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Difference between LED and Photodiode
LED and Photodiode are both semiconductor-based electronic devices that function by the interaction of light energy and electricity. LED converts the electricity into light, whereas a photodiode converts the light energy into electricity. Therefore, LEDs and photodiodes are quite different in the way they operate.
In this article, we will highlight all the major differences between LEDs and photodiodes. Let's start with some basics first so that it becomes easy to understand the differences between them.
What is an LED?
LED stands for Light Emitting Diode. An LED is basically a PN junction semiconductor diode which emits light when electrical energy is supplied to it. The operation of the LED is based on the principle of electroluminescence. The principle of electroluminescence states that when an electric field interacts with the interatomic particles of the substance, then it emits light. This emitted light may be visible, ultraviolet or infrared.
An LED consists of a PN junction just like an ordinary semiconductor diode, the only difference is that the PN junction of an LED is covered with a transparent material to direct the emitted light from the junction when it is forward biased. In practice, the most common semiconductor materials used for manufacturing the LEDs are gallium arsenide, indium phosphide, gallium phosphide, etc. The selection of material used for LED manufacture is based on the color of light emitted.
A typical LED consists of two electrodes namely anode and cathode. The anode is the metallic terminal attached to the P-region, while the cathode is the metallic contact attached to N-region. When the anode is connected to positive terminal of battery and cathode to the negative terminal of battery, then the LED is said to be forward biased and hence emits the light. It should always be kept in mind that the reverse biasing of LED can permanently damage it.
The emission of light from the LED is mainly due to recombination of charge carriers, i.e. holes and electrons, in the device. That is, when the LED is forward biased, the externally applied electric field accelerates the electrons and holes towards the PN junction, where these recombine. Due to the recombination, energy is released in the form of heat (very small in case of LED) and light.
LEDs are widely used in several applications such as general lighting, vehicle headlamps, torches, display screens, etc.
What is a Photodiode?
A type of PN junction semiconductor diode which converts light energy into electrical energy is called a photodiode. The operation of a photodiode is based on the principle of photo-conductance. According to photo-conductance, when a beam of light having high energy exposes the PN junction of the photodiode, it produces charge carries (electrons and holes) at the junction. These charges carriers receives energy from the light and move. Thus, the movement of charge carriers in the photodiode results in the flow of current through the diode.
Photodiodes are designed to operate in reverse biased mode. Just like an LED, the photodiode also has two terminals, namely, anode and cathode. Where, in case of the photodiode, the anode is connected to negative terminal and the cathode is connected to the positive terminal of the battery to be charged.
Once the PN junction of the photodiode is supplied with the light of high energy, it rises the temperature of the junction and causes production of electrons and holes in the depilation region. These electrons and holes receive energy from the light and move in the opposite direction. Hence, the movement of electrons and holes generates electric current. The electric current produced by the photodiode is directly proportional to intensity of light, i.e. the light of high intensity produces large number electron-hole pairs and hence the large current.
The photodiode are used in several practical applications such as solar panels, safety equipment, automatic shutter control, fiber optic links, X-ray detection, etc.
Difference between LED and Photodiode
Both LEDs and photodiodes are PN junction semiconductor diodes, however there are several differences between them that are listed in the following table −
| Basis of Difference | LED | Photodiode |
|---|---|---|
| Definition | A PN junction semiconductor diode which converts electrical energy into light energy is called an LED (Light Emitting Diode). | A PN junction semiconductor diode which converts light energy into electrical energy is known as photodiode. |
| Principle of operation | LED operates on the principle of electroluminescence. | Photodiode operates on the principle of photoconductance. |
| Circuit symbol |
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| Mode of biasing | LED works in forward bias only. | Photodiodes work in reverse bias only. Although, they may also be forward biased. |
| Main function | The primary function of LED is to emit light. | The primary function of a photodiode is to generate electricity. |
| Semiconductor materials | LEDs are fabricated with semiconductor materials like gallium-arsenide (GaAs), galliumphosphide (GaP), indiumphosphide (InP), etc. | Most photodiodes are fabricated with the materials like silicon, germanium and indium-gallium-arsenide. |
| Leakage current | There is no leakage current in case of LEDs, as these are always forward biased. | In the absence of external light, the leakage current exists in case of photodiode. This leakage current is known as dark current. |
| Effect of reverse biasing | The reverse biasing of an LED can damage it permanently. | The reversing biasing does not damage the photodiode, because it is specially designed to operate in reverse bias. |
| Physical structure | LED is covered with a transparent material like epoxy resin in a dome-shaped structure to scatter the light in all directions. | The structure of photodiode consists of a lens to focus the light on the PN junction. |
| Output in the absence of input | In case of LED, there is no output of light in the absence of electrical input. | In the photodiode, when the input light is not available, a small electrical output present which is the dark current. |
| Applications | LEDs are used in area illumination, display screens, vehicle headlamps, indicators, torch, etc | Photodiodes are used in solar panels, optocouplers, optical communication, automatic shutter control, smoke detectors, and many other light sensing applications. |
Conclusion
The most significant difference between an LED and a Photodiode is in the way they function. An LED converts electrical energy into light, while a photodiode converts light energy into electrical energy.
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