negative terminal of the battery and the n-side the positive. It produces photocurrent by generating electron-hole pairs, due to the absorption of light in the intrinsic or depletion region. The photocurrent thus generated is proportional to the absorbed light intensity.
Working Principle of Photodiodes
When photons of energy greater than 1.1 eV hit the diode, electron-hole pairs are created. The intensity of photon absorption depends on the energy of photons – the lower the energy of photons, the deeper the absorption is. This process is known as the inner photoelectric effect.
If a photon of sufficient energy enters a depleted region of a diode, it could hit an atom with enough energy to release said electron from the atomic structure, thus creating a free electron and a hole ie an atom with an electron space. The electron has a negative charge, and the hole a positive charge. These electron-hole pairs drift apart and are swept from the junction - due to the built-in electric field of the depletion region. As a result, the holes move toward the anode and the electrons move toward the cathode, thereby producing photocurrent.
The sum of photocurrents and dark currents, which flow with or without light, is the total current passing through the photodiode. The sensitivity of the device can be increased by minimizing the dark current.
Modes of Operation
Photodiodes can be operated in different modes:
- Photovoltaic mode – Also known as zero bias mode, this mode exploits the photovoltaic effect. The flow of photocurrent out of the device is restricted, and a voltage is generated by the illuminated photodiode. It provides a very small dynamic range and non-linear dependence of the voltage produced
- Photoconductive mode - The diode used in this mode is more commonly reverse biased ie the cathode becomes positive with respect to the anode. The application of reverse voltage increases the width of the depletion layer, which in turn reduces the response time and capacitance of the junction. This mode is very fast, and exhibits electronic noise
- Avalanche diode mode - Avalanche photodiodes are operated in a high reverse bias condition, which allow multiplication of an avalanche breakdown to each photo-generated electron-hole pair. This results in internal gain within the photodiode, which gradually increases the responsivity of the device
Photodiodes find application in the following:
- Cameras, cd players, TVs and remote controls
- Medical devices eg instruments to analyse samples, detectors for computed tomography and blood gas monitors
- Safety equipment eg smoke detectors
- Optical communication devices
- Position sensors
- Bar code scanners
- Automotive devices
- Surveying instruments
Sources and Further Reading
This article was updated on 31st July, 2018.