Light Emitting Diode (LED)
Table of Contents
Light Emitting Diode (LED)
Light Emitting Diode
One of the best optoelectronic devices available is the light emitting diode, or LED. When the internal diode junction of the device reaches a forward electric current or voltage, it can emit light with a relatively narrow bandwidth, either visible or invisible.
An LED typically emits orange, red, yellow, or green visible light. Infrared light is a part of the invisible light. This device’s high power to light conversion efficiency is by far its greatest benefit. That is, compared to a standard tungsten lamp, the efficiency is nearly 50 times higher.
A tungsten lamp’s response time is 100 milliseconds, whereas the LED’s response time is in the range of 0.1 microseconds. These benefits have led to widespread use of the device as dancing light displays and visual indicators.
We are aware that a P-N junction can convert the absorbed light energy into an electric current that is proportional to it. The procedure is reversed in this case. That is, when energy is applied to the P-N junction, light is produced. The emission of light from a semiconductor under the influence of an electric field is what is known as electro luminance, and it is the phenomenon that is generally being discussed.
As the electrons cross from the N-region and recombine with the holes already present in the P-region, the charge carriers recombine in a forward P-N junction. Hole energy levels are in the valence band, while the conduction band contains free electrons. Due to this, the energy levels of the electrons and holes will be different. In order to recombine the electrons and holes, some of the energy must be lost. Heat and light are two of the emitted forms of this energy.
For silicon and germanium diodes, the electrons release heat as a form of energy dissipation. However, in semiconductors made of galium-arsenide-phosphorous (GaAsP) and galium-phosphorous (GaP), the electrons release photons to release their energy. A light emitting diode is created if the semiconductor is translucent because the junction serves as the source of light as it is released (LED). But when the junction is reverse biassed, the LED won’t produce any light, and it could even sustain damage.
You can use any of the semiconductors on the previous list. The N-type epitaxial layer is grown over a substrate, and diffusion creates the P-region. The top of the image displays the P-region, which contains charge carrier recombination. Thus, the device surface is the P-region. Metal anode connections are made at the P-outer layer’s edges to provide more surface area for light emission.
A gold film is applied to the device’s surface bottom to ensure that light is reflected as much as possible towards the surface. This setting makes it possible to offer a cathode connection as well. Domed lenses for the device are added to address the re-absorption issue. The device’s casing protects all of the wires in its electronic circuits.
The type of semiconductor material used affects the type of light that the device emits. The semiconductor gallium arsenide (GaAs) is used to create infrared light. Using gallium arsenide phosphorus (GaAsP) as a semiconductor results in red or yellow light.
LED Circuit Symbol
The circuit symbol of LED consists of two arrow marks which indicate the radiation emitted by the diode.
1. LED Characteristics
The forward bias Voltage-Current (V-I) curve and the output characteristics curve is shown in the figure above. The V-I curve is practically applicable in burglar alarms. Forward bias of approximately 1 volt is needed to give significant forward current. The second figure is used to represent a radiant power-forward current curve. The output power produced is very small and thus the efficiency in electrical-to-radiant energy conversion is very less.
The commercially used LED‘s have a typical voltage drop between 1.5 Volt to 2.5 Volt or current between 10 to 50 milliamperes. The exact voltage drop depends on the LED current, colour, tolerance, and so on.
2. LED as an Indicator
One of the main uses for LED is shown in the circuit below. To avoid the device being reverse biassed, the circuit is wired in inverse parallel with a standard diode. In comparison to a DC circuit, the value of the series resistance should be cut in half.
LEDS displays are made to display numbers from segments. One such design is the seven-segment display as shown below. Any desired numerals from 0-9 can be displayed by passing current through the correct segments. To connect such segment a common anode or common cathode cathode configuration can be used. Both the connections are shown below. The LED‘s are switched ON and OFF by using transistors.
Advantages of LED
- Very low voltage and current are enough to drive the LED.
- Voltage range – 1 to 2 volts.
- Current – 5 to 20 mill amperes.
- Total power output will be less than 150 mill watts.
- The response time is very less – only about 10 nanoseconds.
- The device does not need any heating and warm up time.
- Miniature in size and hence light weight.
- Have a rugged construction and hence can withstand shock and vibrations.
- An LED has a life span of more than 20 years.
Disadvantages of LED
- A slight excess in voltage or current can damage the device.
- The device is known to have a much wider bandwidth compared to the laser.
- The temperature depends on the radiant output power and wavelength.
A laser diode, also known as an LD, is a semiconductor laser that is electrically pumped and whose active laser medium is a p-n junction formed by a semiconductor diode that is similar to a light-emitting diode.
The most prevalent type of laser produced is the laser diode, which has a wide range of applications including, but not limited to, fibre optic communications, barcode readers, laser pointers, reading and recording CD/DVD/Blu-ray Discs, laser printing, laser scanning, and increasingly directional lighting sources.
A laser diode is electrically a P-i-n diode. The active region of the laser diode is in the intrinsic (I) region, and the carriers, electrons and holes, are pumped into it from the N and P regions respectively.
All contemporary lasers use the double-hetero structure implementation, where the carriers and the photons are confined to maximise their chances for recombination and light generation. The initial diode laser research was carried out on straightforward P-N diodes.
A laser diode’s objective, in contrast to a regular diode used in electronics, is for all carriers to recombine in the I region and produce light. Thus, direct bandgap semiconductors are used to create laser diodes. One of the crystal growth techniques is used to grow the laser diode epitaxial structure, typically beginning with an N-doped substrate and progressing through the I-doped active layer, P-doped cladding, and contact layer. Quantum wells are the most common type of active layer,
Laser diode L-I characteristic
One of the most commonly used and important laser diode specifications or characteristics is the L/I curve. It plots the drive current supplied against the light output.
This laser diode specification is used to determine the current required to obtain a particular level of light output at a given current. It can also be seen that the light output is also very dependent upon the temperature.
From this characteristic, it can be seen that there is a threshold current below which the laser action does not take place. The laser diode should be operated clear of this point to ensure reliable operation over the full operating temperature range as the threshold current rises with increasing temperature. It is typically found that the laser threshold current rises exponentially with temperature.
Laser Diode Specifications & Characteristics
a summary or overview of laser diode specifications, parameters and characteristics used in defining laser diode performance for datasheets.
In this section
- • Laser diode technology
- • Laser diode types
- • Structure & materials
- • Theory & operation
- • Specs & characteristics
- • Lifetime, failure & reliability
- • Other diodes
When using a laser diode it is essential to know its performance characteristics. Accordingly laser diode specifications are required when designing equipment using laser diodes or for maintenance using near equivalents.
Like any electronics components, many of the specifications are relatively generic, but other parameters will tend to be more focussed on the particular component. This is true for laser diode specifications and characteristics.
There are a number of laser diode specifications, or laser diode characteristics that are key to the overall performance and these are outlined.
When the voltage exceeds the breakdown voltage, also referred to as the “Zener knee voltage” or the “Zener voltage,” a Zener diode, a type of diode, will allow current to flow both forward and backward. Clarence Zener, who discovered this electrical characteristic, was honoured with the device’s name.
However, the Zener Diode or “Breakdown Diode” as they are sometimes called, are basically the same as the standard PN junction diode but are specially designed to have a lowpre-determined Reverse Breakdown Voltage that takes advantage of this high reverse voltage.
The point at which a zener diode breaks down or conducts is called the “Zener Voltage” (Vz). The Zener diode is like a general-purpose signal diode consisting of a silicon PN junction.
When biased in the forward direction it behaves just like a normal signal diode passing the rated current, but when a reverse voltage is applied to it the reverse saturation current remains fairly constant over a wide range of voltages.
The reverse voltage increases until the diodes breakdown voltage VB is reached at which point a process called Avalanche Breakdown occurs in the depletion layer and the current flowing through the zener diode increases dramatically to the maximum circuit value (which is usually limited by a series resistor).
This breakdown voltage point is called the “zener voltage” for zener diodes.
Avalanche Breakdown: There is a limit for the reverse voltage. Reverse voltage can increase until the diode breakdown voltage reaches. This point is called Avalanche Break down region. At this stage maximum current will flow through the zener diode. This breakdown point is referred as ―Zener voltage‖.
The point at which current flows can be very accurately controlled (to less than 1%tolerance) in the doping stage of the diodes construction giving the diode a specific zener breakdown voltage, (Vz) ranging from a few volts up to a few hundred volts. This zener breakdown voltage on the I-V curve is almost a vertical straight line.
1. Zener diode characteristics
When used in “reverse bias” or “reverse breakdown” mode, the zener diode’s anode is connected to the negative supply. From the I-V characteristics curve shown above, we can see that the zener diode has a region in its reverse bias characteristics where the negative voltage is almost constant, regardless of the amount of current flowing through the diode, and it stays almost constant even with significant changes in current as long as the current stays within the range of the breakdown current IZ(min) and the maximum current rating IZ (max).
2. Zener Regulator
When zener diode is forward biased it works as a diode and drop across it is 0.7 V. When it works in breakdown region the voltage across it is constant (VZ) and the current through diode is decided by the external resistance. Thus, zener diode can be used as a voltage regulator in the configuration shown in figure 2 for regulating the dc voltage. It maintains the output voltage constant even through the current through it changes.
The load line of the circuit is given by Vs= Is Rs + Vz. The load line is plotted along with zener characteristic in figure The intersection point of the load line and the zener characteristic gives the output voltage and zener current.
Vs must always be greater than Vz in order for the zener to operate in the breakdown region. The current is limited by Rs. Although the voltage across the zener is nearly constant, the operating point changes simultaneously with changes in the Vs voltage. A voltage source of magnitude Vz serves as the zener diode’s first approximation, and the resistance is included in the second approximation. The two roughly equivalent circuits are depicted in the figure below.
If second approximation of zener diode is considered, the output voltage varies slightly as shown in figure The zener ON state resistance produces more I * R drop as the current increases. As the voltage varies form V1 to V2 the operating point shifts from Q1 to Q2.
The voltage at Q1 is V1 = I1 RZ +VZ and at Q2
V2 = I2 RZ +VZ
Thus, change in voltage is V2 – V1 = ( I2 – I1 ) RZ