LED Working Principle

Light Emitting Diode

The light-emitting diode, which is commonly known as LED is a basic P-N junction diode that emits light when the current flows through it in the forward biasing condition. LEDs can be seen in almost every electric component around us, whether it’s mobile displays, remotes, digital clocks, traffic lights or large advertising screens. They are used for various purposes and are available in various colours, shapes, and sizes. Although different types of LEDs are available nowadays, the working principle of each type of LED is the same. In this article, we’ll learn about the working, types, advantages, disadvantages and applications of LED.

Construction of LED

The LED structure consists of three semiconductor layers deposited over the substrate. The top layer is a P-type region, the bottom layer is the n-type region, and the central layer is called the active region of the LED. As the name suggests, the P-region consists of a large number of holes, and the N-region consists of a large number of electrons. The middle layer, i.e., the active region, consists of both the holes and electrons. The N-type region is heavily doped as compared to the P-type region in the LED structure. The P-layer is deposited above the N layer because the electron-hole pair recombination takes place towards the p region, hence if the p region is the upward surface of the LED, the light (photons) emitted due to the electron-hole pair recombination can be clearly observed from the surface. If the P region is placed below the n region, then the light emitted from the LED may not be clearly observable, which is why the p-type layer is always placed above the n-type region. The p-type layer is covered with the thin metallic layer which provides the anode connections, and the N-type region is coated with the thin gold layer, which provides the cathode connection. The gold film also acts as a reflective layer in the LED structure. If some part of the light emitted at the upper surface of the led enters the bottom surface, then the gold layer reflects the light back to the upward direction, hence the gold film increases the efficiency of the LED. A transistor is used in the circuit to switch on or switch off the LED. The LED structure is encapsulated in a transparent dome-shaped case, which ensures that the minimum internal reflections and uniform light distribution of LED. The semiconductor materials that are commonly used for the construction of LEDs are Gallium Phosphide, Gallium Arsenide, or Gallium Arsenide Phosphide.

Construction of LED

Construction of LED

Electric Circuit Symbol of LED

The circuit symbol of LED is similar to that of the normal diode, with the only difference being that the LED symbol consists of two small arrows above the small triangular part of the symbol (as shown in the figure below). The arrows in the symbol represent the emission of light. The Anode (positive) terminal is towards the small triangular part, and the cathode (negative) terminal is towards the straight-line part of the LED’s symbol.

Symbol of LED

Symbol of LED

LED Circuit Diagram

As seen in the figure below, In the electric circuit of the LED, the positive terminal of the LED is connected to the one terminal of the resistor and another terminal of the resistor is connected to the positive terminal of the battery.  The negative terminal of the LED is connected with the negative terminal of the battery. The negative terminal of the battery is also known as the ‘Ground.’

Angle of View of LED

The angle of view of LED

Working Principle of LED

For the operation of the LED, it must be connected in the forward-biasing. Due to the forward biasing the potential barrier between the p and the n region decreases because of the electron-hole pair recombinations in the active layer (depletion region). The working principle of the LED can be understood from the energy bandgap theory. This theory states that the ability to release photons upon the electron-hole pair recombination depends upon the bandgap of the semiconductors, i.e., whether the semiconductor has a direct bandgap or indirect bandgap. If the semiconductor materials used in LEDs have a direct bandgap then they will emit the photons. The semiconductor materials that have the direct bandgap means that the highest energy level of the valence band lies exactly above the topmost energy level of the valance band lies exactly below the lowest energy level of the conduction band on the E-M (energy vs momentum) diagram of the energy levels. According to the energy band gap theory, when the electron energy E equals the energy bandgap difference ∆ (EV) is released in the form of light photons.

The energy released is given by,

E= hν

Where ‘h’ is the Plancks constant, which has the value 6.626×{10}^{-34} J{Hz}^{-1}, and ν is the frequency of the light.

Whereas in the case of the indirect bandgap no photons get emitted, i.e., it’s non-radiative because the upmost level of the valence band does not coincide with the lowest level of the conduction band.

Direct Bandgap and Indirect Bandgap

Light is a type of energy that consists of a large number of small packets of zero mass and only possess momentum and energy, these are called photons. The photons can be released by the atoms due to the fast-moving electrons. As electrons rotate in different orbitals around the nucleus, every electron in different orbitals possesses different energies. Those electrons that are farther away from the nucleus have more energies than those that are nearer to the nucleus. To excite an electron from the lower energy orbital to the higher energy orbital some amount of energy is required, and similarly, when the electrons jump from the higher energy orbital to the lower energy orbital it releases some amount of the energy in the form of photons (E=hν). More the energy difference between the higher and the lower orbitals more will be the energy of the photon. This phenomenon happens in the LED too.

Working Principle of LED

Working Principle of LED

When no voltage is applied across the LED circuit, the electrons and holes in the p-region and the n-region respectively remains stable, but when the voltage is applied to the LED the circuit will be forward biased, hence the electrons present in the N region starts moving towards the holes present in the P region and the electron-hole pair recombination takes place in the active region of the circuit, which is why the active region is also known as the depletion region. As holes are positively charged and electrons are negatively charged the light is emitted by the recombination of the holes and the electrons. The release of the photon is due to the energy differences of the holes and the electron. According to the energy band gap theory, the electrons present in the conduction band have higher energy than the holes in the valence band. The semiconductors materials that are used for the construction of the LEDs have an energy difference between the holes and the electrons equal to the visible light energy range, and when electrons and the holes recombine in the active region, the energy of the electron releases in the form of a photon of visible range. This phenomenon is known as electroluminescence.

Electroluminescence

Electroluminescence

V-I Characteristics of LED

The voltage-current characteristics or simply the V-I characteristics provides important information about the resistance and the breakdown of the electric devices. By understanding the V-I characteristics of a device one can analyze the operating conditions of the device. In other words, the V-I characteristic is the graph between the current and the voltage, in which the electric current is measured with respect to the voltage applied across the device. As it is much easier to control the voltage in the circuit in comparison to the current, hence while plotting the V-I characteristics voltage act as the independent variable, while the current act as the dependent variable, i.e., the value of voltages is plotted on the x-axis and the electric current values are plotted over the y-axis. The following figure represents the V-I characteristics of the LED. As seen in the figure below, the V-I characteristic of the LED is similar to the forward-biased diode.

V-I Characteristics of LED

LED Specifications

Various types of LEDs are available in the market, each type has its own specifications and datasheet. One should understand the specification of the particular LED before choosing it so that an optimum LED can be selected for the specific purpose.

1. Electric Current or Voltage Specifications

LED requires some amount of current to pass through it before emitting the light, the output light intensity of the LED is directly proportional to the forward electric current flowing through it; however, a series resistor is used in the circuit to protect the LED from the passing the excessive electric current through it. One should not connect the LED directly to the power supply because the sudden increase in the electric current can immediately burn out the LED. Each LED has a particular forward voltage drop, which mainly depends upon the semiconductor material used for its construction. Usually, it occurs at the 20mA forward electric current.

Use of Resistor in LED Circuit

2. Reverse Voltage

LEDs can not tolerate the large reverse voltage. They should always be run below the specified maximum reverse voltage. Running the led’s above this value may lead to its permanent destruction. To protect the led from this situation, a normal diode can be fitted in the circuits.

Reverse Breakdown Voltage of LED

3. Angle of View

The angle of view is defined as the angle through which the led spreads its light and it is measured in terms of degrees. Typically, the angle of view of LED is nearly 30 degrees, but some specially designed LED’s have the viewing angle up to 140 degrees. The larger will be the angle of view, the more will be the illumination by the LED.

Angle of View of LED

The angle of view of the LED

4. Brightness/Light Intensity

The intensity of the light emitted by the LED depends upon various factors such as construction material, design of the LED, electric current, and encapsulation of the LED. The brightness of the LED is not an issue if it is being used for indicator purposes; however, brightness is a crucial factor if LED needs to be used for the lighting purpose. The brightness of the LED is measured in terms of lumens(lm) or millicandella (mcd).

5. LED Colour

The colour of the LED is also an important parameter for choosing the LED. The colour radiated by the LED depends upon the type of material used for its construction and the forward voltage of the circuit. For example, Gallium phosphide emits green or red colour at 2.2V, Gallium Arsenide phosphide emits red colour at 2.1 V while yellow colour at 2.2 V, Aluminium Gallium Indium Phosphide (AlInGap) emits amber colour at 2.1v. Visible LED’s that are widely used consist of the larger gap between the valence and the conduction band. The frequency of the photon depends upon the size of the bandgap, i.e., the bandgap determines the colour of the LED. It is to be noted that we can only observe those photons that lie in the range of visible wavelength. For example, in a Silicon diode, the bandgap between the conduction and the valence band is very low due to which the energy emitted by the electrons is very low, hence the frequency of the photons is very low; it lies in the infrared region, which is not visible to human eyes. The infrared LEDs may not be employed for lighting purposes, but they are widely used for remote controls and other security systems.

6. Efficiency of LED

The external efficiency of the LED refers to its ability to convert electrical energy into light. It is the ratio of the output light to the input electrical power.

ηext=Pout / Iv

In the case of a direct bandgap, the external efficiency is even less than 1%, while it is quite significant in the case of indirect band gaps.

The internal efficiency of the LED depends upon the types of layers, structure and quality of the materials used to construct the LED. It is given by,

ηint= rate of electron-hole pair recombination / Total recombinations

7. Operational-Life

The intensity of the light does not remain the same all the time, it gradually starts diminishing. The operational life specifications of the LED are usually defined as L70% (time to 70% of the illumination) and L50% (time to 50% of the illumination). Basically, L70 represents the point of time when the illumination of the led reaches 70% of its original output light. For example, An LED that was producing 10,000 lumens originally, will reduce to 7,000 lumens at some point, and this point in time is called L70. No matter what’s the type of LED, the illumination of every LED fades after sometimes, hence the time difference that an LED takes to reach L70 or L50 is an important specification for buyers to choose the best LED.

Types of LED

LED’s can be manufactured in different shapes, sizes, and designs according to the requirements. Let us discuss some widely used types of LEDs.

1. Surface Mounted Diode LEDs (SMD)

These LED’s are very popular for lighting purposes in households and offices. They are smaller in size, which makes them an ideal choice to use in space-limited circuit boards. They are available in different colours, and packaging types. They consist of more numbers of pins than the through-hole LEDs. Surface Mount LEDs are very lightweight and consists of less parasitic capacitance and inductance, hence it is ideal for RF applications.

2. Through-hole LEDs

Through-hole LEDs, which are also known as leaded LEDs are attached to the circuit board with the two wires. Through-hole LEDs are preferred over the surface mount LEDs because of the much stronger bond between the board and the through-hole LED. They are used in the components that undergo extreme temperature variations and mechanical stress. Their important feature is that they do not give much backward illumination. They are also very budget-friendly.

3. High-Brightness LEDs

High-brightness or high-light intensity LEDs are the latest types of inorganic LEDs. The LEDs demands accurate power dissipation and controlled electric current for the maximum light output. They are widely used in architectural illumination, project displays and signage.

High-Brightness LED

4. Chip on Board LEDs (COB)

Chip on board LEDs is the latest designed LEDs. They provide many benefits over standard LEDs. Basically, these are the multiple LED chips that are directly attached to the substrate to make a single module. When the chip onboard LEDs are packed together they appear like a lightning panel rather than the multiple isolated lights as it appears in the case of closely mounted Surface-mounted LEDs. Chip on board LEDs are more efficient than the SMD and they also consume less power than the SMD.

Chip on Board LED

Chip on Board LED

5. RGB LED

The RGB LED consists of three different colours in one package. These three different colours are Red, Green, and Blue. By combining these colours, one can generate almost every required colour. To generate different colours the light intensity of closely packed Red, blue, and green LEDs are varied. The light intensity of the LEDs is controlled by using a pulse width modulation (PWM) signal. Due to the close packaging of the RGB LEDs, the human eye sees only the final colour generates after the combination of three different colours instead of the three colours separately. For example, to get the blue coloured in LED, the blue LED is set at the high intensity and the other two, i.e., the red and the blue LEDs are kept at the low intensity, and to produce the white coloured LED, all the three LEDs are kept at the highest intensity.

6. Graphene LEDs

In graphene LED’s, the filament of the LED light bulb is coated with graphene. They are considered around 10% more efficient than the standard LEDs. Moreover, they are cheap, easy to manufacture, and have more life span than regular LEDs. Hence, replacing the regular LEDs with graphene LEDs on the commercial and the industrial-scale can be a great advantage.

Graphene LED

Graphene LED

7. Seven Segment LEDs and Alphanumeric LEDs

The seven-segment LEDs are the digital displays module, which displays the numerical data. These LEDs are arranged in numerical shapes. They are often known as the seven-segment indicators or seven-segment displays. Alphanumeric LEDs are the fourteen-segment advanced version of the seven-segment LEDs. The alphanumeric LEDs can display both the numeric characters and the alphabets.

8. Organic Light Emitting Diode (O-LEDs)

As the name suggests, the OLEDs are made of organic components. The regular light-emitting diodes are made of inorganic semiconductors having different levels of dopings. The Organic LED displays are constructed in narrow sheets, which creates a diffused light area. Basically, it is narrow films material that is printed over the glass substrate. The organic light LEDs technology is increasing nowadays.

9. Dimmer Switches LEDs

These types of LEDs come with dimmer switches like the other conventional bulbs, but the dimmer switches LEDs works better than the dimmer switches bulbs. This is the reason that standard bulbs are replaced with dimmer switch LEDs.

Advantages of LED

Advantages of LEDs

LEDs are preferred over other light sources due to their various advantages. Let us discuss some major points.

  • One of the prime reasons for using LEDs over other light sources is the efficiency of the LEDs. In the case of incandescent bulbs, to illuminate the light, the filament of the bulb needs to be warmed first, and due to which a large amount of electricity is getting wasted to heat the light bulb. However, in the case of LEDs, maximum electricity is getting used for generating the light, and a minimal amount of heat is produced during the process. This makes the LEDs more cost-efficient as they help to cut down the electricity bill.
  • LEDs don’t have filaments like other incandescent bulbs that get burned out easily, hence their shelf life is much longer than the conventional bulbs.
  • LED’s have a longer lifetime as compared to the other light sources due to their thermal management properties. LEDs are provided with heat sinks, which absorbs the heat produced by the LEDs and dissipate it outside.
  • LED’s have more efficiency in converting electricity into visible light, hence they give more output than regular light sources. For example, a 60 watt of regular light bulb can produce around 750-900 lumens, while the same light output can be obtained by using only the 6-8 watts of LED; lumens is the measure of the amount of light obtained from the bulb.
  • With the rising concern of climatic change, environment conscious people are shifting to the eco-friendly options of light sources. The fluorescent lights and incandescent bulbs use mercury as a crucial part of manufacturing. When these conventional lights source reaches the end of their shelf life, special handling is required to dispose of them. On the other hand, one need not worry about the LED’s disposal as they do not cause any harm to the environment.
  • In cold weather conditions, the light intensity of conventional light sources such as fluorescent lamps diminishes due to the temperature drop. They require a significant amount of voltage to operate. However, LED’s have the ability to perform better than conventional sources. This is the reason they are used in the lightning required in cold storage spaces, refrigerator displays cases, and freezers. Their remarkable efficiency of working in any temperature conditions makes them an ideal choice to be used in the outdoor premises and parking areas.
  • The small size of the LED’s is a great advantage of LEDs, which makes them easily fit in almost every electric circuit. In fact, LEDs were originally used as indicators in electric circuits. They can be also be combined together to form a long series of light due to which they are widely used for decorative purposes.
  • Incandescent bulbs emit some amount of ultraviolet radiation, the exposure to the uv rays can be cut off by using LEDs instead of incandescent bulbs.
  • Fluorescent bulbs flicker before they are turned on, on the other hand, LEDs can light up instantly; one need not wait for the warm-up period that is required in the conventional metal halide lamps.
  • LED’s release very little amount of heat and most of the light emitted by the led lies in the visible region. Due to this feature of LEDs, many medical experts have preferred the application of LED to deal with Seasonal Affective Disorder (SAD); SAD is the type of depression, which affects people due to seasonal changes, especially during the winters.

Disadvantages of LED

Disadvantages of LED

LED’s have many advantages over conventional light sources, but there exist certain disadvantages too. These are given below.

  • LEDs do not give the white light directly. The commonly available colours of LEDs are blue, green, and red. To obtain the white light, various LEDs of primary colours (Red, blue, and green) are merged together to give the white light, and the other method is coating the LED with the phosphorus layers, which turns the original light colour of LED to white light.
  • The light emitted by LEDs is directional, i.e., LEDs emit light in a particular direction, whereas other light sources such as incandescent bulbs or fluorescent lamps emit light in every direction rather than in a particular direction. Hence, specially designed LED bulbs are needed to spread the light in all directions.
  • LED’s don’t have the filament that burns out; however, LEDs face the lumen depreciation, in which the original power output of the LEDs gets lowers over time.
  • The quality of light emitted by the LED is highly dependent upon the operating temperature. High temperature may result in changes in the various parameters of the LED.

Applications of LED

  • LED indicators can be commonly seen in various industries, these indicators show the operating status of electronic devices. They are also used in digital watches, calculators, and multimeters.
  • LEDs are used as display panels. It is commonly observed in the stadium displays, dynamic messages signs, and dynamic decorative displays. The lightweight and thin displays are used to displays the schedules charts at the railways and the airports.
  • They are used in traffic lights and automotive brake lights due to their long shelf life, clear visibility even in the bright daylight, and fast switching time. The use of LEDs in brake lights is a great advantage as they have a fast rise time (nearly o.1 seconds), hence improves safety.
  • LEDs also find their applications in temporary uses, for example, glowsticks, led art or throwies. Throwies are small LED devices that are used in various types of street art.
  • LEDs are used in the DLP projectors as a light source, and also in the LED television to backlight LCD and in the mobile or laptop displays. The RGB LEDs are used to increase the colour gamut by nearly 45 %. The television screens can be made thinner if LED is being used for the backlighting.
  • LEDs are used in flashlights due to their small size and durability. It is also used in the camera flashes and cameras used in mobile phones as LEDs operates at a low voltage, which is safer than the lightning using the xenon flashlamps, which operates at the voltage of 250 volts or more.
  • LEDs are also used as cap lamps for mining operations. Specially designed LEDs that offer increased illumination and less glare are used for this purpose; this reduces the chances of injuries to the miners.
  • The use of LEDs is also seen in the health departments as the LED light helps to enhance the mood. One of the Researches based on the application of LEDs to promote the astronaut’s health is also sponsored by NASA.
  • Many of the machine vision applications, such as barcode scanners, make use of the red LEDs in place of lasers. LEDs are also used as the light sources in the optical computer mouse for the miniature cameras present in the mouse.
  • LEDs can also be used as photodiodes; hence it is used for the detection and photoemission processes. For example, LED is used in touchscreens, which detects the reflected light from the stylus pen or the finger.
  • The light emitted by the LED can be easily modulated, due to which they are used in the free space optics communications and optical fibre. Infra-red LEDs are used in the remote controls of the television sets.
  • LEDs find their applications in biological systems too. LEDs are used as the grow light to enhance the photosynthesis process in plants. UV LEDs are used for the sterilization process to kill the viruses and bacterias present in the water. Deep ultraviolet LEDs having a spectra range of 247nm to 386 nm are used for various purposes such as epoxy curing, high-performance liquid chromatography, air/water purification, and phototherapy.

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