Physics Point

Light Emitting Diode (LED)

2022-01-13T19:15:00.001+05:30

Light Emitting Diodes or simply LED´s, are among the most widely used of all the different types of semiconductor diodes available today and are commonly used in TV’s and colour displays.

They are the most visible type of diode, that emit a fairly narrow bandwidth of either visible light at different coloured wavelengths, invisible infra-red light for remote controls or laser type light when a forward current is passed through them.

The “Light Emitting Diode” or LED as it is more commonly called, is basically just a specialised type of diode as they have very similar electrical characteristics to a PN junction diode. This means that an LED will pass current in its forward direction but block the flow of current in the reverse direction.

Light-emitting diodes are made from a very thin layer of fairly heavily doped semiconductor material and depending on the semiconductor material used and the amount of doping, when forward biased an LED will emit a coloured light at a particular spectral wavelength.

When the diode is forward biased, electrons from the semiconductors conduction band recombine with holes from the valence band releasing sufficient energy to produce photons that emit a monochromatic (single colour) of light. Because of this thin layer, a reasonable number of these photons can leave the junction and radiate away producing a coloured light output.

LED Construction

Then we can say that when operated in a forward-biased direction Light Emitting Diodes are semiconductor devices that convert electrical energy into light energy.

The construction of a Light Emitting Diode is very different from that of a normal signal diode. The PN junction of an LED is surrounded by a transparent, hard plastic epoxy resin hemispherical shaped shell or body that protects the LED from both vibration and shock.

Surprisingly, an LED junction does not actually emit that much light so the epoxy resin body is constructed in such a way that the photons of light emitted by the junction are reflected away from the surrounding substrate base to which the diode is attached and are focused upwards through the domed top of the LED, which itself acts as a lens concentrating the amount of light. This is why the emitted light appears to be the brightest at the top of the LED.

However, not all LEDs are made with a hemispherical shaped dome for their epoxy shell. Some indication LEDs have a rectangular or cylindrical shaped construction that has a flat surface on top of their body is shaped into a bar or arrow. Generally, all LED’s are manufactured with two legs protruding from the bottom of the body.

Also, nearly all modern light-emitting diodes have their cathode, ( – ) terminal identified by either a notch or flat spot on the body or by the cathode lead being shorter than the other as the anode ( + ) lead is longer than the cathode (k).

Unlike normal incandescent lamps and bulbs which generate large amounts of heat when illuminated, the light-emitting diode produces a “cold” generation of light which leads to high efficiencies than the normal “light bulb” because most of the generated energy radiates away within the visible spectrum. Because LEDs are solid-state devices, they can be extremely small and durable and provide a much longer lamp life than normal light sources.

Light Emitting Diode Colours

So how does a light-emitting diode get its colour? Unlike normal signal diodes which are made for the detection or power rectification, and which are made from either Germanium or Silicon semiconductor materials, Light Emitting Diodes are made from exotic semiconductor compounds such as Gallium Arsenide (GaAs), Gallium Phosphide (GaP), Gallium Arsenide Phosphide (GaAsP), Silicon Carbide (SiC) or Gallium Indium Nitride (GaInN) all mixed together at different ratios to produce a distinct wavelength of colour.

Different LED compounds emit light in specific regions of the visible light spectrum and therefore produce different intensity levels. The exact choice of the semiconductor material used will determine the overall wavelength of the photon light emissions and therefore the resulting colour of the light emitted.

Color of an LED

The color of an LED device is expressed in terms of the dominant wavelength emitted, λd (in nm). AlInGaP LEDs produce the colors red (626 to 630 nm), red-orange (615 to 621 nm), orange (605 nm), and amber (590 to 592 nm). InGaN LEDs produce the colors green (525 nm), blue-green (498 to 505 nm), and blue (470 nm). The color and forward voltage of AlInGaP LEDs depend on the temperature of the LED p-n junction.

As the temperature of the LED p-n junction increases, the luminous intensity decreases, the dominant wavelength shifts towards longer wavelengths, and the forward voltage drops. The variation in luminous intensity of InGaN LEDs with operating ambient temperature is small (about 10%) from − 20°C to 80°C. However, the dominant wavelength of InGaN LEDs does vary with the LED drive current; as the LED drive current increases, the dominant wavelength moves toward shorter wavelengths.

Types of Light Emitting Diode (LED)

Gallium Arsenide (GaAs) – infra-red
Gallium Arsenide Phosphide (GaAsP) – red to infra-red, orange
Aluminium Gallium Arsenide Phosphide (AlGaAsP) – high-brightness red, orange-red, orange, and yellow
Gallium Phosphide (GaP) – red, yellow and green
Aluminium Gallium Phosphide (AlGaP) – green
Gallium Nitride (GaN) – green, emerald green
Gallium Indium Nitride (GaInN) – near-ultraviolet, bluish-green and blue
Silicon Carbide (SiC) – blue as a substrate
Zinc Selenide (ZnSe) – blue
Aluminium Gallium Nitride (AlGaN) – ultraviolet

What is a Photodiode? Working, V-I Characteristics, Applications

2021-10-20T14:05:00.005+05:30

What is a Photodiode?

It is a form of light sensor that converts light energy into electrical energy (voltage or current). Photodiode is a type of semi conducting device with PN junction. Between the p (positive) and n (negative) layers, an intrinsic layer is present. The photo diode accepts light energy as input to generate electric current.

It is also called as Photodetector, Photo Sensor or Light Detector. Photodiode operates in reverse bias condition i.e., the p – side of the photodiode is connected with negative terminal of battery (or the power supply) and n – side to the positive terminal of battery.

Typical photodiode materials are Silicon, Germanium, Indium Gallium Arsenide Phosphide and Indium gallium arsenide.

Internally, a photodiode has optical filters, built in lens and a surface area. When surface area of photodiode increases, it results in less response time. Few photo diodes will look like Light Emitting Diode (LED). It has two terminals as shown below. The smaller terminal acts as cathode and longer terminal acts as anode.

The symbol of the photodiode is similar to that of an LED but the arrows point inwards as opposed to outwards in the LED. The following image shows the symbol of a photodiode.

Working of a Photodiode

Generally, when a light is made to illuminate the PN junction, covalent bonds are ionized. This generates hole and electron pairs. Photocurrents are produced due to generation of electron-hole pairs. Electron hole pairs are formed when photons of energy more than 1.1eV hits the diode. When the photon enters the depletion region of diode, it hits the atom with high energy. This results in release of electron from atom structure. After the electron release, free electrons and hole are produced.

In general, an electron will have a negative charge and holes will have a positive charge. The depletion energy will have built-in electric field. Due to that electric field, electron-hole pairs move away from the junction. Hence, holes move to anode and electrons move to the cathode to produce photocurrent.

The photon absorption intensity and photon energy are directly proportional to each other. When energy of photos is less, the absorption will be more. This entire process is known as Inner Photoelectric Effect.

Intrinsic Excitations and Extrinsic Excitations are the two methods via which the photon excitation happens. The process of intrinsic excitation happens, when an electron in the valence band is excited by photon to conduction band.

Modes of Operation

The operating modes of the photodiode include three modes, namely Photovoltaic mode, Photoconductive mode, an avalanche diode mode

Photovoltaic Mode: This mode is also known as zero-bias mode, in which a voltage is produced by the lightened photodiode. It gives a very small dynamic range & non-linear necessity of the voltage formed.

Photoconductive Mode: The photodiode used in this photoconductive mode is more usually reverse biased. The reverse voltage application will increase the depletion layer’s width, which in turn decreases the response time & the junction capacitance. This mode is too fast and displays electronic noise

Avalanche Diode Mode: Avalanche diodes operate in a high reverse bias condition, which permits the multiplication of an avalanche breakdown to each photo-produced electron-hole pair. This outcome is an internal gain in the photodiode, which slowly increases the device response.

Why is Photodiode Operated in Reverse Bias?

The photodiode operates in the mode of photoconductive. When the diode is connected in reverse bias, then the depletion layer width can be increased. So this will diminish the capacitance of the junction & the response time. In fact, this biasing will cause quicker response times for the diode. So the relation between photocurrent & illuminance is linearly proportional.

V-I Characteristics of Photodiode

Photodiode operates in reverse bias condition. Reverse voltages are plotted along X axis in volts and reverse current are plotted along Y-axis in microampere. Reverse current does not depend on reverse voltage. When there is no light illumination, reverse current will be almost zero. The minimum amount of current present is called as Dark Current. Once when the light illumination increases, reverse current also increases linearly.

Advantages

The advantages of photodiode include the following.

Less resistance
Quick and high operation speed
Long life span
Fastest photodetector
Spectral response is good
Doesn’t use high voltage
Frequency response is good
Solid and low-weight
It is extremely responsive to the light
Dark current is lees
High quantum efficiency
Less noise

Disadvantages

The disadvantages of photodiode include the following.
Temperature stability is poor
Change within current is extremely little, therefore may not be enough to drive the circuit
The active area is small
Usual PN junction photodiode includes a high response time
It has less sensitivity
It mainly works by depending on the temperature
It uses offset voltage

Applications of Photodiode

The applications of photodiodes involve similar applications of photodetectors like charge-coupled devices, photoconductors, and photomultiplier tubes.
These diodes are used in consumer electronics devices like smoke detectors, compact disc players, and televisions and remote controls in VCRs.
In other consumer devices like clock radios, camera light meters, and street lights, photoconductors are more frequently used rather than photodiodes.
Photodiodes are frequently used for exact measurement of the intensity of light in science & industry. Generally, they have an enhanced, more linear response than photoconductors.
Photodiodes are also widely used in numerous medical applications like instruments to analyze samples, detectors for computed tomography, and also used in blood gas monitors.
These diodes are much faster & more complex than normal PN junction diodes and hence are frequently used for lighting regulation and in optical communications.

Zener diode | Definition, Breakdown

2021-08-25T14:58:00.005+05:30

Zener diode

A normal p-n junction diode allows electric current only in forward biased condition. When forward biased voltage is applied to the p-n junction diode, it allows large amount of electric current and blocks only a small amount of electric current. Hence, a forward biased p-n junction diode offer only a small resistance to the electric current.

When reverse biased voltage is applied to the p-n junction diode, it blocks large amount of electric current and allows only a small amount of electric current. Hence, a reverse biased p-n junction diode offer large resistance to the electric current.

If reverse biased voltage applied to the p-n junction diode is highly increased, a sudden rise in current occurs. At this point, a small increase in voltage will rapidly increases the electric current. This sudden rise in electric current causes a junction breakdown called zener or avalanche breakdown. The voltage at which zener breakdown occurs is called zener voltage and the sudden increase in current is called zener current.

A normal p-n junction diode does not operate in breakdown region because the excess current permanently damages the diode. Normal p-n junction diodes are not designed to operate in reverse breakdown region. Therefore, a normal p-n junction diode does not operate in reverse breakdown region.

What is zener diode?

A zener diode is a special type of device designed to operate in the zener breakdown region. Zener diodes acts like normal p-n junction diodes under forward biased condition. When forward biased voltage is applied to the zener diode it allows large amount of electric current and blocks only a small amount of electric current.

Zener diode is heavily doped than the normal p-n junction diode. Hence, it has very thin depletion region. Therefore, zener diodes allow more electric current than the normal p-n junction diodes.

Zener diode allows electric current in forward direction like a normal diode but also allows electric current in the reverse direction if the applied reverse voltage is greater than the zener voltage. Zener diode is always connected in reverse direction because it is specifically designed to work in reverse direction.

Zener Diode in Hindi

Zener diode definition

A zener diode is a p-n junction semiconductor device designed to operate in the reverse breakdown region. The breakdown voltage of a zener diode is carefully set by controlling the doping level during manufacture.

The name zener diode was named after the American physicist Clarance Melvin Zener who discovered the zener effect. Zener diodes are the basic building blocks of electronic circuits. They are widely used in all kinds of electronic equipments. Zener diodes are mainly used to protect electronic circuits from over voltage.

Breakdown in zener diode

There are two types of reverse breakdown regions in a zener diode: avalanche breakdown and zener breakdown.
Avalanche breakdown

The avalanche breakdown occurs in both normal diodes and zener diodes at high reverse voltage. When high reverse voltage is applied to the p-n junction diode, the free electrons (minority carriers) gains large amount of energy and accelerated to greater velocities.

The free electrons moving at high speed will collides with the atoms and knock off more electrons. These electrons are again accelerated and collide with other atoms. Because of this continuous collision with the atoms, a large number of free electrons are generated. As a result, electric current in the diode increases rapidly. This sudden increase in electric current may permanently destroys the normal diode. However, avalanche diodes may not be destroyed because they are carefully designed to operate in avalanche breakdown region. Avalanche breakdown occurs in zener diodes with zener voltage (Vz) greater than 6V.

Zener breakdown

The zener breakdown occurs in heavily doped p-n junction diodes because of their narrow depletion region. When reverse biased voltage applied to the diode is increased, the narrow depletion region generates strong electric field.

When reverse biased voltage applied to the diode reaches close to zener voltage, the electric field in the depletion region is strong enough to pull electrons from their valence band. The valence electrons which gains sufficient energy from the strong electric field of depletion region will breaks bonding with the parent atom. The valance electrons which break bonding with parent atom will become free electrons. This free electrons carry electric current from one place to another place. At zener breakdown region, a small increase in voltage will rapidly increases the electric current.

Zener breakdown occurs at low reverse voltage whereas avalanche breakdown occurs at high reverse voltage.
Zener breakdown occurs in zener diodes because they have very thin depletion region.
Breakdown region is the normal operating region for a zener diode.
Zener breakdown occurs in zener diodes with zener voltage (Vz) less than 6V.

Symbol of zener diode

The symbol of zener diode is shown in below figure. Zener diode consists of two terminals: cathode and anode.

In zener diode, electric current flows from both anode to cathode and cathode to anode.

The symbol of zener diode is similar to the normal p-n junction diode, but with bend edges on the vertical bar.

VI characteristics of zener diode

The VI characteristics of a zener diode is shown in the below figure. When forward biased voltage is applied to the zener diode, it works like a normal diode. However, when reverse biased voltage is applied to the zener diode, it works in different manner.

When reverse biased voltage is applied to a zener diode, it allows only a small amount of leakage current until the voltage is less than zener voltage. When reverse biased voltage applied to the zener diode reaches zener voltage, it starts allowing large amount of electric current. At this point, a small increase in reverse voltage will rapidly increases the electric current. Because of this sudden rise in electric current, breakdown occurs called zener breakdown. However, zener diode exhibits a controlled breakdown that does damage the device.

The zener breakdown voltage of the zener diode is depends on the amount of doping applied. If the diode is heavily doped, zener breakdown occurs at low reverse voltages. On the other hand, if the diode is lightly doped, the zener breakdown occurs at high reverse voltages. Zener diodes are available with zener voltages in the range of 1.8V to 400V.

Advantages of zener diode

Power dissipation capacity is very high
High accuracy
Small size
Low cost

Applications of zener diode

It is normally used as voltage reference
Zener diodes are used in voltage stabilizers or shunt regulators.
Zener diodes are used in switching operations
Zener diodes are used in clipping and clamping circuits.
Zener diodes are used in various protection circuits

Blog World

Conductor, Semiconductor And Insulator

2021-08-17T14:06:00.011+05:30

Levels of conductivity are the main difference between conductors, semiconductors and insulators. Conductors display high conductivity, which means they allow energy, such as electricity, heat or sound, to easily flow through them. Whereas semiconductors allow a moderate flow and insulators exhibit low conductivity.

Band Theory

Band theory is one of the main ways of explaining differences in conduction. This uses the ‘band’ of material to explain a number of physical properties of conduction.

Electrons orbit the positive nucleus of an individual atom within permitted levels of energy. In a lot of atoms, energy levels reorganise into two bands, namely the valence band and the conduction band. The valence band is the lower level of electrons and the conduction band is the higher level of electrons.

An energy gap exists between the bands where electrons can’t exist. When conduction occurs electrons move and for this to happen there has to be spaces in the energy bands for the electrons to move into.

The Definition of a Conductor

A conductor facilitates the easy flow of an electron from one atom to another atom when the proper application of voltage. This is because there are no band gaps between the valence and conduction bands.

In some materials, there is an overlapping of the conductor and valence bands, which means electrons can move between the two overlapping bands. As there is space for elections to move into in the conduction band, valence band electron moves into the other band and conduction is allowed.

Silver is probably the best electrical conductor we encounter in everyday life. Other metals, such as gold, copper, steel, aluminium and brass also represent good conductors. You’ll find these materials in everyday electrical equipment, either in the form of wires or etched onto circuit boards.

Solids are normally the best types of conductors, however, some liquids including liquid metals such as mercury are also good at permitting the transmission of energy through them.

Some materials are classed as superconductors. At extremely low temperatures these materials will conduct without resistance.

The Definition of a Semiconductor

With moderate conductivity, a semiconductor has a conductivity value between that of a conductor such as silver and an insulator, such as the mica we use in Elmelin’s product range.

The resistance of a semiconductor falls as its temperature rises. Elements like silicon (Si), germanium (Ge), selenium (Se); compounds like gallium arsenide (GaAs) and indium antimonide (InSb) are all examples of semiconductor elements. Silicon represents the most widely used semiconductor.

There is a gap between the valance and conduction bands in a semiconductor, however, it’s small enough to facilitate the movement of electrons at room temperature, enabling some conduction.

A rise in temperature increases the conductivity of a semiconductor because more electrons will have enough energy to move into the conduction band.

Ordinarily, gases are poor conductors due to the space between atoms. However, in some circumstances – such as when it contains a large number of ions – gasses can be fair conductors and act as semiconductors.

The Definition of an Insulator

An insulator prevents the flow of energy between two objects. For example, insulators may prevent the flow of electric, heat or sound.

Thermal insulators, reduce the transfer of heat between two objects of differing temperatures. Thermal insulators do this by reflecting thermal energy. The insulative capacity of a material is the inverse of thermal conductivity (k) and therefore those materials with low thermal conductivity will have high insulating capability or resistance value. Other important properties to consider are product density (ρ) and specific heat capacity ©.

A substance that does not conduct electricity is called a dielectric material. These substances can be polarised by an applied electric field so electric charges do not flow through them as they would through a conductor. Therefore, the internal electrical field reduces the overall field within the dielectric.

In insulators, there are larger gaps between the conduction and valence bands. The electrons cannot move into the conduction band and this means the material cannot conduct.

Uses of Conductors, Semiconductors & Insulators

Uses of Conductors

Conductors can be found in a range of everyday situations, for example:

Thermometres: Mercury has traditionally been used in thermometres to measure body temperature.
Radiators: central heating systems traditionally rely on radiators made from conductive metals to quickly transfer the heat of the radiator into the room.
Cooking pans: iron was traditionally used to quickly conduct heat from a flame to the food in the pan.

Uses of Semiconductors

Semiconductors are all around us but perhaps are less obvious than conductors or insulators. Semiconductors use include:

Transistors: very large scale integration(VLSI) technology means that tiny transistors are in almost every gadget we use.
Solar cells: these are made up of p-type and n-type semiconductors, which are used in solar panels to turn sunlight into electricity.

Uses of Insulators

Insulators have a wide range of applications from everyday use through to specialist and high-tech industrial applications. Insulators include:

Wall insulation: this normally comes in the form of thermal insulation to regulate the heat flow between a building and the outside environment.
Furnace insulation: One of Elmelin’s specialities is the use of mica to create thermal and dielectric barriers in foundry environments.
Sound insulation: this is most obviously applied to sound studios, however, it can also be applied to regular buildings to prevent disturbances between rooms and properties.
Electrical insulation: this can range from the coating of wire in household circuitry through to the insulation in capacitors in commercial and consumer goods.

Energy bands in conductors

2021-08-17T13:03:00.007+05:30

The lower completely filled band is called valence band and the upper empty band is called conduction band. The gap between the top of the valence band and bottom of the conduction band is called the energy band gap. It may be large, small, or zero depending on the material.

Valence Energy Band:

They have valence electrons. This band may be partially or completely filled with electrons. This band is never empty. In this band, electrons are not capable of gaining energy from the external electric field. Therefore, the electrons in this band do not contribute to the electric current.

Conduction Energy Band:

Here the electrons are rarely present. This band is either empty or partially filled with electrons. In this band, the electrons can gain energy from the external electric field. Electrons in this band contribute to the electric current.

Forbidden Energy gap:

The separation between the valence band and conduction band is known as forbidden energy gap. If an electron is to be transferred from valence band to conduction band, external energy is required, which is equal to the forbidden energy gap.