What is a Photodiode? Difference Between Photodiode and Light Emitting Diode

What is a Photodiode? Difference Between Photodiode and Light Emitting Diode

    Photodiodes (Photo-Diodes), like ordinary diodes, are also semiconductor devices composed of a PN junction, and also have unidirectional conductivity. But in the circuit, it is not a rectifying element, but a photoelectric sensing device that converts optical signals into electrical signals. If you are interested in what this article is about to cover, then continue reading.

Overview of Photodiodes

    A photodiode is a semiconductor device that converts light into electrical current, and between the p (positive) and n (negative) layers, there is an intrinsic layer. Photodiodes accept light energy as input to generate electrical current. Photodiodes are also known as photodetectors, photosensors or light detectors.

The photodiode operates under reverse bias conditions, that is, the p-side of the photodiode is connected to the negative terminal of the battery (or power supply), and the n-side is connected to the positive terminal of the battery. Typical photodiode materials are silicon, germanium, indium gallium arsenide phosphide, and indium gallium arsenide.

    Internally, a photodiode has a filter, built-in lens, and surface area. When the surface area of the photodiode is increased, the response time is shortened. Few photodiodes look like light emitting diodes (LEDs). It has two terminals. The smaller terminal serves as the cathode and the longer terminal serves as the anode.

Overview of Light Emitting Diodes

Light-emitting diode is a commonly used light-emitting device, which releases energy through the recombination of electrons and acupoints, and is widely used in the field of lighting. Light-emitting diodes can effectively convert electrical energy into light energy, and are widely used in modern society such as lighting, flat panel displays, and medical equipment.

    Light-emitting diodes are also composed of ordinary diodes, which have unidirectional conductivity. When a positive voltage is added to the light-emitting diode, the holes injected from the P region into the N region and the electrons injected from the N region into the P region recombine with the electrons in the N region and the holes in the P region within a few microns near the PN junction, resulting in spontaneous emission of fluorescence. Electrons and holes are in different energy states in different semiconductor materials. When electrons and holes recombine, more energy is released and the wavelength of light is shorter. Commonly used diodes are red, green or yellow. The reverse voltage of the LED is greater than 5 volts. Its positive volt-ampere characteristic curve is very steep, and the current limiting control must pass through the diode in series.

The core part of a light-emitting diode is a wafer composed of a P-type semiconductor and an N-type semiconductor. There is a transition layer between the P-type semiconductor and the N-type semiconductor, which is called a PN junction. In the PN junction of some semiconductor materials, the injected minority carriers and majority carriers release excess energy in the form of light, thereby directly converting electrical energy into light energy. When a reverse voltage is applied to the PN junction, minority carriers are difficult to inject, so they do not emit light. When it is in a positive working state (that is, a positive voltage is applied to both ends), and the current flows from the anode to the cathode, the semiconductor emits light from ultraviolet light to other colors, and the intensity of the light is related to the current.

    Difference Between Light Emitting Diode and Photodiode

Ordinary diodes are in a cut-off state under the action of reverse voltage, and can only flow a weak reverse current. In the design and production process of the photodiode, the PN junction area should be relatively large so as to receive the incident light. Photodiodes work with reverse voltage. When there is no light, the reverse current is very weak, which is called dark current; when there is light, the reverse current rapidly increases to tens of microamps, which is called photocurrent. The greater the intensity of the light, the greater the reverse current. A change in light causes a change in the current of the photodiode, which converts the light signal into an electrical signal and becomes a photosensor.